default.nix: allow passing in compiler

Author: sorki

default.nix

@@ -8,11 +8,19 @@
 #
 # ... and then Nix will supply the correct Haskell development environment for
 # you
+{ compiler ? null
+}:
 let
   config = { };
 
   overlay = pkgsNew: pkgsOld: {
-    haskellPackages = pkgsOld.haskellPackages.override (old: {
+    haskellPackages =
+      let
+        packageSet =
+          if compiler == null
+          then pkgsOld.haskellPackages
+          else pkgsOld.haskell.packages.${compiler};
+      in packageSet.override (old: {
       overrides =
         let
           fromCabal2nix = self: super: {