* Refactoring: put the Nix expression evaluator in its own library so

that it can be used by multiple programs.

src/libexpr/eval.cc

@@ -0,0 +1,265 @@
+#include "eval.hh"
+#include "parser.hh"
+#include "primops.hh"
+
+
+EvalState::EvalState()
+    : normalForms(32768, 75)
+{
+    blackHole = ATmake("BlackHole()");
+    if (!blackHole) throw Error("cannot build black hole");
+    nrEvaluated = nrCached = 0;
+}
+
+
+/* Substitute an argument set into the body of a function. */
+static Expr substArgs(Expr body, ATermList formals, Expr arg)
+{
+    ATMatcher m;
+    ATermMap subs;
+    Expr undefined = ATmake("Undefined");
+
+    /* Get the formal arguments. */
+    for (ATermIterator i(formals); i; ++i) {
+        Expr name, def;
+        if (atMatch(m, *i) >> "NoDefFormal" >> name)
+            subs.set(name, undefined);
+        else if (atMatch(m, *i) >> "DefFormal" >> name >> def)
+            subs.set(name, def);
+        else abort(); /* can't happen */
+    }
+
+    /* Get the actual arguments, and check that they match with the
+       formals. */
+    ATermMap args;
+    queryAllAttrs(arg, args);
+    for (ATermIterator i(args.keys()); i; ++i) {
+        Expr key = *i;
+        Expr cur = subs.get(key);
+        if (!cur)
+            throw badTerm(format("function has no formal argument `%1%'")
+                % aterm2String(key), arg);
+        subs.set(key, args.get(key));
+    }
+
+    /* Check that all arguments are defined. */
+    for (ATermIterator i(subs.keys()); i; ++i)
+        if (subs.get(*i) == undefined)
+            throw badTerm(format("formal argument `%1%' missing")
+                % aterm2String(*i), arg);
+    
+    return substitute(subs, body);
+}
+
+
+/* Transform a mutually recursive set into a non-recursive set.  Each
+   attribute is transformed into an expression that has all references
+   to attributes substituted with selection expressions on the
+   original set.  E.g., e = `rec {x = f x y, y = x}' becomes `{x = f
+   (e.x) (e.y), y = e.x}'. */
+ATerm expandRec(ATerm e, ATermList bnds)
+{
+    ATMatcher m;
+
+    /* Create the substitution list. */
+    ATermMap subs;
+    for (ATermIterator i(bnds); i; ++i) {
+        string s;
+        Expr e2;
+        if (!(atMatch(m, *i) >> "Bind" >> s >> e2))
+            abort(); /* can't happen */
+        subs.set(s, ATmake("Select(<term>, <str>)", e, s.c_str()));
+    }
+
+    /* Create the non-recursive set. */
+    ATermMap as;
+    for (ATermIterator i(bnds); i; ++i) {
+        string s;
+        Expr e2;
+        if (!(atMatch(m, *i) >> "Bind" >> s >> e2))
+            abort(); /* can't happen */
+        as.set(s, substitute(subs, e2));
+    }
+
+    return makeAttrs(as);
+}
+
+
+string evalString(EvalState & state, Expr e)
+{
+    e = evalExpr(state, e);
+    ATMatcher m;
+    string s;
+    if (!(atMatch(m, e) >> "Str" >> s))
+        throw badTerm("string expected", e);
+    return s;
+}
+
+
+Path evalPath(EvalState & state, Expr e)
+{
+    e = evalExpr(state, e);
+    ATMatcher m;
+    string s;
+    if (!(atMatch(m, e) >> "Path" >> s))
+        throw badTerm("path expected", e);
+    return s;
+}
+
+
+bool evalBool(EvalState & state, Expr e)
+{
+    e = evalExpr(state, e);
+    ATMatcher m;
+    if (atMatch(m, e) >> "Bool" >> "True") return true;
+    else if (atMatch(m, e) >> "Bool" >> "False") return false;
+    else throw badTerm("expecting a boolean", e);
+}
+
+
+Expr evalExpr2(EvalState & state, Expr e)
+{
+    ATMatcher m;
+    Expr e1, e2, e3, e4;
+    string s1;
+
+    /* Normal forms. */
+    if (atMatch(m, e) >> "Str" ||
+        atMatch(m, e) >> "Path" ||
+        atMatch(m, e) >> "Uri" ||
+        atMatch(m, e) >> "Bool" ||
+        atMatch(m, e) >> "Function" ||
+        atMatch(m, e) >> "Attrs" ||
+        atMatch(m, e) >> "List")
+        return e;
+
+    /* Any encountered variables must be undeclared or primops. */
+    if (atMatch(m, e) >> "Var" >> s1) {
+        if (s1 == "null") return primNull(state);
+        return e;
+    }
+
+    /* Function application. */
+    if (atMatch(m, e) >> "Call" >> e1 >> e2) {
+
+        ATermList formals;
+        
+        /* Evaluate the left-hand side. */
+        e1 = evalExpr(state, e1);
+
+        /* Is it a primop or a function? */
+        if (atMatch(m, e1) >> "Var" >> s1) {
+            if (s1 == "import") return primImport(state, e2);
+            if (s1 == "derivation") return primDerivation(state, e2);
+            if (s1 == "toString") return primToString(state, e2);
+            if (s1 == "baseNameOf") return primBaseNameOf(state, e2);
+            if (s1 == "isNull") return primIsNull(state, e2);
+            else throw badTerm("undefined variable/primop", e1);
+        }
+
+        else if (atMatch(m, e1) >> "Function" >> formals >> e4)
+            return evalExpr(state, 
+                substArgs(e4, formals, evalExpr(state, e2)));
+        
+        else throw badTerm("expecting a function or primop", e1);
+    }
+
+    /* Attribute selection. */
+    if (atMatch(m, e) >> "Select" >> e1 >> s1) {
+        Expr a = queryAttr(evalExpr(state, e1), s1);
+        if (!a) throw badTerm(format("missing attribute `%1%'") % s1, e);
+        return evalExpr(state, a);
+    }
+
+    /* Mutually recursive sets. */
+    ATermList bnds;
+    if (atMatch(m, e) >> "Rec" >> bnds)
+        return expandRec(e, bnds);
+
+    /* Let expressions `let {..., body = ...}' are just desugared
+       into `(rec {..., body = ...}).body'. */
+    if (atMatch(m, e) >> "LetRec" >> bnds)
+        return evalExpr(state, ATmake("Select(Rec(<term>), \"body\")", bnds));
+
+    /* Conditionals. */
+    if (atMatch(m, e) >> "If" >> e1 >> e2 >> e3) {
+        if (evalBool(state, e1))
+            return evalExpr(state, e2);
+        else
+            return evalExpr(state, e3);
+    }
+
+    /* Assertions. */
+    if (atMatch(m, e) >> "Assert" >> e1 >> e2) {
+        if (!evalBool(state, e1)) throw badTerm("guard failed", e);
+        return evalExpr(state, e2);
+    }
+
+    /* Generic equality. */
+    if (atMatch(m, e) >> "OpEq" >> e1 >> e2)
+        return makeBool(evalExpr(state, e1) == evalExpr(state, e2));
+
+    /* Generic inequality. */
+    if (atMatch(m, e) >> "OpNEq" >> e1 >> e2)
+        return makeBool(evalExpr(state, e1) != evalExpr(state, e2));
+
+    /* Negation. */
+    if (atMatch(m, e) >> "OpNot" >> e1)
+        return makeBool(!evalBool(state, e1));
+
+    /* Implication. */
+    if (atMatch(m, e) >> "OpImpl" >> e1 >> e2)
+        return makeBool(!evalBool(state, e1) || evalBool(state, e2));
+
+    /* Conjunction (logical AND). */
+    if (atMatch(m, e) >> "OpAnd" >> e1 >> e2)
+        return makeBool(evalBool(state, e1) && evalBool(state, e2));
+
+    /* Disjunction (logical OR). */
+    if (atMatch(m, e) >> "OpOr" >> e1 >> e2)
+        return makeBool(evalBool(state, e1) || evalBool(state, e2));
+
+    /* Barf. */
+    throw badTerm("invalid expression", e);
+}
+
+
+Expr evalExpr(EvalState & state, Expr e)
+{
+    startNest(nest, lvlVomit,
+        format("evaluating expression: %1%") % e);
+
+    state.nrEvaluated++;
+
+    /* Consult the memo table to quickly get the normal form of
+       previously evaluated expressions. */
+    Expr nf = state.normalForms.get(e);
+    if (nf) {
+        if (nf == state.blackHole)
+            throw badTerm("infinite recursion", e);
+        state.nrCached++;
+        return nf;
+    }
+
+    /* Otherwise, evaluate and memoize. */
+    state.normalForms.set(e, state.blackHole);
+    nf = evalExpr2(state, e);
+    state.normalForms.set(e, nf);
+    return nf;
+}
+
+
+Expr evalFile(EvalState & state, const Path & path)
+{
+    startNest(nest, lvlTalkative, format("evaluating file `%1%'") % path);
+    Expr e = parseExprFromFile(path);
+    return evalExpr(state, e);
+}
+
+
+void printEvalStats(EvalState & state)
+{
+    debug(format("evaluated %1% expressions, %2% cache hits, %3%%% efficiency")
+        % state.nrEvaluated % state.nrCached
+        % ((float) state.nrCached / (float) state.nrEvaluated * 100));
+}