make stacklock2nix figure out the ghc version it needs automatically

[cdepillabout]

I think it should be possible to make stacklock2nix figure out the ghc version is needs automatically.

Currently, the API of stacklock2nix requires you to pass a full package set, like the following (the baseHaskellPkgSet attr):

stacklock2nix/my-example-haskell-lib-easy/flake.nix

Lines 41 to 56 in ac35783

	my-example-haskell-stacklock = final.stacklock2nix {
	stackYaml = ./stack.yaml;
	# The Haskell package set to use as a base. You should change this
	# based on the compiler version from the resolver in your stack.yaml.
	baseHaskellPkgSet = final.haskell.packages.ghc924;
	# Any additional Haskell package overrides you may want to add.
	additionalHaskellPkgSetOverrides = hfinal: hprev: {
	# The servant-cassava.cabal file is malformed on GitHub:
	# https://github.com/haskell-servant/servant-cassava/pull/29
	servant-cassava =
	final.haskell.lib.compose.overrideCabal
	{ editedCabalFile = null; revision = null; }
	hprev.servant-cassava;
	};

It should be possible for stacklock2nix to figure out the GHC version automatically from the stackage package set, and then use that internally. I imagine an API like the following would be possible:

final.stacklock2nix {
  stackYaml = ./stack.yaml;
  haskell = final.haskell;
}

stacklock2nix would internally look in the resolver in stack.yaml.lock, figure out it uses GHC-9.2.4, translate that to haskell.packages.ghc924, and then just use that.

If a user wanted to use a different package set (like haskell.packages.native-bignum.ghc924), then they could just fall-back to using the baseHaskellPkgSet argument.

[cdepillabout]

In Nixpkgs, ghc has a haskellCompilerName attribute. For instance, haskell.compiler.ghc924.haskellCompilerName is "ghc-9.2.4".

This value may be a good starting point for figuring out the Nixpkgs attribute to use for a given stack resolver GHC.

[silky]

please forgive my extreme inability, but i would love to have a go at this, but my nix development skills are such that i can't even figure out how to run the tests, run a repl that does anything useful, or otherwise get a dev environment going :D 😅 ; for example, i had assumed i could run nix develop in the root dir and get a shell, but i can't ...

error: flake 'git+file:///home/noon/dev/ext/stacklock2nix' does not provide attribute 'devShells.x86_64-linux.default', 'devShell.x86_64-linux', 'packages.x86_64-linux.default' or 'defaultPackage.x86_64-linux'

if you could put together a very small developing... guide, then i could give it a go :) thanks!

[cdepillabout]

@silky I added a short explanation of how to run the tests in #14.

If this doesn't help, or you'd like a few more pointers, feel free to send me an email and we could do a video chat or some pair programming or something.

My email address (and other ways of getting in touch) is on this page: https://functor.tokyo/about