Fix evaluation with old versions of nixpkgs
Using an old version of nixpkgs with ghc.nix fails to evaluate:
error: undefined variable 'happy'
at /nix/store/g8fnx3r5jccwgwhiridj6rlmqwd80cmi-source/ghc.nix:270:20:
269| export GHCPKG=$NIX_GHCPKG
270| export HAPPY=${happy}/bin/happy
| ^
271| export ALEX=${alex}/bin/alex
Using hspkgs.happy
instead of happy
(which defers to pkgs.happy
due to the with pkgs;
clause on line 85 fixes this.