auto-call error

1 files changed, 22 insertions, 1 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index d6366050c..e52e8dcf2 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1370,7 +1370,28 @@ void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
             if (j != args.end()) {
                 actualArgs->attrs->push_back(*j);
             } else if (!i.def) {
-                throwTypeError("cannot auto-call a function that has an argument without a default value ('%1%')", i.name);
+                throwUndefinedVarError(R"(cannot auto-call a function that has an argument without a default value ('%1%')
+  An 'auto-call' is when a nix expression is evaluated without any external arguments.  If that
+  nix expression is a function, and that function's arguments all have default values, then all is well.
+
+  But if the function arguments don't have default values, then evaluation fails.
+
+  The classic case for this error is evaluating a nix file with nix-build that expects to be evaluated by callPackage.
+    # in 'callPackage' format: expression is a function that takes an argument 'stdenv'.
+    # callPackage would implicitly pull 'stdenv' from nixpkgs, then call this function.
+    { stdenv }:
+    stdenv.mkDerivation  {
+    ...
+
+    # in 'auto-call' format: nixpkgs is imported explicitly, and used directly.
+    let
+      nixpkgs = import <nixpkgs> {};
+    in
+      nixpkgs.stdenv.mkDerivation {
+    ...
+
+  See this nix pill for more information re callPackage format:
+  https://nixos.org/guides/nix-pills/callpackage-design-pattern.html)", i.name);
             }
         }
     }