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-rw-r--r--third_party/nix/src/libexpr/primops.cc2456
1 files changed, 2456 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..713c31e4fa79
--- /dev/null
+++ b/third_party/nix/src/libexpr/primops.cc
@@ -0,0 +1,2456 @@
+#include "primops.hh"
+
+#include <algorithm>
+#include <cstring>
+#include <regex>
+
+#include <dlfcn.h>
+#include <glog/logging.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#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 "json.hh"
+#include "names.hh"
+#include "store-api.hh"
+#include "util.hh"
+#include "value-to-json.hh"
+#include "value-to-xml.hh"
+
+namespace nix {
+
+/*************************************************************
+ * Miscellaneous
+ *************************************************************/
+
+/* Decode a context string ‘!<name>!<path>’ into a pair <path,
+   name>. */
+std::pair<string, string> decodeContext(const std::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));
+  }
+  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);
+        auto 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;
+  PathSet willSubstitute;
+  PathSet unknown;
+  unsigned long long downloadSize;
+  unsigned long long 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);
+    }
+    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.second.name] = displ;
+        env->values[displ++] = attr.second.value;
+      }
+
+      DLOG(INFO) << "evaluating file '" << 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" using ValueInitializer = void(*)(EvalState&, Value&);
+
+/* 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);
+
+  std::string sym = state.forceStringNoCtx(*args[1], pos);
+
+  void* handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
+  if (handle == nullptr) {
+    throw EvalError(format("could not open '%1%': %2%") % path % dlerror());
+  }
+
+  dlerror();
+  auto func = (ValueInitializer)dlsym(handle, sym.c_str());
+  if (func == nullptr) {
+    char* message = dlerror();
+    if (message != nullptr) {
+      throw EvalError(format("could not load symbol '%1%' from '%2%': %3%") %
+                      sym % path % message);
+    }
+    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]);
+  std::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->second.value, pos);
+
+  ValueList workSet;
+  for (unsigned int n = 0; n < startSet->second.value->listSize(); ++n) {
+    workSet.push_back(startSet->second.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->second.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->second.value);
+
+    if (doneKeys.find(key->second.value) != doneKeys.end()) {
+      continue;
+    }
+    doneKeys.insert(key->second.value);
+    res.push_back(e);
+
+    /* Call the `operator' function with `e' as argument. */
+    Value call;
+    mkApp(call, *op->second.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;
+  std::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;
+  std::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);
+  }
+}
+
+/* Return an environment variable.  Use with care. */
+static void prim_getEnv(EvalState& state, const Pos& pos, Value** args,
+                        Value& v) {
+  std::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) {
+    DLOG(INFO) << "trace: " << args[0]->string.s;
+  } else {
+    DLOG(INFO) << "trace: " << *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);
+  }
+  std::string drvName;
+  Pos& posDrvName(*attr->second.pos);
+  try {
+    drvName = state.forceStringNoCtx(*attr->second.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->second.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->second.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 std::string& key = i->name;
+
+    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) {
+          std::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.empty()) {
+    throw EvalError(format("required attribute 'builder' missing, at %1%") %
+                    posDrvName);
+  }
+  if (drv.platform.empty()) {
+    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.empty()) {
+        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);
+
+  DLOG(INFO) << "instantiated '" << 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});
+  }
+}
+
+/* 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);
+  }
+  std::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);
+
+    std::string prefix;
+    Bindings::iterator i = v2.attrs->find(state.symbols.Create("prefix"));
+    if (i != v2.attrs->end()) {
+      prefix = state.forceStringNoCtx(*i->second.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;
+    std::string path =
+        state.coerceToString(pos, *i->second.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);
+  }
+
+  std::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) {
+  std::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");
+  }
+}
+
+/*************************************************************
+ * 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) {
+  std::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;
+  std::string name = state.forceStringNoCtx(*args[0], pos);
+  std::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 std::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 != nullptr ? ([&](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;
+  std::string name;
+  Value* filterFun = nullptr;
+  auto recursive = true;
+  Hash expectedHash;
+
+  for (auto& attr : *args[0]->attrs) {
+    const std::string& n(attr.second.name);
+    if (n == "path") {
+      PathSet context;
+      path = state.coerceToPath(*attr.second.pos, *attr.second.value, context);
+      if (!context.empty()) {
+        throw EvalError(
+            format("string '%1%' cannot refer to other paths, at %2%") % path %
+            *attr.second.pos);
+      }
+    } else if (attr.second.name == state.sName) {
+      name = state.forceStringNoCtx(*attr.second.value, *attr.second.pos);
+    } else if (n == "filter") {
+      state.forceValue(*attr.second.value);
+      filterFun = attr.second.value;
+    } else if (n == "recursive") {
+      recursive = state.forceBool(*attr.second.value, *attr.second.pos);
+    } else if (n == "sha256") {
+      expectedHash =
+          Hash(state.forceStringNoCtx(*attr.second.value, *attr.second.pos),
+               htSHA256);
+    } else {
+      throw EvalError(
+          format("unsupported argument '%1%' to 'addPath', at %2%") %
+          attr.second.name % *attr.second.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.second.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) {
+  std::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->second.pos != nullptr)) {
+    state.attrSelects[*i->second.pos]++;
+  }
+  state.forceValue(*i->second.value);
+  v = *i->second.value;
+}
+
+/* Return position information of the specified attribute. */
+void prim_unsafeGetAttrPos(EvalState& state, const Pos& pos, Value** args,
+                           Value& v) {
+  std::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->second.pos);
+  }
+}
+
+/* Dynamic version of the `?' operator. */
+static void prim_hasAttr(EvalState& state, const Pos& pos, Value** args,
+                         Value& v) {
+  std::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.second.name) == names.end()) {
+      v.attrs->push_back(i.second);
+    }
+  }
+}
+
+/* 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);
+    }
+    std::string name = state.forceStringNoCtx(*j->second.value, pos);
+
+    Symbol sym = state.symbols.Create(name);
+    if (seen.find(sym) == seen.end()) {
+      Bindings::iterator j2 =
+          // TODO(tazjin): this line used to construct the symbol again:
+          // state.symbols.Create(state.sValue));
+          // Why?
+          v2.attrs->find(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->second.value, j2->second.pos));
+      seen.insert(sym);
+    }
+  }
+}
+
+/* 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.second.name);
+    if (j != args[1]->attrs->end()) {
+      v.attrs->push_back(j->second);
+    }
+  }
+}
+
+/* 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->second.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)
+    // TODO(tazjin): figure out what the above comment means
+    mkBool(*state.allocAttr(v, i.name), i.def != nullptr);
+  }
+}
+
+/* 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.second.name);
+    mkApp(*vFun2, *args[0], *vName);
+    mkApp(*state.allocAttr(v, i.second.name), *vFun2, *i.second.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() != 0u) {
+    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;
+    Value 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;
+  ValueVector 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 != 0u) {
+    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 != 0u) {
+    memcpy(vWrong->listElems(), wrong.data(), sizeof(Value*) * wsize);
+  }
+}
+
+/* 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 != 0u) {
+      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;
+  std::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;
+  std::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;
+  std::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) {
+  std::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
+  std::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);
+    }
+    throw EvalError("invalid regular expression '%s', at %s", re, pos);
+  }
+}
+
+/* Split a std::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);
+    }
+    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);
+
+  std::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.emplace_back(std::move(s), std::move(ctx));
+  }
+
+  PathSet context;
+  auto s = state.forceString(*args[2], context, pos);
+
+  std::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) {
+  std::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);
+}
+
+static void prim_compareVersions(EvalState& state, const Pos& pos, Value** args,
+                                 Value& v) {
+  std::string version1 = state.forceStringNoCtx(*args[0], pos);
+  std::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) {
+  std::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, component);
+  }
+}
+
+/*************************************************************
+ * Networking
+ *************************************************************/
+
+void fetch(EvalState& state, const Pos& pos, Value** args, Value& v,
+           const std::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) {
+      std::string n(attr.second.name);
+      if (n == "url") {
+        request.uri =
+            state.forceStringNoCtx(*attr.second.value, *attr.second.pos);
+      } else if (n == "sha256") {
+        request.expectedHash =
+            Hash(state.forceStringNoCtx(*attr.second.value, *attr.second.pos),
+                 htSHA256);
+      } else if (n == "name") {
+        request.name =
+            state.forceStringNoCtx(*attr.second.value, *attr.second.pos);
+      } else {
+        throw EvalError(format("unsupported argument '%1%' to '%2%', at %3%") %
+                        attr.second.name % who % attr.second.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(const std::string& name, size_t arity,
+                               PrimOpFun fun) {
+  if (primOps == nullptr) {
+    primOps = new PrimOps;
+  }
+  primOps->emplace_back(name, arity, fun);
+}
+
+void EvalState::createBaseEnv() {
+  baseEnv.up = nullptr;
+
+  /* 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(nullptr));
+    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. */
+  std::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);
+  }
+  addConstant("__nixPath", v);
+
+  if (RegisterPrimOp::primOps != nullptr) {
+    for (auto& primOp : *RegisterPrimOp::primOps) {
+      addPrimOp(std::get<0>(primOp), std::get<1>(primOp), std::get<2>(primOp));
+    }
+  }
+}
+
+}  // namespace nix