Add 'third_party/nix/' from commit 'be66c7a6b24e3c3c6157fd37b86c7203d14acf10' r/724

git-subtree-dir: third_party/nix
git-subtree-mainline: cf8cd640c1adf74a3706efbcb0ea4625da106fb2
git-subtree-split: be66c7a6b24e3c3c6157fd37b86c7203d14acf10

1 files changed, 2329 insertions, 0 deletions

diff --git a/third_party/nix/src/libexpr/primops.cc b/third_party/nix/src/libexpr/primops.cc
new file mode 100644
index 000000000000..d4c60f870ed2
--- /dev/null
+++ b/third_party/nix/src/libexpr/primops.cc
@@ -0,0 +1,2329 @@
+#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 (!evalSettings.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]);
+    bool res;
+    switch (args[0]->type) {
+        case tLambda:
+        case tPrimOp:
+        case tPrimOpApp:
+            res = true;
+            break;
+        default:
+            res = false;
+            break;
+    }
+    mkBool(v, res);
+}
+
+
+/* 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);
+}
+
+/* Determine whether the argument is a path. */
+static void prim_isPath(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[0]);
+    mkBool(v, args[0]->type == tPath);
+}
+
+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, evalSettings.restrictEval || evalSettings.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::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);
+            }
+        }
+
+        /* 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);
+        }
+
+        /* 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 = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
+        Hash h(*outputHash, ht);
+
+        Path outPath = state.store->makeFixedOutputPath(outputHashRecursive, h, drvName);
+        if (!jsonObject) drv.env["out"] = outPath;
+        drv.outputs["out"] = DerivationOutput(outPath,
+            (outputHashRecursive ? "r:" : "") + printHashType(h.type),
+            h.to_string(Base16, false));
+    }
+
+    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);
+    try {
+        state.realiseContext(context);
+    } catch (InvalidPathError & e) {
+        throw EvalError(format(
+                "cannot check the existence of '%1%', since path '%2%' is not valid, at %3%")
+            % path % e.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.coerceToString(pos, *args[0], context, false, false));
+    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());
+}
+
+/* Return the cryptographic hash of a file in base-16. */
+static void prim_hashFile(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
+    Path p = state.coerceToPath(pos, *args[1], context);
+
+    mkString(v, hashFile(ht, state.checkSourcePath(p)).to_string(Base16, false), context);
+}
+
+/* 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) != '/')
+            throw EvalError(format("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%") % name % pos);
+        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 = evalSettings.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();
+}
+
+
+/* Apply a function to every element of an attribute set. */
+static void prim_mapAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceAttrs(*args[1], pos);
+
+    state.mkAttrs(v, args[1]->attrs->size());
+
+    for (auto & i : *args[1]->attrs) {
+        Value * vName = state.allocValue();
+        Value * vFun2 = state.allocValue();
+        mkString(*vName, i.name);
+        mkApp(*vFun2, *args[0], *vName);
+        mkApp(*state.allocAttr(v, i.name), *vFun2, *i.value);
+    }
+}
+
+
+
+/*************************************************************
+ * 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.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);
+
+    if (args[2]->listSize()) {
+        Value * vCur = args[1];
+
+        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);
+        }
+        state.forceValue(v);
+    } else {
+        state.forceValue(*args[1]);
+        v = *args[1];
+    }
+}
+
+
+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)
+{
+    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();
+}
+
+
+/* concatMap = f: list: concatLists (map f list); */
+/* C++-version is to avoid allocating `mkApp', call `f' eagerly */
+static void prim_concatMap(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceFunction(*args[0], pos);
+    state.forceList(*args[1], pos);
+    auto nrLists = args[1]->listSize();
+
+    Value lists[nrLists];
+    size_t len = 0;
+
+    for (unsigned int n = 0; n < nrLists; ++n) {
+        Value * vElem = args[1]->listElems()[n];
+        state.callFunction(*args[0], *vElem, lists[n], pos);
+        state.forceList(lists[n], pos);
+        len += lists[n].listSize();
+    }
+
+    state.mkList(v, len);
+    auto out = v.listElems();
+    for (unsigned int n = 0, pos = 0; n < nrLists; ++n) {
+        auto l = lists[n].listSize();
+        if (l)
+            memcpy(out + pos, lists[n].listElems(), l * sizeof(Value *));
+        pos += l;
+    }
+}
+
+
+/*************************************************************
+ * Integer arithmetic
+ *************************************************************/
+
+
+static void prim_add(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[0], pos);
+    state.forceValue(*args[1], pos);
+    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)
+{
+    state.forceValue(*args[0], pos);
+    state.forceValue(*args[1], pos);
+    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)
+{
+    state.forceValue(*args[0], pos);
+    state.forceValue(*args[1], pos);
+    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)
+{
+    state.forceValue(*args[0], pos);
+    state.forceValue(*args[1], pos);
+
+    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_bitAnd(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    mkInt(v, state.forceInt(*args[0], pos) & state.forceInt(*args[1], pos));
+}
+
+static void prim_bitOr(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    mkInt(v, state.forceInt(*args[0], pos) | state.forceInt(*args[1], pos));
+}
+
+static void prim_bitXor(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    mkInt(v, state.forceInt(*args[0], pos) ^ state.forceInt(*args[1], pos));
+}
+
+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());
+}
+
+
+/* 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)
+{
+    CachedDownloadRequest request("");
+    request.unpack = unpack;
+    request.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")
+                request.uri = state.forceStringNoCtx(*attr.value, *attr.pos);
+            else if (n == "sha256")
+                request.expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
+            else if (n == "name")
+                request.name = state.forceStringNoCtx(*attr.value, *attr.pos);
+            else
+                throw EvalError(format("unsupported argument '%1%' to '%2%', at %3%") % attr.name % who % attr.pos);
+        }
+
+        if (request.uri.empty())
+            throw EvalError(format("'url' argument required, at %1%") % pos);
+
+    } else
+        request.uri = state.forceStringNoCtx(*args[0], pos);
+
+    state.checkURI(request.uri);
+
+    if (evalSettings.pureEval && !request.expectedHash)
+        throw Error("in pure evaluation mode, '%s' requires a 'sha256' argument", who);
+
+    auto res = getDownloader()->downloadCached(state.store, request);
+
+    if (state.allowedPaths)
+        state.allowedPaths->insert(res.path);
+
+    mkString(v, res.storePath, PathSet({res.storePath}));
+}
+
+
+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 (!evalSettings.pureEval) {
+        mkInt(v, time(0));
+        addConstant("__currentTime", v);
+    }
+
+    if (!evalSettings.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 (evalSettings.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("__isPath", 1, prim_isPath);
+    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 (evalSettings.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);
+    addPrimOp("__hashFile", 2, prim_hashFile);
+
+    // 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);
+    addPrimOp("__mapAttrs", 2, prim_mapAttrs);
+
+    // 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);
+    addPrimOp("__concatMap", 2, prim_concatMap);
+
+    // Integer arithmetic
+    addPrimOp("__add", 2, prim_add);
+    addPrimOp("__sub", 2, prim_sub);
+    addPrimOp("__mul", 2, prim_mul);
+    addPrimOp("__div", 2, prim_div);
+    addPrimOp("__bitAnd", 2, prim_bitAnd);
+    addPrimOp("__bitOr", 2, prim_bitOr);
+    addPrimOp("__bitXor", 2, prim_bitXor);
+    addPrimOp("__lessThan", 2, prim_lessThan);
+
+    // String manipulation
+    addPrimOp("toString", 1, prim_toString);
+    addPrimOp("__substring", 3, prim_substring);
+    addPrimOp("__stringLength", 1, prim_stringLength);
+    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();
+}
+
+
+}