Add functors (callable attribute sets).

With this, attribute sets with a `__functor` attribute can be applied
just like normal functions. This can be used to attach arbitrary
metadata to a function without callers needing to treat it specially.

src/libexpr/eval.cc

@@ -180,6 +180,7 @@ EvalState::EvalState(const Strings & _searchPath)
     , sFile(symbols.create("file"))
     , sLine(symbols.create("line"))
     , sColumn(symbols.create("column"))
+    , sFunctor(symbols.create("__functor"))
     , repair(false)
     , baseEnv(allocEnv(128))
     , staticBaseEnv(false, 0)
@@ -885,6 +886,17 @@ void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & po
         return;
     }
 
+    if (fun.type == tAttrs) {
+      auto found = fun.attrs->find(sFunctor);
+      if (found != fun.attrs->end()) {
+        forceValue(*found->value);
+        Value * v2 = allocValue();
+        callFunction(*found->value, fun, *v2, pos);
+        forceValue(*v2);
+        return callFunction(*v2, arg, v, pos);
+      }
+    }
+
     if (fun.type != tLambda)
         throwTypeError("attempt to call something which is not a function but %1%, at %2%", fun, pos);
 

src/libexpr/eval.hh

@@ -126,7 +126,7 @@ public:
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
-        sFile, sLine, sColumn;
+        sFile, sLine, sColumn, sFunctor;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they

tests/lang/eval-okay-callable-attrs.exp

@@ -0,0 +1 @@
+true

tests/lang/eval-okay-callable-attrs.nix

@@ -0,0 +1 @@
+({ __functor = self: x: self.foo && x; foo = false; } // { foo = true; }) true