POC implementation of lazy attribute names

This makes the following work: nix-instantiate --eval -E '(throw "" // { x = 0; }).x' Does not work yet for e.g. nix-instantiate --eval -E 'let a = (throw "" // { x = 0; }); in a.x'
infinisil · Sep 28, 2020 · afeda4f · afeda4f
1 parent 5080d4e
commit afeda4f
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Showing 3 changed files with 61 additions and 13 deletions.
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
@@ -1104,13 +1104,13 @@ static string showAttrPath(EvalState & state, Env & env, const AttrPath & attrPa
 
 unsigned long nrLookups = 0;
 
-void ExprSelect::eval(EvalState & state, Env & env, Value & v)
+bool Expr::select(EvalState & state, Env & env, const AttrPath & attrPath, Expr * def, const Pos & pos, bool required, Value & v)
 {
     Value vTmp;
     Pos * pos2 = 0;
     Value * vAttrs = &vTmp;
 
-    e->eval(state, env, vTmp);
+    this->eval(state, env, vTmp);
 
     try {
 
@@ -1124,19 +1124,24 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     (j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
                 {
                     def->eval(state, env, v);
-                    return;
+                    return true;
                 }
             } else {
                 state.forceAttrs(*vAttrs, pos);
-                if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
-                    throwEvalError(pos, "attribute '%1%' missing", name);
+                if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end()) {
+                    if (required) {
+                        throwEvalError(pos, "attribute '%1%' missing", name);
+                    } else {
+                        return false;
+                    }
+                }
             }
             vAttrs = j->value;
             pos2 = j->pos;
             if (state.countCalls && pos2) state.attrSelects[*pos2]++;
         }
 
-        state.forceValue(*vAttrs, ( pos2 != NULL ? *pos2 : this->pos ) );
+        state.forceValue(*vAttrs, ( pos2 != NULL ? *pos2 : pos ) );
 
     } catch (Error & e) {
         if (pos2 && pos2->file != state.sDerivationNix)
@@ -1146,15 +1151,20 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     }
 
     v = *vAttrs;
+    return true;
 }
 
+void ExprSelect::eval(EvalState & state, Env & env, Value & v)
+{
+    e->select(state, env, attrPath, def, pos, true, v);
+}
 
-void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
+bool Expr::hasAttr(EvalState & state, Env & env, const AttrPath & attrPath)
 {
     Value vTmp;
     Value * vAttrs = &vTmp;
 
-    e->eval(state, env, vTmp);
+    this->eval(state, env, vTmp);
 
     for (auto & i : attrPath) {
         state.forceValue(*vAttrs);
@@ -1163,16 +1173,19 @@ void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
         if (vAttrs->type != tAttrs ||
             (j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
         {
-            mkBool(v, false);
-            return;
+            return false;
         } else {
             vAttrs = j->value;
         }
     }
 
-    mkBool(v, true);
+    return true;
 }
 
+void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
+{
+    mkBool(v, e->hasAttr(state, env, attrPath));
+}
 
 void ExprLambda::eval(EvalState & state, Env & env, Value & v)
 {
@@ -1449,8 +1462,20 @@ void ExprOpImpl::eval(EvalState & state, Env & env, Value & v)
 }
 
 
+bool ExprOpUpdate::hasAttr(EvalState & state, Env & env, const AttrPath & attrPath)
+{
+    return e2->hasAttr(state, env, attrPath) || e1->hasAttr(state, env, attrPath);
+}
+
+
+bool ExprOpUpdate::select(EvalState & state, Env & env, const AttrPath & attrPath, Expr * def, const Pos & pos, bool required, Value & v)
+{
+    return e2->select(state, env, attrPath, def, pos, false, v) || e1->select(state, env, attrPath, def, pos, required, v);
+}
+
 void ExprOpUpdate::eval(EvalState & state, Env & env, Value & v)
 {
+
     Value v1, v2;
     state.evalAttrs(env, e1, v1);
     state.evalAttrs(env, e2, v2);

diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
@@ -335,7 +335,7 @@ private:
 
     friend struct ExprOpUpdate;
     friend struct ExprOpConcatLists;
-    friend struct ExprSelect;
+    friend bool Expr::select(EvalState & state, Env & env, const AttrPath & attrPath, Expr * def, const Pos & pos, bool required, Value & v);
     friend void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v);
     friend void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v);
 };

diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
@@ -82,6 +82,8 @@ struct Expr
     virtual void eval(EvalState & state, Env & env, Value & v);
     virtual Value * maybeThunk(EvalState & state, Env & env);
     virtual void setName(Symbol & name);
+    virtual bool hasAttr(EvalState & state, Env & env, const AttrPath & attrPath);
+    virtual bool select(EvalState & state, Env & env, const AttrPath & attrPath, Expr * def, const Pos & pos, bool required, Value & v);
 };
 
 std::ostream & operator << (std::ostream & str, const Expr & e);
@@ -311,9 +313,30 @@ MakeBinOp(ExprOpNEq, "!=")
 MakeBinOp(ExprOpAnd, "&&")
 MakeBinOp(ExprOpOr, "||")
 MakeBinOp(ExprOpImpl, "->")
-MakeBinOp(ExprOpUpdate, "//")
+//MakeBinOp(ExprOpUpdate, "//")
 MakeBinOp(ExprOpConcatLists, "++")
 
+
+struct ExprOpUpdate : Expr
+{
+    Pos pos;
+    Expr * e1, * e2;
+    ExprOpUpdate(Expr * e1, Expr * e2) : e1(e1), e2(e2) { };
+    ExprOpUpdate(const Pos & pos, Expr * e1, Expr * e2) : pos(pos), e1(e1), e2(e2) { };
+    void show(std::ostream & str) const
+    {
+        str << "(" << *e1 << " " "//" " " << *e2 << ")";
+    }
+    void bindVars(const StaticEnv & env)
+    {
+        e1->bindVars(env); e2->bindVars(env);
+    }
+    void eval(EvalState & state, Env & env, Value & v);
+
+    bool hasAttr(EvalState & state, Env & env, const AttrPath & attrPath);
+    bool select(EvalState & state, Env & env, const AttrPath & attrPath, Expr * def, const Pos & pos, bool required, Value & v);
+};
+
 struct ExprConcatStrings : Expr
 {
     Pos pos;