As some may already know, I'm working on cross compiling Haskell to Windows using nix.

I'll try to keep track of the issues I run into, and then add the relevant PRs as I manage to
clean them up.

Minor

• mingw64 is too old for GHC. GHC requires mingw-w64 >= 5
• cctools asserts targetPlatform.isDarwin, which fails.
• ghc binary expressions assert stdenv.targetPlatform == stdenv.hostPlatform, which fails during validation.
• The buildMK in the ghc expressions is wrong and needs to also include the relevant $flavour.mk
• The generic builder uses hsc2hs from the nativeGhc, but should use $targetPrefix-hsc2hs.
• haskellSetupDepends should come from the buildPackages package set as they are compiled and run by the native compiler.

Major

• packageOverrides and buildPackages don't play nice together. Injecting a custom ghc via config doesn't work anymore, as buildPackages is fixed prior to the overrides being applied, and buildPackages doesn't see any added packages from the packageOverrides.

I might be missing some philosophical foundation with respect to nix here. Please bear with me and help me better understand.

/cc @Ericson2314