path: root/src/libexpr/primops.cc

#include "eval.hh"
#include "misc.hh"
#include "globals.hh"
#include "store-api.hh"
#include "util.hh"
#include "archive.hh"
#include "value-to-xml.hh"
#include "names.hh"

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

#include <algorithm>
#include <cstring>


namespace nix {


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


/* Load and evaluate an expression from path specified by the
   argument. */ 
static void prim_import(EvalState & state, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(*args[0], context);

    for (PathSet::iterator i = context.begin(); i != context.end(); ++i) {
        assert(isStorePath(*i));
        if (!store->isValidPath(*i))
            throw EvalError(format("cannot import `%1%', since path `%2%' is not valid")
                % path % *i);
        if (isDerivation(*i))
            try {
                store->buildDerivations(singleton<PathSet>(*i));
            } catch (Error & e) {
                throw ImportError(e.msg());
            }
    }

    state.evalFile(path, v);
}


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


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


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


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


struct CompareValues
{
    bool operator () (const Value & v1, const Value & v2) const
    {
        if (v1.type != v2.type)
            throw EvalError("cannot compare values of different types");
        switch (v1.type) {
            case tInt:
                return v1.integer < v2.integer;
            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));
        }
    }
};


static void prim_genericClosure(EvalState & state, Value * * args, Value & v)
{
    startNest(nest, lvlDebug, "finding dependencies");

    state.forceAttrs(*args[0]);

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

    list<Value *> workSet;
    for (unsigned int n = 0; n < startSet->value->list.length; ++n)
        workSet.push_back(startSet->value->list.elems[n]);

    /* Get the operator. */
    Bindings::iterator op =
        args[0]->attrs->find(state.symbols.create("operator"));
    if (op == args[0]->attrs->end())
        throw EvalError("attribute `operator' required");
    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. */
    list<Value> res;
    set<Value, CompareValues> doneKeys; // !!! use Value *?
    while (!workSet.empty()) {
	Value * e = *(workSet.begin());
	workSet.pop_front();

        state.forceAttrs(*e);

        Bindings::iterator key =
            e->attrs->find(state.symbols.create("key"));
        if (key == e->attrs->end())
            throw EvalError("attribute `key' required");
        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);

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

    /* Create the result list. */
    state.mkList(v, res.size());
    unsigned int n = 0;
    foreach (list<Value>::iterator, i, res)
        *(v.list.elems[n++] = state.allocValue()) = *i;
}


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


static void prim_throw(EvalState & state, Value * * args, Value & v)
{
    PathSet context;
    throw ThrownError(format("user-thrown exception: %1%") %
        state.coerceToString(*args[0], context));
}


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


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


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


/*************************************************************
 * 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, Value * * args, Value & v)
{
    startNest(nest, lvlVomit, "evaluating derivation");

    state.forceAttrs(*args[0]);

    /* 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("required attribute `name' missing");
    string drvName;
    Pos & posDrvName(*attr->pos);
    try {        
        drvName = state.forceStringNoCtx(*attr->value);
    } catch (Error & e) {
        e.addPrefix(format("while evaluating the derivation attribute `name' at %1%:\n") % posDrvName);
        throw;
    }
    
    /* Build the derivation expression by processing the attributes. */
    Derivation drv;
    
    PathSet context;

    string outputHash, outputHashAlgo;
    bool outputHashRecursive = false;

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

    foreach (Bindings::iterator, i, *args[0]->attrs) {
        string key = i->name;
        startNest(nest, lvlVomit, format("processing attribute `%1%'") % key);

        try {

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

            /* All other attributes are passed to the builder through
               the environment. */
            else {
                string s = state.coerceToString(*i->value, context, true);
                drv.env[key] = s;
                if (key == "builder") drv.builder = s;
                else if (i->name == state.sSystem) drv.platform = s;
                else if (i->name == state.sName) drvName = s;
                else if (key == "outputHash") outputHash = s;
                else if (key == "outputHashAlgo") outputHashAlgo = s;
                else if (key == "outputHashMode") {
                    if (s == "recursive") outputHashRecursive = true; 
                    else if (s == "flat") outputHashRecursive = false;
                    else throw EvalError(format("invalid value `%1%' for `outputHashMode' attribute") % s);
                }
                else if (key == "outputs") {
                    Strings tmp = tokenizeString(s);
                    outputs.clear();
                    foreach (Strings::iterator, j, tmp) {
                        if (outputs.find(*j) != outputs.end())
                            throw EvalError(format("duplicate derivation output `%1%'") % *j);
                        /* !!! 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'") % *j);
                        outputs.insert(*j);
                    }
                    if (outputs.empty())
                        throw EvalError("derivation cannot have an empty set of outputs");
                }
            }

        } catch (Error & e) {
            e.addPrefix(format("while evaluating the derivation attribute `%1%' at %2%:\n")
                % key % *i->pos);
            e.addPrefix(format("while instantiating the derivation named `%1%' at %2%:\n")
                % drvName % posDrvName);
            throw;
        }
    }
    
    /* Everything in the context of the strings in the derivation
       attributes should be added as dependencies of the resulting
       derivation. */
    foreach (PathSet::iterator, i, context) {
        Path path = *i;
        bool explicitlyPassed = false;
        string output = "out";
        
        /* 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) == '=') {
            path = string(path, 1);
            PathSet refs; computeFSClosure(*store, path, refs);
            foreach (PathSet::iterator, j, refs) {
                drv.inputSrcs.insert(*j);
                if (isDerivation(*j))
                    drv.inputDrvs[*j] = store -> queryDerivationOutputNames(*j);
            }

            explicitlyPassed = true;
        } else if (path.at(0) == '!') {
            size_t index;
            path = string(path, 1);
            index = path.find("!");
            output = path.substr(0, index);
            path = string(path, index + 1);
        }

        /* See prim_unsafeDiscardOutputDependency. */
        bool useDrvAsSrc = false;
        if (path.at(0) == '~') {
            path = string(path, 1);
            useDrvAsSrc = true;
        }

        assert(isStorePath(path));

        debug(format("derivation uses `%1%'") % path);
        if (!useDrvAsSrc && isDerivation(path))
            if (explicitlyPassed)
                drv.inputDrvs[path] = store -> queryDerivationOutputNames(path);
            else if (drv.inputDrvs.find(path) == drv.inputDrvs.end())
                drv.inputDrvs[path] = singleton<StringSet>(output);
            else
                drv.inputDrvs[path].insert(output);
        else
            drv.inputSrcs.insert(path);
    }
            
    /* Do we have all required attributes? */
    if (drv.builder == "")
        throw EvalError("required attribute `builder' missing");
    if (drv.platform == "")
        throw EvalError("required attribute `system' missing");

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

    if (outputHash != "") {
        /* Handle fixed-output derivations. */
        if (outputs.size() != 1 || *(outputs.begin()) != "out")
            throw Error("multiple outputs are not supported in fixed-output derivations");
        
        HashType ht = parseHashType(outputHashAlgo);
        if (ht == htUnknown)
            throw EvalError(format("unknown hash algorithm `%1%'") % outputHashAlgo);
        Hash h(ht);
        if (outputHash.size() == h.hashSize * 2)
            /* hexadecimal representation */
            h = parseHash(ht, outputHash);
        else if (outputHash.size() == hashLength32(h))
            /* base-32 representation */
            h = parseHash32(ht, outputHash);
        else
            throw Error(format("hash `%1%' has wrong length for hash type `%2%'")
                % outputHash % outputHashAlgo);
        string s = outputHash;
        outputHash = printHash(h);
        if (outputHashRecursive) outputHashAlgo = "r:" + outputHashAlgo;
        
        Path outPath = makeFixedOutputPath(outputHashRecursive, ht, h, drvName);
        drv.env["out"] = outPath;
        drv.outputs["out"] = DerivationOutput(outPath, outputHashAlgo, outputHash);
    }

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

        /* Use the masked derivation expression to compute the output
           path. */
        Hash h = hashDerivationModulo(*store, drv);
        
        foreach (DerivationOutputs::iterator, i, drv.outputs)
            if (i->second.path == "") {
                Path outPath = makeOutputPath(i->first, h, drvName);
                drv.env[i->first] = outPath;
                i->second.path = outPath;
            }
    }

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

    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(*store, drv);

    state.mkAttrs(v, 1 + drv.outputs.size());
    mkString(*state.allocAttr(v, state.sDrvPath), drvPath, singleton<PathSet>("=" + drvPath));
    foreach (DerivationOutputs::iterator, i, drv.outputs) {
        /* The output path of an output X is ‘<X>Path’,
           e.g. ‘outPath’. */
        mkString(*state.allocAttr(v, state.symbols.create(i->first + "Path")),
            i->second.path, singleton<PathSet>("!" + i->first + "!" + drvPath));
    }
    v.attrs->sort();
}


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


/* Convert the argument to a path.  !!! obsolete? */
static void prim_toPath(EvalState & state, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(*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, Value * * args, Value & v)
{
    PathSet context;
    Path path = canonPath(state.coerceToPath(*args[0], context));
    if (!isInStore(path))
        throw EvalError(format("path `%1%' is not in the Nix store") % path);
    Path path2 = toStorePath(path);
    if (!store->isValidPath(path2))
        throw EvalError(format("store path `%1%' is not valid") % path2);
    context.insert(path2);
    mkString(v, path, context);
}


static void prim_pathExists(EvalState & state, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(*args[0], context);
    if (!context.empty())
        throw EvalError(format("string `%1%' cannot refer to other paths") % path);
    mkBool(v, pathExists(path));
}


/* Return the base name of the given string, i.e., everything
   following the last slash. */
static void prim_baseNameOf(EvalState & state, Value * * args, Value & v)
{
    PathSet context;
    mkString(v, baseNameOf(state.coerceToString(*args[0], context)), 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, Value * * args, Value & v)
{
    PathSet context;
    Path dir = dirOf(state.coerceToPath(*args[0], context));
    if (args[0]->type == tPath) mkPath(v, dir.c_str()); else mkString(v, dir, context);
}


/* Return the contents of a file as a string. */
static void prim_readFile(EvalState & state, Value * * args, Value & v)
{
    PathSet context;
    Path path = state.coerceToPath(*args[0], context);
    if (!context.empty())
        throw EvalError(format("string `%1%' cannot refer to other paths") % path);
    mkString(v, readFile(path).c_str());
}


/*************************************************************
 * 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, Value * * args, Value & v)
{
    std::ostringstream out;
    PathSet context;
    printValueAsXML(state, true, false, *args[0], out, context);
    mkString(v, out.str(), context);
}


/* 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, Value * * args, Value & v)
{
    PathSet context;
    string name = state.forceStringNoCtx(*args[0]);
    string contents = state.forceString(*args[1], context);

    PathSet refs;

    foreach (PathSet::iterator, i, context) {
        Path path = *i;
        if (path.at(0) == '=') path = string(path, 1);
        if (isDerivation(path))
            throw EvalError(format("in `toFile': the file `%1%' cannot refer to derivation outputs") % name);
        refs.insert(path);
    }
    
    Path storePath = readOnlyMode
        ? computeStorePathForText(name, contents, refs)
        : store->addTextToStore(name, contents, refs);

    /* 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, singleton<PathSet>(storePath));
}


struct FilterFromExpr : PathFilter
{
    EvalState & state;
    Value & filter;
    
    FilterFromExpr(EvalState & state, Value & filter)
        : state(state), filter(filter)
    {
    }

    bool operator () (const Path & path)
    {
        struct stat st;
        if (lstat(path.c_str(), &st))
            throw SysError(format("getting attributes of path `%1%'") % 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(filter, arg1, fun2);

        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);

        return state.forceBool(res);
    }
};


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

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

    FilterFromExpr filter(state, *args[0]);

    Path dstPath = readOnlyMode
        ? computeStorePathForPath(path, true, htSHA256, filter).first
        : store->addToStore(path, true, htSHA256, filter);

    mkString(v, dstPath, singleton<PathSet>(dstPath));
}


/*************************************************************
 * Attribute sets
 *************************************************************/


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

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

    StringSet names;
    foreach (Bindings::iterator, i, *args[0]->attrs)
        names.insert(i->name);

    unsigned int n = 0;
    foreach (StringSet::iterator, i, names)
        mkString(*(v.list.elems[n++] = state.allocValue()), *i);
}


/* Dynamic version of the `.' operator. */
static void prim_getAttr(EvalState & state, Value * * args, Value & v)
{
    string attr = state.forceStringNoCtx(*args[0]);
    state.forceAttrs(*args[1]);
    // !!! 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") % attr);
    // !!! add to stack trace?
    state.forceValue(*i->value);
    v = *i->value;
}


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


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


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

    /* Get the attribute names to be removed. */
    std::set<Symbol> names;
    for (unsigned int i = 0; i < args[1]->list.length; ++i) {
        state.forceStringNoCtx(*args[1]->list.elems[i]);
        names.insert(state.symbols.create(args[1]->list.elems[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());
    foreach (Bindings::iterator, i, *args[0]->attrs) {
        if (names.find(i->name) == names.end())
            v.attrs->push_back(*i);
    }
}


/* Builds an attribute 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;}. */
static void prim_listToAttrs(EvalState & state, Value * * args, Value & v)
{
    state.forceList(*args[0]);

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

    std::set<Symbol> seen;

    for (unsigned int i = 0; i < args[0]->list.length; ++i) {
        Value & v2(*args[0]->list.elems[i]);
        state.forceAttrs(v2);
        
        Bindings::iterator j = v2.attrs->find(state.sName);
        if (j == v2.attrs->end())
            throw TypeError("`name' attribute missing in a call to `listToAttrs'");
        string name = state.forceStringNoCtx(*j->value);
        
        Bindings::iterator j2 = v2.attrs->find(state.symbols.create("value"));
        if (j2 == v2.attrs->end())
            throw TypeError("`value' attribute missing in a call to `listToAttrs'");

        Symbol sym = state.symbols.create(name);
        if (seen.find(sym) == seen.end()) {
            v.attrs->push_back(Attr(sym, j2->value, j2->pos));
            seen.insert(sym);
        }
        /* !!! Throw an error if `name' already exists? */
    }

    v.attrs->sort();
}


/* Return the right-biased intersection of two attribute 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, Value * * args, Value & v)
{
    state.forceAttrs(*args[0]);
    state.forceAttrs(*args[1]);
        
    state.mkAttrs(v, std::min(args[0]->attrs->size(), args[1]->attrs->size()));

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


/* 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 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, Value * * args, Value & v)
{
    state.forceValue(*args[0]);
    if (args[0]->type != tLambda)
        throw TypeError("`functionArgs' requires a function");

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

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


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


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


/* Return the first element of a list. */
static void prim_head(EvalState & state, Value * * args, Value & v)
{
    state.forceList(*args[0]);
    if (args[0]->list.length == 0)
        throw Error("`head' called on an empty list");
    state.forceValue(*args[0]->list.elems[0]);
    v = *args[0]->list.elems[0];
}


/* Return a list consisting of everything but the the first element of
   a list. */
static void prim_tail(EvalState & state, Value * * args, Value & v)
{
    state.forceList(*args[0]);
    if (args[0]->list.length == 0)
        throw Error("`tail' called on an empty list");
    state.mkList(v, args[0]->list.length - 1);
    for (unsigned int n = 0; n < v.list.length; ++n)
        v.list.elems[n] = args[0]->list.elems[n + 1];
}


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

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

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


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


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


static void prim_add(EvalState & state, Value * * args, Value & v)
{
    mkInt(v, state.forceInt(*args[0]) + state.forceInt(*args[1]));
}


static void prim_sub(EvalState & state, Value * * args, Value & v)
{
    mkInt(v, state.forceInt(*args[0]) - state.forceInt(*args[1]));
}


static void prim_mul(EvalState & state, Value * * args, Value & v)
{
    mkInt(v, state.forceInt(*args[0]) * state.forceInt(*args[1]));
}


static void prim_div(EvalState & state, Value * * args, Value & v)
{
    int i2 = state.forceInt(*args[1]);
    if (i2 == 0) throw EvalError("division by zero");
    mkInt(v, state.forceInt(*args[0]) / i2);
}


static void prim_lessThan(EvalState & state, Value * * args, Value & v)
{
    mkBool(v, state.forceInt(*args[0]) < state.forceInt(*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, Value * * args, Value & v)
{
    PathSet context;
    string s = state.coerceToString(*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, Value * * args, Value & v)
{
    int start = state.forceInt(*args[0]);
    int len = state.forceInt(*args[1]);
    PathSet context;
    string s = state.coerceToString(*args[2], context);

    if (start < 0) throw EvalError("negative start position in `substring'");

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


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


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


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

    PathSet context2;
    foreach (PathSet::iterator, i, context) {
        Path p = *i;
        if (p.at(0) == '=') p = "~" + string(p, 1);
        context2.insert(p);
    }
    
    mkString(v, s, context2);
}


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


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


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


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);
    
    v.type = tNull;
    addConstant("null", v);

    mkInt(v, time(0));
    addConstant("__currentTime", v);

    mkString(v, thisSystem.c_str());
    addConstant("__currentSystem", v);

    // Miscellaneous
    addPrimOp("import", 1, prim_import);
    addPrimOp("isNull", 1, prim_isNull);
    addPrimOp("__isFunction", 1, prim_isFunction);
    addPrimOp("__isString", 1, prim_isString);
    addPrimOp("__isInt", 1, prim_isInt);
    addPrimOp("__isBool", 1, prim_isBool);
    addPrimOp("__genericClosure", 1, prim_genericClosure);
    addPrimOp("abort", 1, prim_abort);
    addPrimOp("throw", 1, prim_throw);
    addPrimOp("__addErrorContext", 2, prim_addErrorContext);
    addPrimOp("__tryEval", 1, prim_tryEval);
    addPrimOp("__getEnv", 1, prim_getEnv);
    addPrimOp("__trace", 2, prim_trace);

    // Paths
    addPrimOp("__toPath", 1, prim_toPath);
    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);

    // Creating files
    addPrimOp("__toXML", 1, prim_toXML);
    addPrimOp("__toFile", 2, prim_toFile);
    addPrimOp("__filterSource", 2, prim_filterSource);

    // Attribute sets
    addPrimOp("__attrNames", 1, prim_attrNames);
    addPrimOp("__getAttr", 2, prim_getAttr);
    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("__functionArgs", 1, prim_functionArgs);

    // Lists
    addPrimOp("__isList", 1, prim_isList);
    addPrimOp("__head", 1, prim_head);
    addPrimOp("__tail", 1, prim_tail);
    addPrimOp("map", 2, prim_map);
    addPrimOp("__length", 1, prim_length);
    
    // Integer arithmetic
    addPrimOp("__add", 2, prim_add);
    addPrimOp("__sub", 2, prim_sub);
    addPrimOp("__mul", 2, prim_mul);
    addPrimOp("__div", 2, prim_div);
    addPrimOp("__lessThan", 2, prim_lessThan);

    // String manipulation
    addPrimOp("toString", 1, prim_toString);
    addPrimOp("__substring", 3, prim_substring);
    addPrimOp("__stringLength", 1, prim_stringLength);
    addPrimOp("__unsafeDiscardStringContext", 1, prim_unsafeDiscardStringContext);
    addPrimOp("__unsafeDiscardOutputDependency", 1, prim_unsafeDiscardOutputDependency);

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

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

    /* Add a wrapper around the derivation primop that computes the
       `drvPath' and `outPath' attributes lazily. */
    string s = "attrs: \
      let \
        strict = derivationStrict attrs; \
        attrValues = attrs: \
          map (name: builtins.getAttr name attrs) (builtins.attrNames attrs); \
        outputToAttrListElement = output: \
          { \
            name = output; \
            value = attrs // { \
              outPath = builtins.getAttr (output + \"Path\") strict; \
              drvPath = strict.drvPath; \
              type = \"derivation\"; \
              currentOutput = output; \
            } // outputsAttrs // { all = allList; }; \
          }; \
        outputsList = if attrs ? outputs then \
          map outputToAttrListElement attrs.outputs else \
          [ (outputToAttrListElement \"out\") ]; \
        outputsAttrs = builtins.listToAttrs outputsList; \
        allList = attrValues outputsAttrs; \
        head = if attrs ? outputs then builtins.head attrs.outputs else \"out\"; \
      in builtins.getAttr head outputsAttrs";
    mkThunk_(v, parseExprFromString(s, "/"));
    addConstant("derivation", v);

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


}