`stack-to-nix` fails to forward compiler information.

[angerman]

I didn't add it here, but the reason it was needed is due to a limitation of nix-tools. stack-to-nix doesn't output the LTS compiler version, unless it's explicitly defined in stack.yaml. And you need this if you are setting Haskell.nix module options which depend on the compiler version.

Originally posted by @rvl in cardano-foundation/cardano-wallet#1138 (comment)

[rvl]

It's possible to get the compiler after the Haskell package set has been made using pkgSet.config.ghc.package.

However, if you need the compiler version in order to determine the arguments of mkStackPkgSet, there is no good way to do it.

[angerman]

So it's for this usecase:
https://github.com/input-output-hk/cardano-wallet/blob/d7716dfa9d7c477dc48fe03806696951e785923f/nix/pkgs.nix#L89-L104

and then used as the module parameter to

haskell.nix/overlays/haskell.nix

Lines 94 to 97 in a28e362

	mkStackPkgSet =
	{ stack-pkgs # Path to the output of stack-to-nix
	, pkg-def-extras ? []
	, modules ? []

where we check the compiler to choose an integer library?

I see two options:

• the integer-library selection shouldn't be this hideous. That's just aweful. If you choose the integer-lirbary you shouldn't have to concern yourself with the compiler. They are in lockstep, you won't be able to choose an integer library and use a different compiler while expecting them to properly work.
• allow modules to be of the form ({compiler, ...}: { ... }); I believe we do allow something like that for modules already, I'm just not sure if we pass the compiler information there.

[rvl]

Having both would be good. :-)

I need to double-check whether cardano-wallet will actually need integer-simple. If not, we can remove compiler from stack.yaml.