Add the ExprBuiltin Expr type to the AST

Certain desugaring schemes may require the parser to use some builtin
function to do some of the work (e.g. currently `throw` is used to
lazily cause an error if a `<>`-style path is not in the search path)
Unfortunately, these names are not reserved keywords, so an expression
that uses such a syntactic sugar will not see the expected behavior
(see tests/lang/eval-okay-redefine-builtin.nix for an example).

This adds the ExprBuiltin AST type, which when evaluated uses the value
from the rootmost variable scope (which of course is initialized
internally and can't shadow any of the builtins).

Signed-off-by: Shea Levy <shea@shealevy.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index f4b4295e290c..c7ac967968d7 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -121,6 +121,11 @@ void ExprOpNot::show(std::ostream & str)
     str << "! " << *e;
 }
 
+void ExprBuiltin::show(std::ostream & str)
+{
+    str << "builtins." << name;
+}
+
 void ExprConcatStrings::show(std::ostream & str)
 {
     bool first = true;
@@ -314,6 +319,10 @@ void ExprOpNot::bindVars(const StaticEnv & env)
     e->bindVars(env);
 }
 
+void ExprBuiltin::bindVars(const StaticEnv & env)
+{
+}
+
 void ExprConcatStrings::bindVars(const StaticEnv & env)
 {
     foreach (vector<Expr *>::iterator, i, *es)