about summary refs log tree commit diff
path: root/src/libexpr/primops.cc
blob: 9e7651da725f2333087e77799d790cf7f566ca2e (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
#include "archive.hh"
#include "derivations.hh"
#include "download.hh"
#include "eval-inline.hh"
#include "eval.hh"
#include "globals.hh"
#include "json-to-value.hh"
#include "names.hh"
#include "store-api.hh"
#include "util.hh"
#include "json.hh"
#include "value-to-json.hh"
#include "value-to-xml.hh"
#include "primops.hh"

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

#include <algorithm>
#include <cstring>
#include <regex>
#include <dlfcn.h>


namespace nix {


/*************************************************************
 * Miscellaneous
 *************************************************************/


/* Decode a context string ‘!<name>!<path>’ into a pair <path,
   name>. */
std::pair<string, string> decodeContext(const string & s)
{
    if (s.at(0) == '!') {
        size_t index = s.find("!", 1);
        return std::pair<string, string>(string(s, index + 1), string(s, 1, index - 1));
    } else
        return std::pair<string, string>(s.at(0) == '/' ? s : string(s, 1), "");
}


InvalidPathError::InvalidPathError(const Path & path) :
    EvalError(format("path '%1%' is not valid") % path), path(path) {}

void EvalState::realiseContext(const PathSet & context)
{
    PathSet drvs;

    for (auto & i : context) {
        std::pair<string, string> decoded = decodeContext(i);
        Path ctx = decoded.first;
        assert(store->isStorePath(ctx));
        if (!store->isValidPath(ctx))
            throw InvalidPathError(ctx);
        if (!decoded.second.empty() && nix::isDerivation(ctx)) {
            drvs.insert(decoded.first + "!" + decoded.second);

            /* Add the output of this derivation to the allowed
               paths. */
            if (allowedPaths) {
                auto drv = store->derivationFromPath(decoded.first);
                DerivationOutputs::iterator i = drv.outputs.find(decoded.second);
                if (i == drv.outputs.end())
                    throw Error("derivation '%s' does not have an output named '%s'", decoded.first, decoded.second);
                allowedPaths->insert(i->second.path);
            }
        }
    }

    if (drvs.empty()) return;

    if (!settings.enableImportFromDerivation)
        throw EvalError(format("attempted to realize '%1%' during evaluation but 'allow-import-from-derivation' is false") % *(drvs.begin()));

    /* For performance, prefetch all substitute info. */
    PathSet willBuild, willSubstitute, unknown;
    unsigned long long downloadSize, narSize;
    store->queryMissing(drvs, willBuild, willSubstitute, unknown, downloadSize, narSize);
    store->buildPaths(drvs);
}


/* Load and evaluate an expression from path specified by the
   argument. */
static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(pos, *args[1], context);

    try {
        state.realiseContext(context);
    } catch (InvalidPathError & e) {
        throw EvalError(format("cannot import '%1%', since path '%2%' is not valid, at %3%")
            % path % e.path % pos);
    }

    Path realPath = state.checkSourcePath(state.toRealPath(path, context));

    if (state.store->isStorePath(path) && state.store->isValidPath(path) && isDerivation(path)) {
        Derivation drv = readDerivation(realPath);
        Value & w = *state.allocValue();
        state.mkAttrs(w, 3 + drv.outputs.size());
        Value * v2 = state.allocAttr(w, state.sDrvPath);
        mkString(*v2, path, {"=" + path});
        v2 = state.allocAttr(w, state.sName);
        mkString(*v2, drv.env["name"]);
        Value * outputsVal =
            state.allocAttr(w, state.symbols.create("outputs"));
        state.mkList(*outputsVal, drv.outputs.size());
        unsigned int outputs_index = 0;

        for (const auto & o : drv.outputs) {
            v2 = state.allocAttr(w, state.symbols.create(o.first));
            mkString(*v2, o.second.path, {"!" + o.first + "!" + path});
            outputsVal->listElems()[outputs_index] = state.allocValue();
            mkString(*(outputsVal->listElems()[outputs_index++]), o.first);
        }
        w.attrs->sort();
        Value fun;
        state.evalFile(settings.nixDataDir + "/nix/corepkgs/imported-drv-to-derivation.nix", fun);
        state.forceFunction(fun, pos);
        mkApp(v, fun, w);
        state.forceAttrs(v, pos);
    } else {
        state.forceAttrs(*args[0]);
        if (args[0]->attrs->empty())
            state.evalFile(realPath, v);
        else {
            Env * env = &state.allocEnv(args[0]->attrs->size());
            env->up = &state.baseEnv;

            StaticEnv staticEnv(false, &state.staticBaseEnv);

            unsigned int displ = 0;
            for (auto & attr : *args[0]->attrs) {
                staticEnv.vars[attr.name] = displ;
                env->values[displ++] = attr.value;
            }

            printTalkative("evaluating file '%1%'", realPath);
            Expr * e = state.parseExprFromFile(resolveExprPath(realPath), staticEnv);

            e->eval(state, *env, v);
        }
    }
}


/* Want reasonable symbol names, so extern C */
/* !!! Should we pass the Pos or the file name too? */
extern "C" typedef void (*ValueInitializer)(EvalState & state, Value & v);

/* Load a ValueInitializer from a DSO and return whatever it initializes */
void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(pos, *args[0], context);

    try {
        state.realiseContext(context);
    } catch (InvalidPathError & e) {
        throw EvalError(format("cannot import '%1%', since path '%2%' is not valid, at %3%")
            % path % e.path % pos);
    }

    path = state.checkSourcePath(path);

    string sym = state.forceStringNoCtx(*args[1], pos);

    void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
    if (!handle)
        throw EvalError(format("could not open '%1%': %2%") % path % dlerror());

    dlerror();
    ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
    if(!func) {
        char *message = dlerror();
        if (message)
            throw EvalError(format("could not load symbol '%1%' from '%2%': %3%") % sym % path % message);
        else
            throw EvalError(format("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected")
                    % sym % path);
    }

    (func)(state, v);

    /* We don't dlclose because v may be a primop referencing a function in the shared object file */
}


/* Execute a program and parse its output */
void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);
    auto elems = args[0]->listElems();
    auto count = args[0]->listSize();
    if (count == 0) {
        throw EvalError(format("at least one argument to 'exec' required, at %1%") % pos);
    }
    PathSet context;
    auto program = state.coerceToString(pos, *elems[0], context, false, false);
    Strings commandArgs;
    for (unsigned int i = 1; i < args[0]->listSize(); ++i) {
        commandArgs.emplace_back(state.coerceToString(pos, *elems[i], context, false, false));
    }
    try {
        state.realiseContext(context);
    } catch (InvalidPathError & e) {
        throw EvalError(format("cannot execute '%1%', since path '%2%' is not valid, at %3%")
            % program % e.path % pos);
    }

    auto output = runProgram(program, true, commandArgs);
    Expr * parsed;
    try {
        parsed = state.parseExprFromString(output, pos.file);
    } catch (Error & e) {
        e.addPrefix(format("While parsing the output from '%1%', at %2%\n") % program % pos);
        throw;
    }
    try {
        state.eval(parsed, v);
    } catch (Error & e) {
        e.addPrefix(format("While evaluating the output from '%1%', at %2%\n") % program % pos);
        throw;
    }
}


/* Return a string representing the type of the expression. */
static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    string t;
    switch (args[0]->type) {
        case tInt: t = "int"; break;
        case tBool: t = "bool"; break;
        case tString: t = "string"; break;
        case tPath: t = "path"; break;
        case tNull: t = "null"; break;
        case tAttrs: t = "set"; break;
        case tList1: case tList2: case tListN: t = "list"; break;
        case tLambda:
        case tPrimOp:
        case tPrimOpApp:
            t = "lambda";
            break;
        case tExternal:
            t = args[0]->external->typeOf();
            break;
        case tFloat: t = "float"; break;
        default: abort();
    }
    mkString(v, state.symbols.create(t));
}


/* Determine whether the argument is the null value. */
static void prim_isNull(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tNull);
}


/* Determine whether the argument is a function. */
static void prim_isFunction(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tLambda);
}


/* Determine whether the argument is an integer. */
static void prim_isInt(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tInt);
}

/* Determine whether the argument is a float. */
static void prim_isFloat(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tFloat);
}

/* Determine whether the argument is a string. */
static void prim_isString(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tString);
}


/* Determine whether the argument is a Boolean. */
static void prim_isBool(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tBool);
}


struct CompareValues
{
    bool operator () (const Value * v1, const Value * v2) const
    {
        if (v1->type == tFloat && v2->type == tInt)
            return v1->fpoint < v2->integer;
        if (v1->type == tInt && v2->type == tFloat)
            return v1->integer < v2->fpoint;
        if (v1->type != v2->type)
            throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
        switch (v1->type) {
            case tInt:
                return v1->integer < v2->integer;
            case tFloat:
                return v1->fpoint < v2->fpoint;
            case tString:
                return strcmp(v1->string.s, v2->string.s) < 0;
            case tPath:
                return strcmp(v1->path, v2->path) < 0;
            default:
                throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
        }
    }
};


#if HAVE_BOEHMGC
typedef list<Value *, gc_allocator<Value *> > ValueList;
#else
typedef list<Value *> ValueList;
#endif


static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);

    /* Get the start set. */
    Bindings::iterator startSet =
        args[0]->attrs->find(state.symbols.create("startSet"));
    if (startSet == args[0]->attrs->end())
        throw EvalError(format("attribute 'startSet' required, at %1%") % pos);
    state.forceList(*startSet->value, pos);

    ValueList workSet;
    for (unsigned int n = 0; n < startSet->value->listSize(); ++n)
        workSet.push_back(startSet->value->listElems()[n]);

    /* Get the operator. */
    Bindings::iterator op =
        args[0]->attrs->find(state.symbols.create("operator"));
    if (op == args[0]->attrs->end())
        throw EvalError(format("attribute 'operator' required, at %1%") % pos);
    state.forceValue(*op->value);

    /* Construct the closure by applying the operator to element of
       `workSet', adding the result to `workSet', continuing until
       no new elements are found. */
    ValueList res;
    // `doneKeys' doesn't need to be a GC root, because its values are
    // reachable from res.
    set<Value *, CompareValues> doneKeys;
    while (!workSet.empty()) {
        Value * e = *(workSet.begin());
        workSet.pop_front();

        state.forceAttrs(*e, pos);

        Bindings::iterator key =
            e->attrs->find(state.symbols.create("key"));
        if (key == e->attrs->end())
            throw EvalError(format("attribute 'key' required, at %1%") % pos);
        state.forceValue(*key->value);

        if (doneKeys.find(key->value) != doneKeys.end()) continue;
        doneKeys.insert(key->value);
        res.push_back(e);

        /* Call the `operator' function with `e' as argument. */
        Value call;
        mkApp(call, *op->value, *e);
        state.forceList(call, pos);

        /* Add the values returned by the operator to the work set. */
        for (unsigned int n = 0; n < call.listSize(); ++n) {
            state.forceValue(*call.listElems()[n]);
            workSet.push_back(call.listElems()[n]);
        }
    }

    /* Create the result list. */
    state.mkList(v, res.size());
    unsigned int n = 0;
    for (auto & i : res)
        v.listElems()[n++] = i;
}


static void prim_abort(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(pos, *args[0], context);
    throw Abort(format("evaluation aborted with the following error message: '%1%'") % s);
}


static void prim_throw(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(pos, *args[0], context);
    throw ThrownError(s);
}


static void prim_addErrorContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    try {
        state.forceValue(*args[1]);
        v = *args[1];
    } catch (Error & e) {
        PathSet context;
        e.addPrefix(format("%1%\n") % state.coerceToString(pos, *args[0], context));
        throw;
    }
}


/* Try evaluating the argument. Success => {success=true; value=something;},
 * else => {success=false; value=false;} */
static void prim_tryEval(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.mkAttrs(v, 2);
    try {
        state.forceValue(*args[0]);
        v.attrs->push_back(Attr(state.sValue, args[0]));
        mkBool(*state.allocAttr(v, state.symbols.create("success")), true);
    } catch (AssertionError & e) {
        mkBool(*state.allocAttr(v, state.sValue), false);
        mkBool(*state.allocAttr(v, state.symbols.create("success")), false);
    }
    v.attrs->sort();
}


/* Return an environment variable.  Use with care. */
static void prim_getEnv(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string name = state.forceStringNoCtx(*args[0], pos);
    mkString(v, settings.restrictEval || settings.pureEval ? "" : getEnv(name));
}


/* Evaluate the first argument, then return the second argument. */
static void prim_seq(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    state.forceValue(*args[1]);
    v = *args[1];
}


/* Evaluate the first argument deeply (i.e. recursing into lists and
   attrsets), then return the second argument. */
static void prim_deepSeq(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValueDeep(*args[0]);
    state.forceValue(*args[1]);
    v = *args[1];
}


/* Evaluate the first expression and print it on standard error.  Then
   return the second expression.  Useful for debugging. */
static void prim_trace(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    if (args[0]->type == tString)
        printError(format("trace: %1%") % args[0]->string.s);
    else
        printError(format("trace: %1%") % *args[0]);
    state.forceValue(*args[1]);
    v = *args[1];
}


void prim_valueSize(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    /* We're not forcing the argument on purpose. */
    mkInt(v, valueSize(*args[0]));
}


/*************************************************************
 * Derivations
 *************************************************************/


/* Construct (as a unobservable side effect) a Nix derivation
   expression that performs the derivation described by the argument
   set.  Returns the original set extended with the following
   attributes: `outPath' containing the primary output path of the
   derivation; `drvPath' containing the path of the Nix expression;
   and `type' set to `derivation' to indicate that this is a
   derivation. */
static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);

    /* Figure out the name first (for stack backtraces). */
    Bindings::iterator attr = args[0]->attrs->find(state.sName);
    if (attr == args[0]->attrs->end())
        throw EvalError(format("required attribute 'name' missing, at %1%") % pos);
    string drvName;
    Pos & posDrvName(*attr->pos);
    try {
        drvName = state.forceStringNoCtx(*attr->value, pos);
    } catch (Error & e) {
        e.addPrefix(format("while evaluating the derivation attribute 'name' at %1%:\n") % posDrvName);
        throw;
    }

    /* Check whether attributes should be passed as a JSON file. */
    std::ostringstream jsonBuf;
    std::unique_ptr<JSONObject> jsonObject;
    attr = args[0]->attrs->find(state.sStructuredAttrs);
    if (attr != args[0]->attrs->end() && state.forceBool(*attr->value, pos))
        jsonObject = std::make_unique<JSONObject>(jsonBuf);

    /* Check whether null attributes should be ignored. */
    bool ignoreNulls = false;
    attr = args[0]->attrs->find(state.sIgnoreNulls);
    if (attr != args[0]->attrs->end())
        ignoreNulls = state.forceBool(*attr->value, pos);

    /* Build the derivation expression by processing the attributes. */
    Derivation drv;

    PathSet context;

    std::experimental::optional<std::string> outputHash;
    std::string outputHashAlgo;
    bool outputHashRecursive = false;

    StringSet outputs;
    outputs.insert("out");

    for (auto & i : args[0]->attrs->lexicographicOrder()) {
        if (i->name == state.sIgnoreNulls) continue;
        const string & key = i->name;
        vomit("processing attribute '%1%'", key);

        auto handleHashMode = [&](const std::string & s) {
            if (s == "recursive") outputHashRecursive = true;
            else if (s == "flat") outputHashRecursive = false;
            else throw EvalError("invalid value '%s' for 'outputHashMode' attribute, at %s", s, posDrvName);
        };

        auto handleOutputs = [&](const Strings & ss) {
            outputs.clear();
            for (auto & j : ss) {
                if (outputs.find(j) != outputs.end())
                    throw EvalError(format("duplicate derivation output '%1%', at %2%") % j % posDrvName);
                /* !!! Check whether j is a valid attribute
                   name. */
                /* Derivations cannot be named ‘drv’, because
                   then we'd have an attribute ‘drvPath’ in
                   the resulting set. */
                if (j == "drv")
                    throw EvalError(format("invalid derivation output name 'drv', at %1%") % posDrvName);
                outputs.insert(j);
            }
            if (outputs.empty())
                throw EvalError(format("derivation cannot have an empty set of outputs, at %1%") % posDrvName);
        };

        try {

            if (ignoreNulls) {
                state.forceValue(*i->value);
                if (i->value->type == tNull) continue;
            }

            /* The `args' attribute is special: it supplies the
               command-line arguments to the builder. */
            if (i->name == state.sArgs) {
                state.forceList(*i->value, pos);
                for (unsigned int n = 0; n < i->value->listSize(); ++n) {
                    string s = state.coerceToString(posDrvName, *i->value->listElems()[n], context, true);
                    drv.args.push_back(s);
                }
            }

            /* All other attributes are passed to the builder through
               the environment. */
            else {

                if (jsonObject) {

                    if (i->name == state.sStructuredAttrs) continue;

                    auto placeholder(jsonObject->placeholder(key));
                    printValueAsJSON(state, true, *i->value, placeholder, context);

                    if (i->name == state.sBuilder)
                        drv.builder = state.forceString(*i->value, context, posDrvName);
                    else if (i->name == state.sSystem)
                        drv.platform = state.forceStringNoCtx(*i->value, posDrvName);
                    else if (i->name == state.sOutputHash)
                        outputHash = state.forceStringNoCtx(*i->value, posDrvName);
                    else if (i->name == state.sOutputHashAlgo)
                        outputHashAlgo = state.forceStringNoCtx(*i->value, posDrvName);
                    else if (i->name == state.sOutputHashMode)
                        handleHashMode(state.forceStringNoCtx(*i->value, posDrvName));
                    else if (i->name == state.sOutputs) {
                        /* Require ‘outputs’ to be a list of strings. */
                        state.forceList(*i->value, posDrvName);
                        Strings ss;
                        for (unsigned int n = 0; n < i->value->listSize(); ++n)
                            ss.emplace_back(state.forceStringNoCtx(*i->value->listElems()[n], posDrvName));
                        handleOutputs(ss);
                    }

                } else {
                    auto s = state.coerceToString(posDrvName, *i->value, context, true);
                    drv.env.emplace(key, s);
                    if (i->name == state.sBuilder) drv.builder = s;
                    else if (i->name == state.sSystem) drv.platform = s;
                    else if (i->name == state.sOutputHash) outputHash = s;
                    else if (i->name == state.sOutputHashAlgo) outputHashAlgo = s;
                    else if (i->name == state.sOutputHashMode) handleHashMode(s);
                    else if (i->name == state.sOutputs)
                        handleOutputs(tokenizeString<Strings>(s));
                }

            }

        } catch (Error & e) {
            e.addPrefix(format("while evaluating the attribute '%1%' of the derivation '%2%' at %3%:\n")
                % key % drvName % posDrvName);
            throw;
        }
    }

    if (jsonObject) {
        jsonObject.reset();
        drv.env.emplace("__json", jsonBuf.str());
    }

    /* Everything in the context of the strings in the derivation
       attributes should be added as dependencies of the resulting
       derivation. */
    for (auto & path : context) {

        /* Paths marked with `=' denote that the path of a derivation
           is explicitly passed to the builder.  Since that allows the
           builder to gain access to every path in the dependency
           graph of the derivation (including all outputs), all paths
           in the graph must be added to this derivation's list of
           inputs to ensure that they are available when the builder
           runs. */
        if (path.at(0) == '=') {
            /* !!! This doesn't work if readOnlyMode is set. */
            PathSet refs;
            state.store->computeFSClosure(string(path, 1), refs);
            for (auto & j : refs) {
                drv.inputSrcs.insert(j);
                if (isDerivation(j))
                    drv.inputDrvs[j] = state.store->queryDerivationOutputNames(j);
            }
        }

        /* See prim_unsafeDiscardOutputDependency. */
        else if (path.at(0) == '~')
            drv.inputSrcs.insert(string(path, 1));

        /* Handle derivation outputs of the form ‘!<name>!<path>’. */
        else if (path.at(0) == '!') {
            std::pair<string, string> ctx = decodeContext(path);
            drv.inputDrvs[ctx.first].insert(ctx.second);
        }

        /* Handle derivation contexts returned by
           ‘builtins.storePath’. */
        else if (isDerivation(path))
            drv.inputDrvs[path] = state.store->queryDerivationOutputNames(path);

        /* Otherwise it's a source file. */
        else
            drv.inputSrcs.insert(path);
    }

    /* Do we have all required attributes? */
    if (drv.builder == "")
        throw EvalError(format("required attribute 'builder' missing, at %1%") % posDrvName);
    if (drv.platform == "")
        throw EvalError(format("required attribute 'system' missing, at %1%") % posDrvName);

    /* Check whether the derivation name is valid. */
    checkStoreName(drvName);
    if (isDerivation(drvName))
        throw EvalError(format("derivation names are not allowed to end in '%1%', at %2%")
            % drvExtension % posDrvName);

    if (outputHash) {
        /* Handle fixed-output derivations. */
        if (outputs.size() != 1 || *(outputs.begin()) != "out")
            throw Error(format("multiple outputs are not supported in fixed-output derivations, at %1%") % posDrvName);

        HashType ht = parseHashType(outputHashAlgo);
        if (ht == htUnknown)
            throw EvalError(format("unknown hash algorithm '%1%', at %2%") % outputHashAlgo % posDrvName);
        Hash h(*outputHash, ht);
        outputHash = h.to_string(Base16, false);
        if (outputHashRecursive) outputHashAlgo = "r:" + outputHashAlgo;

        Path outPath = state.store->makeFixedOutputPath(outputHashRecursive, h, drvName);
        if (!jsonObject) drv.env["out"] = outPath;
        drv.outputs["out"] = DerivationOutput(outPath, outputHashAlgo, *outputHash);
    }

    else {
        /* Construct the "masked" store derivation, which is the final
           one except that in the list of outputs, the output paths
           are empty, and the corresponding environment variables have
           an empty value.  This ensures that changes in the set of
           output names do get reflected in the hash. */
        for (auto & i : outputs) {
            if (!jsonObject) drv.env[i] = "";
            drv.outputs[i] = DerivationOutput("", "", "");
        }

        /* Use the masked derivation expression to compute the output
           path. */
        Hash h = hashDerivationModulo(*state.store, drv);

        for (auto & i : drv.outputs)
            if (i.second.path == "") {
                Path outPath = state.store->makeOutputPath(i.first, h, drvName);
                if (!jsonObject) drv.env[i.first] = outPath;
                i.second.path = outPath;
            }
    }

    /* Write the resulting term into the Nix store directory. */
    Path drvPath = writeDerivation(state.store, drv, drvName, state.repair);

    printMsg(lvlChatty, format("instantiated '%1%' -> '%2%'")
        % drvName % drvPath);

    /* Optimisation, but required in read-only mode! because in that
       case we don't actually write store derivations, so we can't
       read them later. */
    drvHashes[drvPath] = hashDerivationModulo(*state.store, drv);

    state.mkAttrs(v, 1 + drv.outputs.size());
    mkString(*state.allocAttr(v, state.sDrvPath), drvPath, {"=" + drvPath});
    for (auto & i : drv.outputs) {
        mkString(*state.allocAttr(v, state.symbols.create(i.first)),
            i.second.path, {"!" + i.first + "!" + drvPath});
    }
    v.attrs->sort();
}


/* Return a placeholder string for the specified output that will be
   substituted by the corresponding output path at build time. For
   example, 'placeholder "out"' returns the string
   /1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9. At build
   time, any occurence of this string in an derivation attribute will
   be replaced with the concrete path in the Nix store of the output
   ‘out’. */
static void prim_placeholder(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    mkString(v, hashPlaceholder(state.forceStringNoCtx(*args[0], pos)));
}


/*************************************************************
 * Paths
 *************************************************************/


/* Convert the argument to a path.  !!! obsolete? */
static void prim_toPath(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(pos, *args[0], context);
    mkString(v, canonPath(path), context);
}


/* Allow a valid store path to be used in an expression.  This is
   useful in some generated expressions such as in nix-push, which
   generates a call to a function with an already existing store path
   as argument.  You don't want to use `toPath' here because it copies
   the path to the Nix store, which yields a copy like
   /nix/store/newhash-oldhash-oldname.  In the past, `toPath' had
   special case behaviour for store paths, but that created weird
   corner cases. */
static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));
    /* Resolve symlinks in ‘path’, unless ‘path’ itself is a symlink
       directly in the store.  The latter condition is necessary so
       e.g. nix-push does the right thing. */
    if (!state.store->isStorePath(path)) path = canonPath(path, true);
    if (!state.store->isInStore(path))
        throw EvalError(format("path '%1%' is not in the Nix store, at %2%") % path % pos);
    Path path2 = state.store->toStorePath(path);
    if (!settings.readOnlyMode)
        state.store->ensurePath(path2);
    context.insert(path2);
    mkString(v, path, context);
}


static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(pos, *args[0], context);
    if (!context.empty())
        throw EvalError(format("string '%1%' cannot refer to other paths, at %2%") % path % pos);
    try {
        mkBool(v, pathExists(state.checkSourcePath(path)));
    } catch (SysError & e) {
        /* Don't give away info from errors while canonicalising
           ‘path’ in restricted mode. */
        mkBool(v, false);
    } catch (RestrictedPathError & e) {
        mkBool(v, false);
    }
}


/* Return the base name of the given string, i.e., everything
   following the last slash. */
static void prim_baseNameOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    mkString(v, baseNameOf(state.coerceToString(pos, *args[0], context, false, false)), context);
}


/* Return the directory of the given path, i.e., everything before the
   last slash.  Return either a path or a string depending on the type
   of the argument. */
static void prim_dirOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path dir = dirOf(state.coerceToPath(pos, *args[0], context));
    if (args[0]->type == tPath) mkPath(v, dir.c_str()); else mkString(v, dir, context);
}


/* Return the contents of a file as a string. */
static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(pos, *args[0], context);
    try {
        state.realiseContext(context);
    } catch (InvalidPathError & e) {
        throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
            % path % e.path % pos);
    }
    string s = readFile(state.checkSourcePath(state.toRealPath(path, context)));
    if (s.find((char) 0) != string::npos)
        throw Error(format("the contents of the file '%1%' cannot be represented as a Nix string") % path);
    mkString(v, s.c_str());
}


/* Find a file in the Nix search path. Used to implement <x> paths,
   which are desugared to 'findFile __nixPath "x"'. */
static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);

    SearchPath searchPath;

    for (unsigned int n = 0; n < args[0]->listSize(); ++n) {
        Value & v2(*args[0]->listElems()[n]);
        state.forceAttrs(v2, pos);

        string prefix;
        Bindings::iterator i = v2.attrs->find(state.symbols.create("prefix"));
        if (i != v2.attrs->end())
            prefix = state.forceStringNoCtx(*i->value, pos);

        i = v2.attrs->find(state.symbols.create("path"));
        if (i == v2.attrs->end())
            throw EvalError(format("attribute 'path' missing, at %1%") % pos);

        PathSet context;
        string path = state.coerceToString(pos, *i->value, context, false, false);

        try {
            state.realiseContext(context);
        } catch (InvalidPathError & e) {
            throw EvalError(format("cannot find '%1%', since path '%2%' is not valid, at %3%")
                % path % e.path % pos);
        }

        searchPath.emplace_back(prefix, path);
    }

    string path = state.forceStringNoCtx(*args[1], pos);

    mkPath(v, state.checkSourcePath(state.findFile(searchPath, path, pos)).c_str());
}

/* Read a directory (without . or ..) */
static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet ctx;
    Path path = state.coerceToPath(pos, *args[0], ctx);
    try {
        state.realiseContext(ctx);
    } catch (InvalidPathError & e) {
        throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
            % path % e.path % pos);
    }

    DirEntries entries = readDirectory(state.checkSourcePath(path));
    state.mkAttrs(v, entries.size());

    for (auto & ent : entries) {
        Value * ent_val = state.allocAttr(v, state.symbols.create(ent.name));
        if (ent.type == DT_UNKNOWN)
            ent.type = getFileType(path + "/" + ent.name);
        mkStringNoCopy(*ent_val,
            ent.type == DT_REG ? "regular" :
            ent.type == DT_DIR ? "directory" :
            ent.type == DT_LNK ? "symlink" :
            "unknown");
    }

    v.attrs->sort();
}


/*************************************************************
 * Creating files
 *************************************************************/


/* Convert the argument (which can be any Nix expression) to an XML
   representation returned in a string.  Not all Nix expressions can
   be sensibly or completely represented (e.g., functions). */
static void prim_toXML(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    std::ostringstream out;
    PathSet context;
    printValueAsXML(state, true, false, *args[0], out, context);
    mkString(v, out.str(), context);
}


/* Convert the argument (which can be any Nix expression) to a JSON
   string.  Not all Nix expressions can be sensibly or completely
   represented (e.g., functions). */
static void prim_toJSON(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    std::ostringstream out;
    PathSet context;
    printValueAsJSON(state, true, *args[0], out, context);
    mkString(v, out.str(), context);
}


/* Parse a JSON string to a value. */
static void prim_fromJSON(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string s = state.forceStringNoCtx(*args[0], pos);
    parseJSON(state, s, v);
}


/* Store a string in the Nix store as a source file that can be used
   as an input by derivations. */
static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string name = state.forceStringNoCtx(*args[0], pos);
    string contents = state.forceString(*args[1], context, pos);

    PathSet refs;

    for (auto path : context) {
        if (path.at(0) == '=') path = string(path, 1);
        if (isDerivation(path)) {
            /* See prim_unsafeDiscardOutputDependency. */
            if (path.at(0) != '~')
                throw EvalError(format("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%") % name % pos);
            path = string(path, 1);
        }
        refs.insert(path);
    }

    Path storePath = settings.readOnlyMode
        ? state.store->computeStorePathForText(name, contents, refs)
        : state.store->addTextToStore(name, contents, refs, state.repair);

    /* Note: we don't need to add `context' to the context of the
       result, since `storePath' itself has references to the paths
       used in args[1]. */

    mkString(v, storePath, {storePath});
}


static void addPath(EvalState & state, const Pos & pos, const string & name, const Path & path_,
    Value * filterFun, bool recursive, const Hash & expectedHash, Value & v)
{
    const auto path = settings.pureEval && expectedHash ?
        path_ :
        state.checkSourcePath(path_);
    PathFilter filter = filterFun ? ([&](const Path & path) {
        auto st = lstat(path);

        /* Call the filter function.  The first argument is the path,
           the second is a string indicating the type of the file. */
        Value arg1;
        mkString(arg1, path);

        Value fun2;
        state.callFunction(*filterFun, arg1, fun2, noPos);

        Value arg2;
        mkString(arg2,
            S_ISREG(st.st_mode) ? "regular" :
            S_ISDIR(st.st_mode) ? "directory" :
            S_ISLNK(st.st_mode) ? "symlink" :
            "unknown" /* not supported, will fail! */);

        Value res;
        state.callFunction(fun2, arg2, res, noPos);

        return state.forceBool(res, pos);
    }) : defaultPathFilter;

    Path expectedStorePath;
    if (expectedHash) {
        expectedStorePath =
            state.store->makeFixedOutputPath(recursive, expectedHash, name);
    }
    Path dstPath;
    if (!expectedHash || !state.store->isValidPath(expectedStorePath)) {
        dstPath = settings.readOnlyMode
            ? state.store->computeStorePathForPath(name, path, recursive, htSHA256, filter).first
            : state.store->addToStore(name, path, recursive, htSHA256, filter, state.repair);
        if (expectedHash && expectedStorePath != dstPath) {
            throw Error(format("store path mismatch in (possibly filtered) path added from '%1%'") % path);
        }
    } else
        dstPath = expectedStorePath;

    mkString(v, dstPath, {dstPath});
}


static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(pos, *args[1], context);
    if (!context.empty())
        throw EvalError(format("string '%1%' cannot refer to other paths, at %2%") % path % pos);

    state.forceValue(*args[0]);
    if (args[0]->type != tLambda)
        throw TypeError(format("first argument in call to 'filterSource' is not a function but %1%, at %2%") % showType(*args[0]) % pos);

    addPath(state, pos, baseNameOf(path), path, args[0], true, Hash(), v);
}

static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);
    Path path;
    string name;
    Value * filterFun = nullptr;
    auto recursive = true;
    Hash expectedHash;

    for (auto & attr : *args[0]->attrs) {
        const string & n(attr.name);
        if (n == "path") {
            PathSet context;
            path = state.coerceToPath(*attr.pos, *attr.value, context);
            if (!context.empty())
                throw EvalError(format("string '%1%' cannot refer to other paths, at %2%") % path % *attr.pos);
        } else if (attr.name == state.sName)
            name = state.forceStringNoCtx(*attr.value, *attr.pos);
        else if (n == "filter") {
            state.forceValue(*attr.value);
            filterFun = attr.value;
        } else if (n == "recursive")
            recursive = state.forceBool(*attr.value, *attr.pos);
        else if (n == "sha256")
            expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
        else
            throw EvalError(format("unsupported argument '%1%' to 'addPath', at %2%") % attr.name % *attr.pos);
    }
    if (path.empty())
        throw EvalError(format("'path' required, at %1%") % pos);
    if (name.empty())
        name = baseNameOf(path);

    addPath(state, pos, name, path, filterFun, recursive, expectedHash, v);
}


/*************************************************************
 * Sets
 *************************************************************/


/* Return the names of the attributes in a set as a sorted list of
   strings. */
static void prim_attrNames(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);

    state.mkList(v, args[0]->attrs->size());

    size_t n = 0;
    for (auto & i : *args[0]->attrs)
        mkString(*(v.listElems()[n++] = state.allocValue()), i.name);

    std::sort(v.listElems(), v.listElems() + n,
              [](Value * v1, Value * v2) { return strcmp(v1->string.s, v2->string.s) < 0; });
}


/* Return the values of the attributes in a set as a list, in the same
   order as attrNames. */
static void prim_attrValues(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);

    state.mkList(v, args[0]->attrs->size());

    unsigned int n = 0;
    for (auto & i : *args[0]->attrs)
        v.listElems()[n++] = (Value *) &i;

    std::sort(v.listElems(), v.listElems() + n,
        [](Value * v1, Value * v2) { return (string) ((Attr *) v1)->name < (string) ((Attr *) v2)->name; });

    for (unsigned int i = 0; i < n; ++i)
        v.listElems()[i] = ((Attr *) v.listElems()[i])->value;
}


/* Dynamic version of the `.' operator. */
void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string attr = state.forceStringNoCtx(*args[0], pos);
    state.forceAttrs(*args[1], pos);
    // !!! Should we create a symbol here or just do a lookup?
    Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
    if (i == args[1]->attrs->end())
        throw EvalError(format("attribute '%1%' missing, at %2%") % attr % pos);
    // !!! add to stack trace?
    if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
    state.forceValue(*i->value);
    v = *i->value;
}


/* Return position information of the specified attribute. */
void prim_unsafeGetAttrPos(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string attr = state.forceStringNoCtx(*args[0], pos);
    state.forceAttrs(*args[1], pos);
    Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
    if (i == args[1]->attrs->end())
        mkNull(v);
    else
        state.mkPos(v, i->pos);
}


/* Dynamic version of the `?' operator. */
static void prim_hasAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string attr = state.forceStringNoCtx(*args[0], pos);
    state.forceAttrs(*args[1], pos);
    mkBool(v, args[1]->attrs->find(state.symbols.create(attr)) != args[1]->attrs->end());
}


/* Determine whether the argument is a set. */
static void prim_isAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->type == tAttrs);
}


static void prim_removeAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);
    state.forceList(*args[1], pos);

    /* Get the attribute names to be removed. */
    std::set<Symbol> names;
    for (unsigned int i = 0; i < args[1]->listSize(); ++i) {
        state.forceStringNoCtx(*args[1]->listElems()[i], pos);
        names.insert(state.symbols.create(args[1]->listElems()[i]->string.s));
    }

    /* Copy all attributes not in that set.  Note that we don't need
       to sort v.attrs because it's a subset of an already sorted
       vector. */
    state.mkAttrs(v, args[0]->attrs->size());
    for (auto & i : *args[0]->attrs) {
        if (names.find(i.name) == names.end())
            v.attrs->push_back(i);
    }
}


/* Builds a set from a list specifying (name, value) pairs.  To be
   precise, a list [{name = "name1"; value = value1;} ... {name =
   "nameN"; value = valueN;}] is transformed to {name1 = value1;
   ... nameN = valueN;}.  In case of duplicate occurences of the same
   name, the first takes precedence. */
static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);

    state.mkAttrs(v, args[0]->listSize());

    std::set<Symbol> seen;

    for (unsigned int i = 0; i < args[0]->listSize(); ++i) {
        Value & v2(*args[0]->listElems()[i]);
        state.forceAttrs(v2, pos);

        Bindings::iterator j = v2.attrs->find(state.sName);
        if (j == v2.attrs->end())
            throw TypeError(format("'name' attribute missing in a call to 'listToAttrs', at %1%") % pos);
        string name = state.forceStringNoCtx(*j->value, pos);

        Symbol sym = state.symbols.create(name);
        if (seen.find(sym) == seen.end()) {
            Bindings::iterator j2 = v2.attrs->find(state.symbols.create(state.sValue));
            if (j2 == v2.attrs->end())
                throw TypeError(format("'value' attribute missing in a call to 'listToAttrs', at %1%") % pos);

            v.attrs->push_back(Attr(sym, j2->value, j2->pos));
            seen.insert(sym);
        }
    }

    v.attrs->sort();
}


/* Return the right-biased intersection of two sets as1 and as2,
   i.e. a set that contains every attribute from as2 that is also a
   member of as1. */
static void prim_intersectAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceAttrs(*args[0], pos);
    state.forceAttrs(*args[1], pos);

    state.mkAttrs(v, std::min(args[0]->attrs->size(), args[1]->attrs->size()));

    for (auto & i : *args[0]->attrs) {
        Bindings::iterator j = args[1]->attrs->find(i.name);
        if (j != args[1]->attrs->end())
            v.attrs->push_back(*j);
    }
}


/* Collect each attribute named `attr' from a list of attribute sets.
   Sets that don't contain the named attribute are ignored.

   Example:
     catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
     => [1 2]
*/
static void prim_catAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    Symbol attrName = state.symbols.create(state.forceStringNoCtx(*args[0], pos));
    state.forceList(*args[1], pos);

    Value * res[args[1]->listSize()];
    unsigned int found = 0;

    for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
        Value & v2(*args[1]->listElems()[n]);
        state.forceAttrs(v2, pos);
        Bindings::iterator i = v2.attrs->find(attrName);
        if (i != v2.attrs->end())
            res[found++] = i->value;
    }

    state.mkList(v, found);
    for (unsigned int n = 0; n < found; ++n)
        v.listElems()[n] = res[n];
}


/* Return a set containing the names of the formal arguments expected
   by the function `f'.  The value of each attribute is a Boolean
   denoting whether the corresponding argument has a default value.  For instance,

      functionArgs ({ x, y ? 123}: ...)
   => { x = false; y = true; }

   "Formal argument" here refers to the attributes pattern-matched by
   the function.  Plain lambdas are not included, e.g.

      functionArgs (x: ...)
   => { }
*/
static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    if (args[0]->type != tLambda)
        throw TypeError(format("'functionArgs' requires a function, at %1%") % pos);

    if (!args[0]->lambda.fun->matchAttrs) {
        state.mkAttrs(v, 0);
        return;
    }

    state.mkAttrs(v, args[0]->lambda.fun->formals->formals.size());
    for (auto & i : args[0]->lambda.fun->formals->formals)
        // !!! should optimise booleans (allocate only once)
        mkBool(*state.allocAttr(v, i.name), i.def);
    v.attrs->sort();
}


/*************************************************************
 * Lists
 *************************************************************/


/* Determine whether the argument is a list. */
static void prim_isList(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    mkBool(v, args[0]->isList());
}


static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Value & v)
{
    state.forceList(list, pos);
    if (n < 0 || (unsigned int) n >= list.listSize())
        throw Error(format("list index %1% is out of bounds, at %2%") % n % pos);
    state.forceValue(*list.listElems()[n]);
    v = *list.listElems()[n];
}


/* Return the n-1'th element of a list. */
static void prim_elemAt(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    elemAt(state, pos, *args[0], state.forceInt(*args[1], pos), v);
}


/* Return the first element of a list. */
static void prim_head(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    elemAt(state, pos, *args[0], 0, v);
}


/* Return a list consisting of everything but the first element of
   a list.  Warning: this function takes O(n) time, so you probably
   don't want to use it!  */
static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);
    if (args[0]->listSize() == 0)
        throw Error(format("'tail' called on an empty list, at %1%") % pos);
    state.mkList(v, args[0]->listSize() - 1);
    for (unsigned int n = 0; n < v.listSize(); ++n)
        v.listElems()[n] = args[0]->listElems()[n + 1];
}


/* Apply a function to every element of a list. */
static void prim_map(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    state.forceList(*args[1], pos);

    state.mkList(v, args[1]->listSize());

    for (unsigned int n = 0; n < v.listSize(); ++n)
        mkApp(*(v.listElems()[n] = state.allocValue()),
            *args[0], *args[1]->listElems()[n]);
}


/* Filter a list using a predicate; that is, return a list containing
   every element from the list for which the predicate function
   returns true. */
static void prim_filter(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    state.forceList(*args[1], pos);

    // FIXME: putting this on the stack is risky.
    Value * vs[args[1]->listSize()];
    unsigned int k = 0;

    bool same = true;
    for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
        Value res;
        state.callFunction(*args[0], *args[1]->listElems()[n], res, noPos);
        if (state.forceBool(res, pos))
            vs[k++] = args[1]->listElems()[n];
        else
            same = false;
    }

    if (same)
        v = *args[1];
    else {
        state.mkList(v, k);
        for (unsigned int n = 0; n < k; ++n) v.listElems()[n] = vs[n];
    }
}


/* Return true if a list contains a given element. */
static void prim_elem(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    bool res = false;
    state.forceList(*args[1], pos);
    for (unsigned int n = 0; n < args[1]->listSize(); ++n)
        if (state.eqValues(*args[0], *args[1]->listElems()[n])) {
            res = true;
            break;
        }
    mkBool(v, res);
}


/* Concatenate a list of lists. */
static void prim_concatLists(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);
    state.concatLists(v, args[0]->listSize(), args[0]->listElems(), pos);
}


/* Return the length of a list.  This is an O(1) time operation. */
static void prim_length(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);
    mkInt(v, args[0]->listSize());
}


/* Reduce a list by applying a binary operator, from left to
   right. The operator is applied strictly. */
static void prim_foldlStrict(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    state.forceList(*args[2], pos);

    Value * vCur = args[1];

    if (args[2]->listSize())
        for (unsigned int n = 0; n < args[2]->listSize(); ++n) {
            Value vTmp;
            state.callFunction(*args[0], *vCur, vTmp, pos);
            vCur = n == args[2]->listSize() - 1 ? &v : state.allocValue();
            state.callFunction(vTmp, *args[2]->listElems()[n], *vCur, pos);
        }
    else
        v = *vCur;

    state.forceValue(v);
}


static void anyOrAll(bool any, EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    state.forceList(*args[1], pos);

    Value vTmp;
    for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
        state.callFunction(*args[0], *args[1]->listElems()[n], vTmp, pos);
        bool res = state.forceBool(vTmp, pos);
        if (res == any) {
            mkBool(v, any);
            return;
        }
    }

    mkBool(v, !any);
}


static void prim_any(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    anyOrAll(true, state, pos, args, v);
}


static void prim_all(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    anyOrAll(false, state, pos, args, v);
}


static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    auto len = state.forceInt(*args[1], pos);

    if (len < 0)
        throw EvalError(format("cannot create list of size %1%, at %2%") % len % pos);

    state.mkList(v, len);

    for (unsigned int n = 0; n < (unsigned int) len; ++n) {
        Value * arg = state.allocValue();
        mkInt(*arg, n);
        mkApp(*(v.listElems()[n] = state.allocValue()), *args[0], *arg);
    }
}


static void prim_lessThan(EvalState & state, const Pos & pos, Value * * args, Value & v);


static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    state.forceList(*args[1], pos);

    auto len = args[1]->listSize();
    state.mkList(v, len);
    for (unsigned int n = 0; n < len; ++n) {
        state.forceValue(*args[1]->listElems()[n]);
        v.listElems()[n] = args[1]->listElems()[n];
    }


    auto comparator = [&](Value * a, Value * b) {
        /* Optimization: if the comparator is lessThan, bypass
           callFunction. */
        if (args[0]->type == tPrimOp && args[0]->primOp->fun == prim_lessThan)
            return CompareValues()(a, b);

        Value vTmp1, vTmp2;
        state.callFunction(*args[0], *a, vTmp1, pos);
        state.callFunction(vTmp1, *b, vTmp2, pos);
        return state.forceBool(vTmp2, pos);
    };

    /* FIXME: std::sort can segfault if the comparator is not a strict
       weak ordering. What to do? std::stable_sort() seems more
       resilient, but no guarantees... */
    std::stable_sort(v.listElems(), v.listElems() + len, comparator);
}


static void prim_partition(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceFunction(*args[0], pos);
    state.forceList(*args[1], pos);

    auto len = args[1]->listSize();

    ValueVector right, wrong;

    for (unsigned int n = 0; n < len; ++n) {
        auto vElem = args[1]->listElems()[n];
        state.forceValue(*vElem);
        Value res;
        state.callFunction(*args[0], *vElem, res, pos);
        if (state.forceBool(res, pos))
            right.push_back(vElem);
        else
            wrong.push_back(vElem);
    }

    state.mkAttrs(v, 2);

    Value * vRight = state.allocAttr(v, state.sRight);
    auto rsize = right.size();
    state.mkList(*vRight, rsize);
    if (rsize)
        memcpy(vRight->listElems(), right.data(), sizeof(Value *) * rsize);

    Value * vWrong = state.allocAttr(v, state.sWrong);
    auto wsize = wrong.size();
    state.mkList(*vWrong, wsize);
    if (wsize)
        memcpy(vWrong->listElems(), wrong.data(), sizeof(Value *) * wsize);

    v.attrs->sort();
}


/*************************************************************
 * Integer arithmetic
 *************************************************************/


static void prim_add(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    if (args[0]->type == tFloat || args[1]->type == tFloat)
        mkFloat(v, state.forceFloat(*args[0], pos) + state.forceFloat(*args[1], pos));
    else
        mkInt(v, state.forceInt(*args[0], pos) + state.forceInt(*args[1], pos));
}


static void prim_sub(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    if (args[0]->type == tFloat || args[1]->type == tFloat)
        mkFloat(v, state.forceFloat(*args[0], pos) - state.forceFloat(*args[1], pos));
    else
        mkInt(v, state.forceInt(*args[0], pos) - state.forceInt(*args[1], pos));
}


static void prim_mul(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    if (args[0]->type == tFloat || args[1]->type == tFloat)
        mkFloat(v, state.forceFloat(*args[0], pos) * state.forceFloat(*args[1], pos));
    else
        mkInt(v, state.forceInt(*args[0], pos) * state.forceInt(*args[1], pos));
}


static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    NixFloat f2 = state.forceFloat(*args[1], pos);
    if (f2 == 0) throw EvalError(format("division by zero, at %1%") % pos);

    if (args[0]->type == tFloat || args[1]->type == tFloat) {
        mkFloat(v, state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
    } else {
        NixInt i1 = state.forceInt(*args[0], pos);
        NixInt i2 = state.forceInt(*args[1], pos);
        /* Avoid division overflow as it might raise SIGFPE. */
        if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
            throw EvalError(format("overflow in integer division, at %1%") % pos);
        mkInt(v, i1 / i2);
    }
}


static void prim_lessThan(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    state.forceValue(*args[1]);
    CompareValues comp;
    mkBool(v, comp(args[0], args[1]));
}


/*************************************************************
 * String manipulation
 *************************************************************/


/* Convert the argument to a string.  Paths are *not* copied to the
   store, so `toString /foo/bar' yields `"/foo/bar"', not
   `"/nix/store/whatever..."'. */
static void prim_toString(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(pos, *args[0], context, true, false);
    mkString(v, s, context);
}


/* `substring start len str' returns the substring of `str' starting
   at character position `min(start, stringLength str)' inclusive and
   ending at `min(start + len, stringLength str)'.  `start' must be
   non-negative. */
static void prim_substring(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    int start = state.forceInt(*args[0], pos);
    int len = state.forceInt(*args[1], pos);
    PathSet context;
    string s = state.coerceToString(pos, *args[2], context);

    if (start < 0) throw EvalError(format("negative start position in 'substring', at %1%") % pos);

    mkString(v, (unsigned int) start >= s.size() ? "" : string(s, start, len), context);
}


static void prim_stringLength(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(pos, *args[0], context);
    mkInt(v, s.size());
}


static void prim_unsafeDiscardStringContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(pos, *args[0], context);
    mkString(v, s, PathSet());
}


static void prim_hasContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    state.forceString(*args[0], context, pos);
    mkBool(v, !context.empty());
}


/* Sometimes we want to pass a derivation path (i.e. pkg.drvPath) to a
   builder without causing the derivation to be built (for instance,
   in the derivation that builds NARs in nix-push, when doing
   source-only deployment).  This primop marks the string context so
   that builtins.derivation adds the path to drv.inputSrcs rather than
   drv.inputDrvs. */
static void prim_unsafeDiscardOutputDependency(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(pos, *args[0], context);

    PathSet context2;
    for (auto & p : context)
        context2.insert(p.at(0) == '=' ? "~" + string(p, 1) : p);

    mkString(v, s, context2);
}


/* Return the cryptographic hash of a string in base-16. */
static void prim_hashString(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string type = state.forceStringNoCtx(*args[0], pos);
    HashType ht = parseHashType(type);
    if (ht == htUnknown)
      throw Error(format("unknown hash type '%1%', at %2%") % type % pos);

    PathSet context; // discarded
    string s = state.forceString(*args[1], context, pos);

    mkString(v, hashString(ht, s).to_string(Base16, false), context);
}


/* Match a regular expression against a string and return either
   ‘null’ or a list containing substring matches. */
static void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    auto re = state.forceStringNoCtx(*args[0], pos);

    try {

        std::regex regex(re, std::regex::extended);

        PathSet context;
        const std::string str = state.forceString(*args[1], context, pos);

        std::smatch match;
        if (!std::regex_match(str, match, regex)) {
            mkNull(v);
            return;
        }

        // the first match is the whole string
        const size_t len = match.size() - 1;
        state.mkList(v, len);
        for (size_t i = 0; i < len; ++i) {
            if (!match[i+1].matched)
                mkNull(*(v.listElems()[i] = state.allocValue()));
            else
                mkString(*(v.listElems()[i] = state.allocValue()), match[i + 1].str().c_str());
        }

    } catch (std::regex_error &e) {
        if (e.code() == std::regex_constants::error_space) {
          // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
          throw EvalError("memory limit exceeded by regular expression '%s', at %s", re, pos);
        } else {
          throw EvalError("invalid regular expression '%s', at %s", re, pos);
        }
    }
}


/* Split a string with a regular expression, and return a list of the
   non-matching parts interleaved by the lists of the matching groups. */
static void prim_split(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    auto re = state.forceStringNoCtx(*args[0], pos);

    try {

        std::regex regex(re, std::regex::extended);

        PathSet context;
        const std::string str = state.forceString(*args[1], context, pos);

        auto begin = std::sregex_iterator(str.begin(), str.end(), regex);
        auto end = std::sregex_iterator();

        // Any matches results are surrounded by non-matching results.
        const size_t len = std::distance(begin, end);
        state.mkList(v, 2 * len + 1);
        size_t idx = 0;
        Value * elem;

        if (len == 0) {
            v.listElems()[idx++] = args[1];
            return;
        }

        for (std::sregex_iterator i = begin; i != end; ++i) {
            assert(idx <= 2 * len + 1 - 3);
            std::smatch match = *i;

            // Add a string for non-matched characters.
            elem = v.listElems()[idx++] = state.allocValue();
            mkString(*elem, match.prefix().str().c_str());

            // Add a list for matched substrings.
            const size_t slen = match.size() - 1;
            elem = v.listElems()[idx++] = state.allocValue();

            // Start at 1, beacause the first match is the whole string.
            state.mkList(*elem, slen);
            for (size_t si = 0; si < slen; ++si) {
                if (!match[si + 1].matched)
                    mkNull(*(elem->listElems()[si] = state.allocValue()));
                else
                    mkString(*(elem->listElems()[si] = state.allocValue()), match[si + 1].str().c_str());
            }

            // Add a string for non-matched suffix characters.
            if (idx == 2 * len) {
                elem = v.listElems()[idx++] = state.allocValue();
                mkString(*elem, match.suffix().str().c_str());
            }
        }
        assert(idx == 2 * len + 1);

    } catch (std::regex_error &e) {
        if (e.code() == std::regex_constants::error_space) {
          // limit is _GLIBCXX_REGEX_STATE_LIMIT for libstdc++
          throw EvalError("memory limit exceeded by regular expression '%s', at %s", re, pos);
        } else {
          throw EvalError("invalid regular expression '%s', at %s", re, pos);
        }
    }
}


static void prim_concatStringSep(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    PathSet context;

    auto sep = state.forceString(*args[0], context, pos);
    state.forceList(*args[1], pos);

    string res;
    res.reserve((args[1]->listSize() + 32) * sep.size());
    bool first = true;

    for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
        if (first) first = false; else res += sep;
        res += state.coerceToString(pos, *args[1]->listElems()[n], context);
    }

    mkString(v, res, context);
}


static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    state.forceList(*args[0], pos);
    state.forceList(*args[1], pos);
    if (args[0]->listSize() != args[1]->listSize())
        throw EvalError(format("'from' and 'to' arguments to 'replaceStrings' have different lengths, at %1%") % pos);

    vector<string> from;
    from.reserve(args[0]->listSize());
    for (unsigned int n = 0; n < args[0]->listSize(); ++n)
        from.push_back(state.forceString(*args[0]->listElems()[n], pos));

    vector<std::pair<string, PathSet>> to;
    to.reserve(args[1]->listSize());
    for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
        PathSet ctx;
        auto s = state.forceString(*args[1]->listElems()[n], ctx, pos);
        to.push_back(std::make_pair(std::move(s), std::move(ctx)));
    }

    PathSet context;
    auto s = state.forceString(*args[2], context, pos);

    string res;
    // Loops one past last character to handle the case where 'from' contains an empty string.
    for (size_t p = 0; p <= s.size(); ) {
        bool found = false;
        auto i = from.begin();
        auto j = to.begin();
        for (; i != from.end(); ++i, ++j)
            if (s.compare(p, i->size(), *i) == 0) {
                found = true;
                res += j->first;
                if (i->empty()) {
                    if (p < s.size())
                        res += s[p];
                    p++;
                } else {
                    p += i->size();
                }
                for (auto& path : j->second)
                    context.insert(path);
                j->second.clear();
                break;
            }
        if (!found) {
            if (p < s.size())
                res += s[p];
            p++;
        }
    }

    mkString(v, res, context);
}


/*************************************************************
 * Versions
 *************************************************************/


static void prim_parseDrvName(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string name = state.forceStringNoCtx(*args[0], pos);
    DrvName parsed(name);
    state.mkAttrs(v, 2);
    mkString(*state.allocAttr(v, state.sName), parsed.name);
    mkString(*state.allocAttr(v, state.symbols.create("version")), parsed.version);
    v.attrs->sort();
}


static void prim_compareVersions(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string version1 = state.forceStringNoCtx(*args[0], pos);
    string version2 = state.forceStringNoCtx(*args[1], pos);
    mkInt(v, compareVersions(version1, version2));
}


static void prim_splitVersion(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    string version = state.forceStringNoCtx(*args[0], pos);
    auto iter = version.cbegin();
    Strings components;
    while (iter != version.cend()) {
        auto component = nextComponent(iter, version.cend());
        if (component.empty())
            break;
        components.emplace_back(std::move(component));
    }
    state.mkList(v, components.size());
    unsigned int n = 0;
    for (auto & component : components) {
        auto listElem = v.listElems()[n++] = state.allocValue();
        mkString(*listElem, std::move(component));
    }
}


/*************************************************************
 * Networking
 *************************************************************/


void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
    const string & who, bool unpack, const std::string & defaultName)
{
    string url;
    Hash expectedHash;
    string name = defaultName;

    state.forceValue(*args[0]);

    if (args[0]->type == tAttrs) {

        state.forceAttrs(*args[0], pos);

        for (auto & attr : *args[0]->attrs) {
            string n(attr.name);
            if (n == "url")
                url = state.forceStringNoCtx(*attr.value, *attr.pos);
            else if (n == "sha256")
                expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
            else if (n == "name")
                name = state.forceStringNoCtx(*attr.value, *attr.pos);
            else
                throw EvalError(format("unsupported argument '%1%' to '%2%', at %3%") % attr.name % who % attr.pos);
        }

        if (url.empty())
            throw EvalError(format("'url' argument required, at %1%") % pos);

    } else
        url = state.forceStringNoCtx(*args[0], pos);

    state.checkURI(url);

    if (settings.pureEval && !expectedHash)
        throw Error("in pure evaluation mode, '%s' requires a 'sha256' argument", who);

    Path res = getDownloader()->downloadCached(state.store, url, unpack, name, expectedHash);

    if (state.allowedPaths)
        state.allowedPaths->insert(res);

    mkString(v, res, PathSet({res}));
}


static void prim_fetchurl(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    fetch(state, pos, args, v, "fetchurl", false, "");
}


static void prim_fetchTarball(EvalState & state, const Pos & pos, Value * * args, Value & v)
{
    fetch(state, pos, args, v, "fetchTarball", true, "source");
}


/*************************************************************
 * Primop registration
 *************************************************************/


RegisterPrimOp::PrimOps * RegisterPrimOp::primOps;


RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun)
{
    if (!primOps) primOps = new PrimOps;
    primOps->emplace_back(name, arity, fun);
}


void EvalState::createBaseEnv()
{
    baseEnv.up = 0;

    /* Add global constants such as `true' to the base environment. */
    Value v;

    /* `builtins' must be first! */
    mkAttrs(v, 128);
    addConstant("builtins", v);

    mkBool(v, true);
    addConstant("true", v);

    mkBool(v, false);
    addConstant("false", v);

    mkNull(v);
    addConstant("null", v);

    auto vThrow = addPrimOp("throw", 1, prim_throw);

    auto addPurityError = [&](const std::string & name) {
        Value * v2 = allocValue();
        mkString(*v2, fmt("'%s' is not allowed in pure evaluation mode", name));
        mkApp(v, *vThrow, *v2);
        addConstant(name, v);
    };

    if (!settings.pureEval) {
        mkInt(v, time(0));
        addConstant("__currentTime", v);
    }

    if (!settings.pureEval) {
        mkString(v, settings.thisSystem);
        addConstant("__currentSystem", v);
    }

    mkString(v, nixVersion);
    addConstant("__nixVersion", v);

    mkString(v, store->storeDir);
    addConstant("__storeDir", v);

    /* Language version.  This should be increased every time a new
       language feature gets added.  It's not necessary to increase it
       when primops get added, because you can just use `builtins ?
       primOp' to check. */
    mkInt(v, 5);
    addConstant("__langVersion", v);

    // Miscellaneous
    auto vScopedImport = addPrimOp("scopedImport", 2, prim_scopedImport);
    Value * v2 = allocValue();
    mkAttrs(*v2, 0);
    mkApp(v, *vScopedImport, *v2);
    forceValue(v);
    addConstant("import", v);
    if (settings.enableNativeCode) {
        addPrimOp("__importNative", 2, prim_importNative);
        addPrimOp("__exec", 1, prim_exec);
    }
    addPrimOp("__typeOf", 1, prim_typeOf);
    addPrimOp("isNull", 1, prim_isNull);
    addPrimOp("__isFunction", 1, prim_isFunction);
    addPrimOp("__isString", 1, prim_isString);
    addPrimOp("__isInt", 1, prim_isInt);
    addPrimOp("__isFloat", 1, prim_isFloat);
    addPrimOp("__isBool", 1, prim_isBool);
    addPrimOp("__genericClosure", 1, prim_genericClosure);
    addPrimOp("abort", 1, prim_abort);
    addPrimOp("__addErrorContext", 2, prim_addErrorContext);
    addPrimOp("__tryEval", 1, prim_tryEval);
    addPrimOp("__getEnv", 1, prim_getEnv);

    // Strictness
    addPrimOp("__seq", 2, prim_seq);
    addPrimOp("__deepSeq", 2, prim_deepSeq);

    // Debugging
    addPrimOp("__trace", 2, prim_trace);
    addPrimOp("__valueSize", 1, prim_valueSize);

    // Paths
    addPrimOp("__toPath", 1, prim_toPath);
    if (settings.pureEval)
        addPurityError("__storePath");
    else
        addPrimOp("__storePath", 1, prim_storePath);
    addPrimOp("__pathExists", 1, prim_pathExists);
    addPrimOp("baseNameOf", 1, prim_baseNameOf);
    addPrimOp("dirOf", 1, prim_dirOf);
    addPrimOp("__readFile", 1, prim_readFile);
    addPrimOp("__readDir", 1, prim_readDir);
    addPrimOp("__findFile", 2, prim_findFile);

    // Creating files
    addPrimOp("__toXML", 1, prim_toXML);
    addPrimOp("__toJSON", 1, prim_toJSON);
    addPrimOp("__fromJSON", 1, prim_fromJSON);
    addPrimOp("__toFile", 2, prim_toFile);
    addPrimOp("__filterSource", 2, prim_filterSource);
    addPrimOp("__path", 1, prim_path);

    // Sets
    addPrimOp("__attrNames", 1, prim_attrNames);
    addPrimOp("__attrValues", 1, prim_attrValues);
    addPrimOp("__getAttr", 2, prim_getAttr);
    addPrimOp("__unsafeGetAttrPos", 2, prim_unsafeGetAttrPos);
    addPrimOp("__hasAttr", 2, prim_hasAttr);
    addPrimOp("__isAttrs", 1, prim_isAttrs);
    addPrimOp("removeAttrs", 2, prim_removeAttrs);
    addPrimOp("__listToAttrs", 1, prim_listToAttrs);
    addPrimOp("__intersectAttrs", 2, prim_intersectAttrs);
    addPrimOp("__catAttrs", 2, prim_catAttrs);
    addPrimOp("__functionArgs", 1, prim_functionArgs);

    // Lists
    addPrimOp("__isList", 1, prim_isList);
    addPrimOp("__elemAt", 2, prim_elemAt);
    addPrimOp("__head", 1, prim_head);
    addPrimOp("__tail", 1, prim_tail);
    addPrimOp("map", 2, prim_map);
    addPrimOp("__filter", 2, prim_filter);
    addPrimOp("__elem", 2, prim_elem);
    addPrimOp("__concatLists", 1, prim_concatLists);
    addPrimOp("__length", 1, prim_length);
    addPrimOp("__foldl'", 3, prim_foldlStrict);
    addPrimOp("__any", 2, prim_any);
    addPrimOp("__all", 2, prim_all);
    addPrimOp("__genList", 2, prim_genList);
    addPrimOp("__sort", 2, prim_sort);
    addPrimOp("__partition", 2, prim_partition);

    // Integer arithmetic
    addPrimOp("__add", 2, prim_add);
    addPrimOp("__sub", 2, prim_sub);
    addPrimOp("__mul", 2, prim_mul);
    addPrimOp("__div", 2, prim_div);
    addPrimOp("__lessThan", 2, prim_lessThan);

    // String manipulation
    addPrimOp("toString", 1, prim_toString);
    addPrimOp("__substring", 3, prim_substring);
    addPrimOp("__stringLength", 1, prim_stringLength);
    addPrimOp("__hasContext", 1, prim_hasContext);
    addPrimOp("__unsafeDiscardStringContext", 1, prim_unsafeDiscardStringContext);
    addPrimOp("__unsafeDiscardOutputDependency", 1, prim_unsafeDiscardOutputDependency);
    addPrimOp("__hashString", 2, prim_hashString);
    addPrimOp("__match", 2, prim_match);
    addPrimOp("__split", 2, prim_split);
    addPrimOp("__concatStringsSep", 2, prim_concatStringSep);
    addPrimOp("__replaceStrings", 3, prim_replaceStrings);

    // Versions
    addPrimOp("__parseDrvName", 1, prim_parseDrvName);
    addPrimOp("__compareVersions", 2, prim_compareVersions);
    addPrimOp("__splitVersion", 1, prim_splitVersion);

    // Derivations
    addPrimOp("derivationStrict", 1, prim_derivationStrict);
    addPrimOp("placeholder", 1, prim_placeholder);

    // Networking
    addPrimOp("__fetchurl", 1, prim_fetchurl);
    addPrimOp("fetchTarball", 1, prim_fetchTarball);

    /* Add a wrapper around the derivation primop that computes the
       `drvPath' and `outPath' attributes lazily. */
    string path = canonPath(settings.nixDataDir + "/nix/corepkgs/derivation.nix", true);
    sDerivationNix = symbols.create(path);
    evalFile(path, v);
    addConstant("derivation", v);

    /* Add a value containing the current Nix expression search path. */
    mkList(v, searchPath.size());
    int n = 0;
    for (auto & i : searchPath) {
        v2 = v.listElems()[n++] = allocValue();
        mkAttrs(*v2, 2);
        mkString(*allocAttr(*v2, symbols.create("path")), i.second);
        mkString(*allocAttr(*v2, symbols.create("prefix")), i.first);
        v2->attrs->sort();
    }
    addConstant("__nixPath", v);

    if (RegisterPrimOp::primOps)
        for (auto & primOp : *RegisterPrimOp::primOps)
            addPrimOp(std::get<0>(primOp), std::get<1>(primOp), std::get<2>(primOp));

    /* Now that we've added all primops, sort the `builtins' set,
       because attribute lookups expect it to be sorted. */
    baseEnv.values[0]->attrs->sort();
}


}