nix-super/src/libexpr/eval.cc

#include "eval.hh"
#include "hash.hh"
#include "util.hh"
#include "store-api.hh"
#include "derivations.hh"
#include "globals.hh"
#include "eval-inline.hh"

#include <algorithm>
#include <cstring>
#include <unistd.h>
#include <sys/time.h>
#include <sys/resource.h>

#if HAVE_BOEHMGC

#include <gc/gc.h>
#include <gc/gc_cpp.h>

#define NEW new (UseGC)

#else

#define GC_STRDUP strdup
#define GC_MALLOC malloc

#define NEW new

#endif


namespace nix {


void Bindings::sort()
{
    std::sort(begin(), end());
}


static void printValue(std::ostream & str, std::set<const Value *> & seen, const Value & v)
{
    if (seen.find(&v) != seen.end()) {
        str << "<CYCLE>";
        return;
    }
    seen.insert(&v);

    switch (v.type) {
    case tInt:
        str << v.integer;
        break;
    case tBool:
        str << (v.boolean ? "true" : "false");
        break;
    case tString:
        str << "\"";
        for (const char * i = v.string.s; *i; i++)
            if (*i == '\"' || *i == '\\') str << "\\" << *i;
            else if (*i == '\n') str << "\\n";
            else if (*i == '\r') str << "\\r";
            else if (*i == '\t') str << "\\t";
            else str << *i;
        str << "\"";
        break;
    case tPath:
        str << v.path; // !!! escaping?
        break;
    case tNull:
        str << "null";
        break;
    case tAttrs: {
        str << "{ ";
        typedef std::map<string, Value *> Sorted;
        Sorted sorted;
        foreach (Bindings::iterator, i, *v.attrs)
            sorted[i->name] = i->value;
        for (auto & i : sorted) {
            str << i.first << " = ";
            printValue(str, seen, *i.second);
            str << "; ";
        }
        str << "}";
        break;
    }
    case tList:
        str << "[ ";
        for (unsigned int n = 0; n < v.list.length; ++n) {
            printValue(str, seen, *v.list.elems[n]);
            str << " ";
        }
        str << "]";
        break;
    case tThunk:
    case tApp:
        str << "<CODE>";
        break;
    case tLambda:
        str << "<LAMBDA>";
        break;
    case tPrimOp:
        str << "<PRIMOP>";
        break;
    case tPrimOpApp:
        str << "<PRIMOP-APP>";
        break;
    default:
        throw Error("invalid value");
    }
}


std::ostream & operator << (std::ostream & str, const Value & v)
{
    std::set<const Value *> seen;
    printValue(str, seen, v);
    return str;
}


string showType(const Value & v)
{
    switch (v.type) {
        case tInt: return "an integer";
        case tBool: return "a boolean";
        case tString: return "a string";
        case tPath: return "a path";
        case tNull: return "null";
        case tAttrs: return "a set";
        case tList: return "a list";
        case tThunk: return "a thunk";
        case tApp: return "a function application";
        case tLambda: return "a function";
        case tBlackhole: return "a black hole";
        case tPrimOp: return "a built-in function";
        case tPrimOpApp: return "a partially applied built-in function";
    }
    abort();
}


#if HAVE_BOEHMGC
/* Called when the Boehm GC runs out of memory. */
static void * oomHandler(size_t requested)
{
    /* Convert this to a proper C++ exception. */
    throw std::bad_alloc();
}
#endif


static Symbol getName(const AttrName & name, EvalState & state, Env & env)
{
    if (name.symbol.set()) {
        return name.symbol;
    } else {
        Value nameValue;
        name.expr->eval(state, env, nameValue);
        state.forceStringNoCtx(nameValue);
        return state.symbols.create(nameValue.string.s);
    }
}


EvalState::EvalState(const Strings & _searchPath)
    : sWith(symbols.create("<with>"))
    , sOutPath(symbols.create("outPath"))
    , sDrvPath(symbols.create("drvPath"))
    , sType(symbols.create("type"))
    , sMeta(symbols.create("meta"))
    , sName(symbols.create("name"))
    , sValue(symbols.create("value"))
    , sSystem(symbols.create("system"))
    , sOverrides(symbols.create("__overrides"))
    , sOutputs(symbols.create("outputs"))
    , sOutputName(symbols.create("outputName"))
    , sIgnoreNulls(symbols.create("__ignoreNulls"))
    , sFile(symbols.create("file"))
    , sLine(symbols.create("line"))
    , sColumn(symbols.create("column"))
    , repair(false)
    , baseEnv(allocEnv(128))
    , staticBaseEnv(false, 0)
    , baseEnvDispl(0)
{
    nrEnvs = nrValuesInEnvs = nrValues = nrListElems = 0;
    nrAttrsets = nrAttrsInAttrsets = nrOpUpdates = nrOpUpdateValuesCopied = 0;
    nrListConcats = nrPrimOpCalls = nrFunctionCalls = 0;
    countCalls = getEnv("NIX_COUNT_CALLS", "0") != "0";

#if HAVE_BOEHMGC
    static bool gcInitialised = false;
    if (!gcInitialised) {

        /* Initialise the Boehm garbage collector.  This isn't
           necessary on most platforms, but for portability we do it
           anyway. */
        GC_INIT();

        GC_oom_fn = oomHandler;

        /* Set the initial heap size to something fairly big (25% of
           physical RAM, up to a maximum of 384 MiB) so that in most
           cases we don't need to garbage collect at all.  (Collection
           has a fairly significant overhead.)  The heap size can be
           overridden through libgc's GC_INITIAL_HEAP_SIZE environment
           variable.  We should probably also provide a nix.conf
           setting for this.  Note that GC_expand_hp() causes a lot of
           virtual, but not physical (resident) memory to be
           allocated.  This might be a problem on systems that don't
           overcommit. */
        if (!getenv("GC_INITIAL_HEAP_SIZE")) {
            size_t maxSize = 384 * 1024 * 1024;
            size_t size = 32 * 1024 * 1024;
#if HAVE_SYSCONF && defined(_SC_PAGESIZE) && defined(_SC_PHYS_PAGES)
            long pageSize = sysconf(_SC_PAGESIZE);
            long pages = sysconf(_SC_PHYS_PAGES);
            if (pageSize != -1)
                size = (pageSize * pages) / 4; // 25% of RAM
            if (size > maxSize) size = maxSize;
#endif
            debug(format("setting initial heap size to %1% bytes") % size);
            GC_expand_hp(size);
        }

        gcInitialised = true;
    }
#endif

    /* Initialise the Nix expression search path. */
    Strings paths = tokenizeString<Strings>(getEnv("NIX_PATH", ""), ":");
    for (auto & i : _searchPath) addToSearchPath(i, true);
    for (auto & i : paths) addToSearchPath(i);
    addToSearchPath("nix=" + settings.nixDataDir + "/nix/corepkgs");

    createBaseEnv();
}


EvalState::~EvalState()
{
    fileEvalCache.clear();
    printCanaries();
}


void EvalState::addConstant(const string & name, Value & v)
{
    Value * v2 = allocValue();
    *v2 = v;
    staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
    baseEnv.values[baseEnvDispl++] = v2;
    string name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
    baseEnv.values[0]->attrs->push_back(Attr(symbols.create(name2), v2));
}


void EvalState::addPrimOp(const string & name,
    unsigned int arity, PrimOpFun primOp)
{
    Value * v = allocValue();
    string name2 = string(name, 0, 2) == "__" ? string(name, 2) : name;
    Symbol sym = symbols.create(name2);
    v->type = tPrimOp;
    v->primOp = NEW PrimOp(primOp, arity, sym);
    staticBaseEnv.vars[symbols.create(name)] = baseEnvDispl;
    baseEnv.values[baseEnvDispl++] = v;
    baseEnv.values[0]->attrs->push_back(Attr(sym, v));
}


void EvalState::getBuiltin(const string & name, Value & v)
{
    v = *baseEnv.values[0]->attrs->find(symbols.create(name))->value;
}


/* Every "format" object (even temporary) takes up a few hundred bytes
   of stack space, which is a real killer in the recursive
   evaluator.  So here are some helper functions for throwing
   exceptions. */

LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2))
{
    throw EvalError(format(s) % s2);
}

LocalNoInlineNoReturn(void throwEvalError(const char * s, const Pos & pos))
{
    throw EvalError(format(s) % pos);
}

LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const Pos & pos))
{
    throw EvalError(format(s) % s2 % pos);
}

LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3))
{
    throw EvalError(format(s) % s2 % s3);
}

LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3, const Pos & pos))
{
    throw EvalError(format(s) % s2 % s3 % pos);
}

LocalNoInlineNoReturn(void throwEvalError(const char * s, const Symbol & sym, const Pos & p1, const Pos & p2))
{
    throw EvalError(format(s) % sym % p1 % p2);
}

LocalNoInlineNoReturn(void throwTypeError(const char * s))
{
    throw TypeError(s);
}

LocalNoInlineNoReturn(void throwTypeError(const char * s, const Pos & pos))
{
    throw TypeError(format(s) % pos);
}

LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
{
    throw TypeError(format(s) % s1);
}

LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1, const string & s2))
{
    throw TypeError(format(s) % s1 % s2);
}

LocalNoInlineNoReturn(void throwTypeError(const char * s, const ExprLambda & fun, const Symbol & s2, const Pos & pos))
{
    throw TypeError(format(s) % fun.showNamePos() % s2 % pos);
}

LocalNoInlineNoReturn(void throwAssertionError(const char * s, const Pos & pos))
{
    throw AssertionError(format(s) % pos);
}

LocalNoInlineNoReturn(void throwUndefinedVarError(const char * s, const string & s1, const Pos & pos))
{
    throw UndefinedVarError(format(s) % s1 % pos);
}

LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2))
{
    e.addPrefix(format(s) % s2);
}

LocalNoInline(void addErrorPrefix(Error & e, const char * s, const ExprLambda & fun, const Pos & pos))
{
    e.addPrefix(format(s) % fun.showNamePos() % pos);
}

LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2, const Pos & pos))
{
    e.addPrefix(format(s) % s2 % pos);
}


void mkString(Value & v, const char * s)
{
    mkStringNoCopy(v, GC_STRDUP(s));
}


void mkString(Value & v, const string & s, const PathSet & context)
{
    mkString(v, s.c_str());
    if (!context.empty()) {
        unsigned int n = 0;
        v.string.context = (const char * *)
            GC_MALLOC((context.size() + 1) * sizeof(char *));
        foreach (PathSet::const_iterator, i, context)
            v.string.context[n++] = GC_STRDUP(i->c_str());
        v.string.context[n] = 0;
    }
}


void mkPath(Value & v, const char * s)
{
    mkPathNoCopy(v, GC_STRDUP(s));
}


inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
{
    for (unsigned int l = var.level; l; --l, env = env->up) ;

    if (!var.fromWith) return env->values[var.displ];

    while (1) {
        if (!env->haveWithAttrs) {
            if (noEval) return 0;
            Value * v = allocValue();
            evalAttrs(*env->up, (Expr *) env->values[0], *v);
            env->values[0] = v;
            env->haveWithAttrs = true;
        }
        Bindings::iterator j = env->values[0]->attrs->find(var.name);
        if (j != env->values[0]->attrs->end()) {
            if (countCalls && j->pos) attrSelects[*j->pos]++;
            return j->value;
        }
        if (!env->prevWith)
            throwUndefinedVarError("undefined variable ‘%1%’ at %2%", var.name, var.pos);
        for (unsigned int l = env->prevWith; l; --l, env = env->up) ;
    }
}


Value * EvalState::allocValue()
{
    nrValues++;
    return (Value *) GC_MALLOC(sizeof(Value));
}


Env & EvalState::allocEnv(unsigned int size)
{
    assert(size <= std::numeric_limits<decltype(Env::size)>::max());

    nrEnvs++;
    nrValuesInEnvs += size;
    Env * env = (Env *) GC_MALLOC(sizeof(Env) + size * sizeof(Value *));
    env->size = size;

    /* Clear the values because maybeThunk() and lookupVar fromWith expects this. */
    for (unsigned i = 0; i < size; ++i)
        env->values[i] = 0;

    return *env;
}


Value * EvalState::allocAttr(Value & vAttrs, const Symbol & name)
{
    Value * v = allocValue();
    vAttrs.attrs->push_back(Attr(name, v));
    return v;
}


Bindings * EvalState::allocBindings(Bindings::size_t capacity)
{
    return new (GC_MALLOC(sizeof(Bindings) + sizeof(Attr) * capacity)) Bindings(capacity);
}


void EvalState::mkList(Value & v, unsigned int length)
{
    v.type = tList;
    v.list.length = length;
    v.list.elems = length ? (Value * *) GC_MALLOC(length * sizeof(Value *)) : 0;
    nrListElems += length;
}


void EvalState::mkAttrs(Value & v, unsigned int expected)
{
    clearValue(v);
    v.type = tAttrs;
    v.attrs = allocBindings(expected);
    nrAttrsets++;
    nrAttrsInAttrsets += expected;
}


unsigned long nrThunks = 0;

static inline void mkThunk(Value & v, Env & env, Expr * expr)
{
    v.type = tThunk;
    v.thunk.env = &env;
    v.thunk.expr = expr;
    nrThunks++;
}


void EvalState::mkThunk_(Value & v, Expr * expr)
{
    mkThunk(v, baseEnv, expr);
}


void EvalState::mkPos(Value & v, Pos * pos)
{
    if (pos) {
        mkAttrs(v, 3);
        mkString(*allocAttr(v, sFile), pos->file);
        mkInt(*allocAttr(v, sLine), pos->line);
        mkInt(*allocAttr(v, sColumn), pos->column);
        v.attrs->sort();
    } else
        mkNull(v);
}


/* Create a thunk for the delayed computation of the given expression
   in the given environment.  But if the expression is a variable,
   then look it up right away.  This significantly reduces the number
   of thunks allocated. */
Value * Expr::maybeThunk(EvalState & state, Env & env)
{
    Value * v = state.allocValue();
    mkThunk(*v, env, this);
    return v;
}


unsigned long nrAvoided = 0;

Value * ExprVar::maybeThunk(EvalState & state, Env & env)
{
    Value * v = state.lookupVar(&env, *this, true);
    /* The value might not be initialised in the environment yet.
       In that case, ignore it. */
    if (v) { nrAvoided++; return v; }
    return Expr::maybeThunk(state, env);
}


Value * ExprString::maybeThunk(EvalState & state, Env & env)
{
    nrAvoided++;
    return &v;
}

Value * ExprInt::maybeThunk(EvalState & state, Env & env)
{
    nrAvoided++;
    return &v;
}

Value * ExprPath::maybeThunk(EvalState & state, Env & env)
{
    nrAvoided++;
    return &v;
}


void EvalState::evalFile(const Path & path, Value & v)
{
    FileEvalCache::iterator i;
    if ((i = fileEvalCache.find(path)) != fileEvalCache.end()) {
        v = i->second;
        return;
    }

    Path path2 = resolveExprPath(path);
    if ((i = fileEvalCache.find(path2)) != fileEvalCache.end()) {
        v = i->second;
        return;
    }

    startNest(nest, lvlTalkative, format("evaluating file ‘%1%’") % path2);
    Expr * e = parseExprFromFile(path2);
    try {
        eval(e, v);
    } catch (Error & e) {
        addErrorPrefix(e, "while evaluating the file ‘%1%’:\n", path2);
        throw;
    }

    fileEvalCache[path2] = v;
    if (path != path2) fileEvalCache[path] = v;
}


void EvalState::resetFileCache()
{
    fileEvalCache.clear();
}


void EvalState::eval(Expr * e, Value & v)
{
    e->eval(*this, baseEnv, v);
}


inline bool EvalState::evalBool(Env & env, Expr * e)
{
    Value v;
    e->eval(*this, env, v);
    if (v.type != tBool)
        throwTypeError("value is %1% while a Boolean was expected", v);
    return v.boolean;
}


inline bool EvalState::evalBool(Env & env, Expr * e, const Pos & pos)
{
    Value v;
    e->eval(*this, env, v);
    if (v.type != tBool)
        throwTypeError("value is %1% while a Boolean was expected, at %2%", v, pos);
    return v.boolean;
}


inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v)
{
    e->eval(*this, env, v);
    if (v.type != tAttrs)
        throwTypeError("value is %1% while a set was expected", v);
}


void Expr::eval(EvalState & state, Env & env, Value & v)
{
    abort();
}


void ExprInt::eval(EvalState & state, Env & env, Value & v)
{
    v = this->v;
}


void ExprString::eval(EvalState & state, Env & env, Value & v)
{
    v = this->v;
}


void ExprPath::eval(EvalState & state, Env & env, Value & v)
{
    v = this->v;
}


void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
{
    state.mkAttrs(v, attrs.size() + dynamicAttrs.size());
    Env *dynamicEnv = &env;

    if (recursive) {
        /* Create a new environment that contains the attributes in
           this `rec'. */
        Env & env2(state.allocEnv(attrs.size()));
        env2.up = &env;
        dynamicEnv = &env2;

        AttrDefs::iterator overrides = attrs.find(state.sOverrides);
        bool hasOverrides = overrides != attrs.end();

        /* The recursive attributes are evaluated in the new
           environment, while the inherited attributes are evaluated
           in the original environment. */
        unsigned int displ = 0;
        foreach (AttrDefs::iterator, i, attrs) {
            Value * vAttr;
            if (hasOverrides && !i->second.inherited) {
                vAttr = state.allocValue();
                mkThunk(*vAttr, env2, i->second.e);
            } else
                vAttr = i->second.e->maybeThunk(state, i->second.inherited ? env : env2);
            env2.values[displ++] = vAttr;
            v.attrs->push_back(Attr(i->first, vAttr, &i->second.pos));
        }

        /* If the rec contains an attribute called `__overrides', then
           evaluate it, and add the attributes in that set to the rec.
           This allows overriding of recursive attributes, which is
           otherwise not possible.  (You can use the // operator to
           replace an attribute, but other attributes in the rec will
           still reference the original value, because that value has
           been substituted into the bodies of the other attributes.
           Hence we need __overrides.) */
        if (hasOverrides) {
            Value * vOverrides = (*v.attrs)[overrides->second.displ].value;
            state.forceAttrs(*vOverrides);
            Bindings * newBnds = state.allocBindings(v.attrs->size() + vOverrides->attrs->size());
            for (auto & i : *v.attrs)
                newBnds->push_back(i);
            for (auto & i : *vOverrides->attrs) {
                AttrDefs::iterator j = attrs.find(i.name);
                if (j != attrs.end()) {
                    (*newBnds)[j->second.displ] = i;
                    env2.values[j->second.displ] = i.value;
                } else
                    newBnds->push_back(i);
            }
            newBnds->sort();
            v.attrs = newBnds;
        }
    }

    else
        foreach (AttrDefs::iterator, i, attrs)
            v.attrs->push_back(Attr(i->first, i->second.e->maybeThunk(state, env), &i->second.pos));

    /* Dynamic attrs apply *after* rec and __overrides. */
    foreach (DynamicAttrDefs::iterator, i, dynamicAttrs) {
        Value nameVal;
        if (i->nameExpr->es->size() == 1) {
            i->nameExpr->es->front()->eval(state, *dynamicEnv, nameVal);
            state.forceValue(nameVal);
            if (nameVal.type == tNull)
                continue;
        }
        i->nameExpr->eval(state, *dynamicEnv, nameVal);
        state.forceStringNoCtx(nameVal);
        Symbol nameSym = state.symbols.create(nameVal.string.s);
        Bindings::iterator j = v.attrs->find(nameSym);
        if (j != v.attrs->end())
            throwEvalError("dynamic attribute ‘%1%’ at %2% already defined at %3%", nameSym, i->pos, *j->pos);

        i->valueExpr->setName(nameSym);
        /* Keep sorted order so find can catch duplicates */
        v.attrs->push_back(Attr(nameSym, i->valueExpr->maybeThunk(state, *dynamicEnv), &i->pos));
        v.attrs->sort(); // FIXME: inefficient
    }
}


void ExprLet::eval(EvalState & state, Env & env, Value & v)
{
    /* Create a new environment that contains the attributes in this
       `let'. */
    Env & env2(state.allocEnv(attrs->attrs.size()));
    env2.up = &env;

    /* The recursive attributes are evaluated in the new environment,
       while the inherited attributes are evaluated in the original
       environment. */
    unsigned int displ = 0;
    foreach (ExprAttrs::AttrDefs::iterator, i, attrs->attrs)
        env2.values[displ++] = i->second.e->maybeThunk(state, i->second.inherited ? env : env2);

    body->eval(state, env2, v);
}


void ExprList::eval(EvalState & state, Env & env, Value & v)
{
    state.mkList(v, elems.size());
    for (unsigned int n = 0; n < v.list.length; ++n)
        v.list.elems[n] = elems[n]->maybeThunk(state, env);
}


void ExprVar::eval(EvalState & state, Env & env, Value & v)
{
    Value * v2 = state.lookupVar(&env, *this, false);
    state.forceValue(*v2);
    v = *v2;
}


unsigned long nrLookups = 0;

void ExprSelect::eval(EvalState & state, Env & env, Value & v)
{
    Value vTmp;
    Pos * pos2 = 0;
    Value * vAttrs = &vTmp;

    e->eval(state, env, vTmp);

    try {

        foreach (AttrPath::const_iterator, i, attrPath) {
            nrLookups++;
            Bindings::iterator j;
            Symbol name = getName(*i, state, env);
            if (def) {
                state.forceValue(*vAttrs);
                if (vAttrs->type != tAttrs ||
                    (j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
                {
                    def->eval(state, env, v);
                    return;
                }
            } else {
                state.forceAttrs(*vAttrs, pos);
                if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end()) {
                    AttrPath staticPath;
                    AttrPath::const_iterator j;
                    for (j = attrPath.begin(); j != i; ++j)
                        staticPath.push_back(AttrName(getName(*j, state, env)));
                    staticPath.push_back(AttrName(getName(*j, state, env)));
                    for (j = j + 1; j != attrPath.end(); ++j)
                        staticPath.push_back(*j);
                    throwEvalError("attribute ‘%1%’ missing, at %2%", showAttrPath(staticPath), pos);
                }
            }
            vAttrs = j->value;
            pos2 = j->pos;
            if (state.countCalls && pos2) state.attrSelects[*pos2]++;
        }

        state.forceValue(*vAttrs);

    } catch (Error & e) {
        if (pos2 && pos2->file != state.sDerivationNix)
            addErrorPrefix(e, "while evaluating the attribute ‘%1%’ at %2%:\n",
                showAttrPath(attrPath), *pos2);
        throw;
    }

    v = *vAttrs;
}


void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
{
    Value vTmp;
    Value * vAttrs = &vTmp;

    e->eval(state, env, vTmp);

    foreach (AttrPath::const_iterator, i, attrPath) {
        state.forceValue(*vAttrs);
        Bindings::iterator j;
        Symbol name = getName(*i, state, env);
        if (vAttrs->type != tAttrs ||
            (j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
        {
            mkBool(v, false);
            return;
        } else {
            vAttrs = j->value;
        }
    }

    mkBool(v, true);
}


void ExprLambda::eval(EvalState & state, Env & env, Value & v)
{
    v.type = tLambda;
    v.lambda.env = &env;
    v.lambda.fun = this;
}


void ExprApp::eval(EvalState & state, Env & env, Value & v)
{
    /* FIXME: vFun prevents GCC from doing tail call optimisation. */
    Value vFun;
    e1->eval(state, env, vFun);
    state.callFunction(vFun, *(e2->maybeThunk(state, env)), v, pos);
}


void EvalState::callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos)
{
    /* Figure out the number of arguments still needed. */
    unsigned int argsDone = 0;
    Value * primOp = &fun;
    while (primOp->type == tPrimOpApp) {
        argsDone++;
        primOp = primOp->primOpApp.left;
    }
    assert(primOp->type == tPrimOp);
    unsigned int arity = primOp->primOp->arity;
    unsigned int argsLeft = arity - argsDone;

    if (argsLeft == 1) {
        /* We have all the arguments, so call the primop. */

        /* Put all the arguments in an array. */
        Value * vArgs[arity];
        unsigned int n = arity - 1;
        vArgs[n--] = &arg;
        for (Value * arg = &fun; arg->type == tPrimOpApp; arg = arg->primOpApp.left)
            vArgs[n--] = arg->primOpApp.right;

        /* And call the primop. */
        nrPrimOpCalls++;
        if (countCalls) primOpCalls[primOp->primOp->name]++;
        primOp->primOp->fun(*this, pos, vArgs, v);
    } else {
        Value * fun2 = allocValue();
        *fun2 = fun;
        v.type = tPrimOpApp;
        v.primOpApp.left = fun2;
        v.primOpApp.right = &arg;
    }
}


void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & pos)
{
    if (fun.type == tPrimOp || fun.type == tPrimOpApp) {
        callPrimOp(fun, arg, v, pos);
        return;
    }

    if (fun.type != tLambda)
        throwTypeError("attempt to call something which is not a function but %1%, at %2%", fun, pos);

    ExprLambda & lambda(*fun.lambda.fun);

    unsigned int size =
        (lambda.arg.empty() ? 0 : 1) +
        (lambda.matchAttrs ? lambda.formals->formals.size() : 0);
    Env & env2(allocEnv(size));
    env2.up = fun.lambda.env;

    unsigned int displ = 0;

    if (!lambda.matchAttrs)
        env2.values[displ++] = &arg;

    else {
        forceAttrs(arg, pos);

        if (!lambda.arg.empty())
            env2.values[displ++] = &arg;

        /* For each formal argument, get the actual argument.  If
           there is no matching actual argument but the formal
           argument has a default, use the default. */
        unsigned int attrsUsed = 0;
        foreach (Formals::Formals_::iterator, i, lambda.formals->formals) {
            Bindings::iterator j = arg.attrs->find(i->name);
            if (j == arg.attrs->end()) {
                if (!i->def) throwTypeError("%1% called without required argument ‘%2%’, at %3%",
                    lambda, i->name, pos);
                env2.values[displ++] = i->def->maybeThunk(*this, env2);
            } else {
                attrsUsed++;
                env2.values[displ++] = j->value;
            }
        }

        /* Check that each actual argument is listed as a formal
           argument (unless the attribute match specifies a `...'). */
        if (!lambda.formals->ellipsis && attrsUsed != arg.attrs->size()) {
            /* Nope, so show the first unexpected argument to the
               user. */
            foreach (Bindings::iterator, i, *arg.attrs)
                if (lambda.formals->argNames.find(i->name) == lambda.formals->argNames.end())
                    throwTypeError("%1% called with unexpected argument ‘%2%’, at %3%", lambda, i->name, pos);
            abort(); // can't happen
        }
    }

    nrFunctionCalls++;
    if (countCalls) incrFunctionCall(&lambda);

    /* Evaluate the body.  This is conditional on showTrace, because
       catching exceptions makes this function not tail-recursive. */
    if (settings.showTrace)
        try {
            lambda.body->eval(*this, env2, v);
        } catch (Error & e) {
            addErrorPrefix(e, "while evaluating %1%, called from %2%:\n", lambda, pos);
            throw;
        }
    else
        fun.lambda.fun->body->eval(*this, env2, v);
}


// Lifted out of callFunction() because it creates a temporary that
// prevents tail-call optimisation.
void EvalState::incrFunctionCall(ExprLambda * fun)
{
    functionCalls[fun]++;
}


void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
{
    forceValue(fun);

    if (fun.type != tLambda || !fun.lambda.fun->matchAttrs) {
        res = fun;
        return;
    }

    Value * actualArgs = allocValue();
    mkAttrs(*actualArgs, fun.lambda.fun->formals->formals.size());

    foreach (Formals::Formals_::iterator, i, fun.lambda.fun->formals->formals) {
        Bindings::iterator j = args.find(i->name);
        if (j != args.end())
            actualArgs->attrs->push_back(*j);
        else if (!i->def)
            throwTypeError("cannot auto-call a function that has an argument without a default value (‘%1%’)", i->name);
    }

    actualArgs->attrs->sort();

    callFunction(fun, *actualArgs, res, noPos);
}


void ExprWith::eval(EvalState & state, Env & env, Value & v)
{
    Env & env2(state.allocEnv(1));
    env2.up = &env;
    env2.prevWith = prevWith;
    env2.haveWithAttrs = false;
    env2.values[0] = (Value *) attrs;

    body->eval(state, env2, v);
}


void ExprIf::eval(EvalState & state, Env & env, Value & v)
{
    (state.evalBool(env, cond) ? then : else_)->eval(state, env, v);
}


void ExprAssert::eval(EvalState & state, Env & env, Value & v)
{
    if (!state.evalBool(env, cond, pos))
        throwAssertionError("assertion failed at %1%", pos);
    body->eval(state, env, v);
}


void ExprOpNot::eval(EvalState & state, Env & env, Value & v)
{
    mkBool(v, !state.evalBool(env, e));
}


void ExprOpEq::eval(EvalState & state, Env & env, Value & v)
{
    Value v1; e1->eval(state, env, v1);
    Value v2; e2->eval(state, env, v2);
    mkBool(v, state.eqValues(v1, v2));
}


void ExprOpNEq::eval(EvalState & state, Env & env, Value & v)
{
    Value v1; e1->eval(state, env, v1);
    Value v2; e2->eval(state, env, v2);
    mkBool(v, !state.eqValues(v1, v2));
}


void ExprOpAnd::eval(EvalState & state, Env & env, Value & v)
{
    mkBool(v, state.evalBool(env, e1, pos) && state.evalBool(env, e2, pos));
}


void ExprOpOr::eval(EvalState & state, Env & env, Value & v)
{
    mkBool(v, state.evalBool(env, e1, pos) || state.evalBool(env, e2, pos));
}


void ExprOpImpl::eval(EvalState & state, Env & env, Value & v)
{
    mkBool(v, !state.evalBool(env, e1, pos) || state.evalBool(env, e2, pos));
}


void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
{
    Value v1, v2;
    state.evalAttrs(env, e1, v1);
    state.evalAttrs(env, e2, v2);

    state.nrOpUpdates++;

    if (v1.attrs->size() == 0) { v = v2; return; }
    if (v2.attrs->size() == 0) { v = v1; return; }

    state.mkAttrs(v, v1.attrs->size() + v2.attrs->size());

    /* Merge the sets, preferring values from the second set.  Make
       sure to keep the resulting vector in sorted order. */
    Bindings::iterator i = v1.attrs->begin();
    Bindings::iterator j = v2.attrs->begin();

    while (i != v1.attrs->end() && j != v2.attrs->end()) {
        if (i->name == j->name) {
            v.attrs->push_back(*j);
            ++i; ++j;
        }
        else if (i->name < j->name)
            v.attrs->push_back(*i++);
        else
            v.attrs->push_back(*j++);
    }

    while (i != v1.attrs->end()) v.attrs->push_back(*i++);
    while (j != v2.attrs->end()) v.attrs->push_back(*j++);

    state.nrOpUpdateValuesCopied += v.attrs->size();
}


void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
{
    Value v1; e1->eval(state, env, v1);
    Value v2; e2->eval(state, env, v2);
    Value * lists[2] = { &v1, &v2 };
    state.concatLists(v, 2, lists, pos);
}


void EvalState::concatLists(Value & v, unsigned int nrLists, Value * * lists, const Pos & pos)
{
    nrListConcats++;

    Value * nonEmpty = 0;
    unsigned int len = 0;
    for (unsigned int n = 0; n < nrLists; ++n) {
        forceList(*lists[n], pos);
        unsigned int l = lists[n]->list.length;
        len += l;
        if (l) nonEmpty = lists[n];
    }

    if (nonEmpty && len == nonEmpty->list.length) {
        v = *nonEmpty;
        return;
    }

    mkList(v, len);
    for (unsigned int n = 0, pos = 0; n < nrLists; ++n) {
        unsigned int l = lists[n]->list.length;
        memcpy(v.list.elems + pos, lists[n]->list.elems, l * sizeof(Value *));
        pos += l;
    }
}


void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
{
    PathSet context;
    std::ostringstream s;
    NixInt n = 0;

    bool first = !forceString;
    ValueType firstType = tString;

    foreach (vector<Expr *>::iterator, i, *es) {
        Value vTmp;
        (*i)->eval(state, env, vTmp);

        /* If the first element is a path, then the result will also
           be a path, we don't copy anything (yet - that's done later,
           since paths are copied when they are used in a derivation),
           and none of the strings are allowed to have contexts. */
        if (first) {
            firstType = vTmp.type;
            first = false;
        }

        if (firstType == tInt) {
            if (vTmp.type != tInt)
                throwEvalError("cannot add %1% to an integer, at %2%", showType(vTmp), pos);
            n += vTmp.integer;
        } else
            s << state.coerceToString(pos, vTmp, context, false, firstType == tString);
    }

    if (firstType == tInt)
        mkInt(v, n);
    else if (firstType == tPath) {
        if (!context.empty())
            throwEvalError("a string that refers to a store path cannot be appended to a path, at %1%", pos);
        mkPath(v, s.str().c_str());
    } else
        mkString(v, s.str(), context);
}


void ExprPos::eval(EvalState & state, Env & env, Value & v)
{
    state.mkPos(v, &pos);
}


void EvalState::forceValueDeep(Value & v)
{
    std::set<const Value *> seen;

    std::function<void(Value & v)> recurse;

    recurse = [&](Value & v) {
        if (seen.find(&v) != seen.end()) return;
        seen.insert(&v);

        forceValue(v);

        if (v.type == tAttrs) {
            foreach (Bindings::iterator, i, *v.attrs)
                recurse(*i->value);
        }

        else if (v.type == tList) {
            for (unsigned int n = 0; n < v.list.length; ++n)
                recurse(*v.list.elems[n]);
        }
    };

    recurse(v);
}


NixInt EvalState::forceInt(Value & v, const Pos & pos)
{
    forceValue(v);
    if (v.type != tInt)
        throwTypeError("value is %1% while an integer was expected, at %2%", v, pos);
    return v.integer;
}


bool EvalState::forceBool(Value & v)
{
    forceValue(v);
    if (v.type != tBool)
        throwTypeError("value is %1% while a Boolean was expected", v);
    return v.boolean;
}


void EvalState::forceFunction(Value & v, const Pos & pos)
{
    forceValue(v);
    if (v.type != tLambda && v.type != tPrimOp && v.type != tPrimOpApp)
        throwTypeError("value is %1% while a function was expected, at %2%", v, pos);
}


string EvalState::forceString(Value & v, const Pos & pos)
{
    forceValue(v);
    if (v.type != tString) {
        if (pos)
            throwTypeError("value is %1% while a string was expected, at %2%", v, pos);
        else
            throwTypeError("value is %1% while a string was expected", v);
    }
    return string(v.string.s);
}


void copyContext(const Value & v, PathSet & context)
{
    if (v.string.context)
        for (const char * * p = v.string.context; *p; ++p)
            context.insert(*p);
}


string EvalState::forceString(Value & v, PathSet & context)
{
    string s = forceString(v);
    copyContext(v, context);
    return s;
}


string EvalState::forceStringNoCtx(Value & v, const Pos & pos)
{
    string s = forceString(v, pos);
    if (v.string.context) {
        if (pos)
            throwEvalError("the string ‘%1%’ is not allowed to refer to a store path (such as ‘%2%’), at %3%",
                v.string.s, v.string.context[0], pos);
        else
            throwEvalError("the string ‘%1%’ is not allowed to refer to a store path (such as ‘%2%’)",
                v.string.s, v.string.context[0]);
    }
    return s;
}


bool EvalState::isDerivation(Value & v)
{
    if (v.type != tAttrs) return false;
    Bindings::iterator i = v.attrs->find(sType);
    if (i == v.attrs->end()) return false;
    forceValue(*i->value);
    if (i->value->type != tString) return false;
    return strcmp(i->value->string.s, "derivation") == 0;
}


string EvalState::coerceToString(const Pos & pos, Value & v, PathSet & context,
    bool coerceMore, bool copyToStore)
{
    forceValue(v);

    string s;

    if (v.type == tString) {
        copyContext(v, context);
        return v.string.s;
    }

    if (v.type == tPath) {
        Path path(canonPath(v.path));
        return copyToStore ? copyPathToStore(context, path) : path;
    }

    if (v.type == tAttrs) {
        Bindings::iterator i = v.attrs->find(sOutPath);
        if (i == v.attrs->end()) throwTypeError("cannot coerce a set to a string, at %1%", pos);
        return coerceToString(pos, *i->value, context, coerceMore, copyToStore);
    }

    if (coerceMore) {

        /* Note that `false' is represented as an empty string for
           shell scripting convenience, just like `null'. */
        if (v.type == tBool && v.boolean) return "1";
        if (v.type == tBool && !v.boolean) return "";
        if (v.type == tInt) return int2String(v.integer);
        if (v.type == tNull) return "";

        if (v.type == tList) {
            string result;
            for (unsigned int n = 0; n < v.list.length; ++n) {
                result += coerceToString(pos, *v.list.elems[n],
                    context, coerceMore, copyToStore);
                if (n < v.list.length - 1
                    /* !!! not quite correct */
                    && (v.list.elems[n]->type != tList || v.list.elems[n]->list.length != 0))
                    result += " ";
            }
            return result;
        }
    }

    throwTypeError("cannot coerce %1% to a string, at %2%", v, pos);
}


string EvalState::copyPathToStore(PathSet & context, const Path & path)
{
    if (nix::isDerivation(path))
        throwEvalError("file names are not allowed to end in ‘%1%’", drvExtension);

    Path dstPath;
    if (srcToStore[path] != "")
        dstPath = srcToStore[path];
    else {
        dstPath = settings.readOnlyMode
            ? computeStorePathForPath(path).first
            : store->addToStore(path, true, htSHA256, defaultPathFilter, repair);
        srcToStore[path] = dstPath;
        printMsg(lvlChatty, format("copied source ‘%1%’ -> ‘%2%’")
            % path % dstPath);
    }

    context.insert(dstPath);
    return dstPath;
}


Path EvalState::coerceToPath(const Pos & pos, Value & v, PathSet & context)
{
    string path = coerceToString(pos, v, context, false, false);
    if (path == "" || path[0] != '/')
        throwEvalError("string ‘%1%’ doesn't represent an absolute path, at %2%", path, pos);
    return path;
}


bool EvalState::eqValues(Value & v1, Value & v2)
{
    forceValue(v1);
    forceValue(v2);

    /* !!! Hack to support some old broken code that relies on pointer
       equality tests between sets.  (Specifically, builderDefs calls
       uniqList on a list of sets.)  Will remove this eventually. */
    if (&v1 == &v2) return true;

    if (v1.type != v2.type) return false;

    switch (v1.type) {

        case tInt:
            return v1.integer == v2.integer;

        case tBool:
            return v1.boolean == v2.boolean;

        case tString:
            return strcmp(v1.string.s, v2.string.s) == 0;

        case tPath:
            return strcmp(v1.path, v2.path) == 0;

        case tNull:
            return true;

        case tList:
            if (v1.list.length != v2.list.length) return false;
            for (unsigned int n = 0; n < v1.list.length; ++n)
                if (!eqValues(*v1.list.elems[n], *v2.list.elems[n])) return false;
            return true;

        case tAttrs: {
            /* If both sets denote a derivation (type = "derivation"),
               then compare their outPaths. */
            if (isDerivation(v1) && isDerivation(v2)) {
                Bindings::iterator i = v1.attrs->find(sOutPath);
                Bindings::iterator j = v2.attrs->find(sOutPath);
                if (i != v1.attrs->end() && j != v2.attrs->end())
                    return eqValues(*i->value, *j->value);
            }

            if (v1.attrs->size() != v2.attrs->size()) return false;

            /* Otherwise, compare the attributes one by one. */
            Bindings::iterator i, j;
            for (i = v1.attrs->begin(), j = v2.attrs->begin(); i != v1.attrs->end(); ++i, ++j)
                if (i->name != j->name || !eqValues(*i->value, *j->value))
                    return false;

            return true;
        }

        /* Functions are incomparable. */
        case tLambda:
        case tPrimOp:
        case tPrimOpApp:
            return false;

        default:
            throwEvalError("cannot compare %1% with %2%", showType(v1), showType(v2));
    }
}


void EvalState::printStats()
{
    bool showStats = getEnv("NIX_SHOW_STATS", "0") != "0";
    Verbosity v = showStats ? lvlInfo : lvlDebug;
    printMsg(v, "evaluation statistics:");

    struct rusage buf;
    getrusage(RUSAGE_SELF, &buf);
    float cpuTime = buf.ru_utime.tv_sec + ((float) buf.ru_utime.tv_usec / 1000000);

    printMsg(v, format("  time elapsed: %1%") % cpuTime);
    printMsg(v, format("  size of a value: %1%") % sizeof(Value));
    printMsg(v, format("  environments allocated: %1% (%2% bytes)")
        % nrEnvs % (nrEnvs * sizeof(Env) + nrValuesInEnvs * sizeof(Value *)));
    printMsg(v, format("  list elements: %1% (%2% bytes)")
        % nrListElems % (nrListElems * sizeof(Value *)));
    printMsg(v, format("  list concatenations: %1%") % nrListConcats);
    printMsg(v, format("  values allocated: %1% (%2% bytes)")
        % nrValues % (nrValues * sizeof(Value)));
    printMsg(v, format("  sets allocated: %1% (%2% bytes)")
        % nrAttrsets % (nrAttrsets * sizeof(Bindings) + nrAttrsInAttrsets * sizeof(Attr)));
    printMsg(v, format("  right-biased unions: %1%") % nrOpUpdates);
    printMsg(v, format("  values copied in right-biased unions: %1%") % nrOpUpdateValuesCopied);
    printMsg(v, format("  symbols in symbol table: %1%") % symbols.size());
    printMsg(v, format("  size of symbol table: %1%") % symbols.totalSize());
    printMsg(v, format("  number of thunks: %1%") % nrThunks);
    printMsg(v, format("  number of thunks avoided: %1%") % nrAvoided);
    printMsg(v, format("  number of attr lookups: %1%") % nrLookups);
    printMsg(v, format("  number of primop calls: %1%") % nrPrimOpCalls);
    printMsg(v, format("  number of function calls: %1%") % nrFunctionCalls);

    if (countCalls) {
        v = lvlInfo;

        printMsg(v, format("calls to %1% primops:") % primOpCalls.size());
        typedef std::multimap<unsigned int, Symbol> PrimOpCalls_;
        PrimOpCalls_ primOpCalls_;
        foreach (PrimOpCalls::iterator, i, primOpCalls)
            primOpCalls_.insert(std::pair<unsigned int, Symbol>(i->second, i->first));
        foreach_reverse (PrimOpCalls_::reverse_iterator, i, primOpCalls_)
            printMsg(v, format("%1$10d %2%") % i->first % i->second);

        printMsg(v, format("calls to %1% functions:") % functionCalls.size());
        typedef std::multimap<unsigned int, ExprLambda *> FunctionCalls_;
        FunctionCalls_ functionCalls_;
        foreach (FunctionCalls::iterator, i, functionCalls)
            functionCalls_.insert(std::pair<unsigned int, ExprLambda *>(i->second, i->first));
        foreach_reverse (FunctionCalls_::reverse_iterator, i, functionCalls_)
            printMsg(v, format("%1$10d %2%") % i->first % i->second->showNamePos());

        printMsg(v, format("evaluations of %1% attributes:") % attrSelects.size());
        typedef std::multimap<unsigned int, Pos> AttrSelects_;
        AttrSelects_ attrSelects_;
        foreach (AttrSelects::iterator, i, attrSelects)
            attrSelects_.insert(std::pair<unsigned int, Pos>(i->second, i->first));
        foreach_reverse (AttrSelects_::reverse_iterator, i, attrSelects_)
            printMsg(v, format("%1$10d %2%") % i->first % i->second);

    }
}


void EvalState::printCanaries()
{
#if HAVE_BOEHMGC
    if (!settings.get("debug-gc", false)) return;

    GC_gcollect();

    if (gcCanaries.empty()) {
        printMsg(lvlError, "all canaries have been garbage-collected");
        return;
    }

    printMsg(lvlError, "the following canaries have not been garbage-collected:");

    for (auto i : gcCanaries)
        printMsg(lvlError, format("  %1%") % i->string.s);
#endif
}


size_t valueSize(Value & v)
{
    std::set<const void *> seen;

    auto doString = [&](const char * s) -> size_t {
        if (seen.find(s) != seen.end()) return 0;
        seen.insert(s);
        return strlen(s) + 1;
    };

    std::function<size_t(Value & v)> doValue;
    std::function<size_t(Env & v)> doEnv;

    doValue = [&](Value & v) -> size_t {
        if (seen.find(&v) != seen.end()) return 0;
        seen.insert(&v);

        size_t sz = sizeof(Value);

        switch (v.type) {
        case tString:
            sz += doString(v.string.s);
            if (v.string.context)
                for (const char * * p = v.string.context; *p; ++p)
                    sz += doString(*p);
            break;
        case tPath:
            sz += doString(v.path);
            break;
        case tAttrs:
            for (auto & i : *v.attrs)
                sz += doValue(*i.value);
            break;
        case tList:
            for (unsigned int n = 0; n < v.list.length; ++n)
                sz += doValue(*v.list.elems[n]);
            break;
        case tThunk:
            sz += doEnv(*v.thunk.env);
            break;
        case tApp:
            sz += doValue(*v.app.left);
            sz += doValue(*v.app.right);
            break;
        case tLambda:
            sz += doEnv(*v.lambda.env);
            break;
        case tPrimOpApp:
            sz += doValue(*v.primOpApp.left);
            sz += doValue(*v.primOpApp.right);
            break;
        default:
            ;
        }

        return sz;
    };

    doEnv = [&](Env & env) -> size_t {
        if (seen.find(&env) != seen.end()) return 0;
        seen.insert(&env);

        size_t sz = sizeof(Env);

        for (unsigned int i = 0; i < env.size; ++i)
            if (env.values[i])
                sz += doValue(*env.values[i]);

        if (env.up) sz += doEnv(*env.up);

        return sz;
    };

    return doValue(v);
}


}