Use environment in evaluation of function definitions

Nix/Eval.hs

@@ -234,7 +234,10 @@ evalExpr = cata phi
         -- environment, but rather should recursively view their own arg
         -- set
         args <- traverse ($ env) a
-        return $ Fix $ NVFunction args b
+        -- The body of a function first looks into the arguments, then into the
+        -- env in the evaluation point.
+        let function x = b (x `Map.union` env)
+        return $ Fix $ NVFunction args function
 
 evalString :: Monad m
            => ValueSet m -> NString (ValueSet m -> m (NValue m)) -> m (NValue m)

tests/EvalTests.hs

@@ -33,6 +33,9 @@ case_function_set_two_arg = constantEqualStr "2" "({ a, b ? 3 }: b - a) { a = 1;
 -- case_function_set_two_arg_default_scope :: Assertion
 -- case_function_set_two_arg_default_scope = constantEqualStr "2" "({ a, b ? a * 3 }: b - a) { a = 1; }"
 
+case_function_definition_uses_environment :: Assertion
+case_function_definition_uses_environment = constantEqualStr "3" "let f = (let a=1; in x: x+a); in f 2"
+
 tests :: TestTree
 tests = $testGroupGenerator