* nix flake check: improve error message if overlay is not a lambda
Suppose you have an overlay like this
{
inputs = { /* ... */ };
outputs = { flake-utils, ... }: flake-utils.lib.eachDefaultSystem
(system: {
overlays.default = final: prev: {
};
});
}
then `nix flake check` (correctly) fails because `overlays` are supposed
to have the structure `overlays.<name> = final: prev: exp`. However, the
error-message is a little bit counter-intuitive:
error: overlay does not take an argument named 'final'
While one might guess where the error actually comes from because the
trace above says `… while checking the overlay 'overlays.x86_64-linux'`
this is still pretty confusing because it complains about an argument
not being named `final` even though that's evidently the case.
With this change, the error-message actually makes it clear what's
wrong:
[ma27@carsten:~/Projects/nix/tmp]$ nix flake check --extra-experimental-features 'nix-command flakes' path:$(pwd)
error:
… while checking flake output 'overlays'
at /nix/store/clgblnxx003hyrq8qkz5ab6kgqkck6qc-source/flake.nix:4:5:
3| outputs = { ... }: {
4| overlays.x86_64-linux.snens = final: prev: {
| ^
5| kek = throw "snens";
… while checking the overlay 'overlays.x86_64-linux'
at /nix/store/clgblnxx003hyrq8qkz5ab6kgqkck6qc-source/flake.nix:4:5:
3| outputs = { ... }: {
4| overlays.x86_64-linux.snens = final: prev: {
| ^
5| kek = throw "snens";
error: overlay is not a lambda, but a set instead
