flake.nix: install cabal-install from head

[d-xo]

Description

This is pretty ugly (and will probably make updating to a newer nixpkgs annoying), but it does give us access to the multiple components features that were added to cabal in haskell/cabal#8726. Which is imo a large enough workflow upgrade that it's worth this temporary hack until we get an official release.

See https://well-typed.com/blog/2023/03/cabal-multi-unit/ for docs.

To spawn a repl session for the tests where :r will reload changes made in lib:hevm, and run a single test:

cabal repl --enable-multi-repl test lib:hevm lib:test-utils
ghci> import Main
ghci> runSubSet "<TESTNAME>"

One unfortunate downside is that Paths.getDataFileName seems to be broken in a multi-component repl, so the tests that trace vs geth and the Dapp-Tests test group are broken. They still work in the normal single component repl.

Checklist

• tested locally
• added automated tests
• updated the docs
• updated the changelog

[msooseth]

Do you think we can add multi-repl: True in the hevm.cabal file? I think it'd make sense to make it default, no?

[msooseth]

lgtm. I see what you mean about updating, seeing the nix file changes, but I think we can go back to released cabal once the multi-component support is released. So I think it's a good workaround, and it's definitely rather slow as-is.

[d-xo]

Do you think we can add multi-repl: True in the hevm.cabal file? I think it'd make sense to make it default, no?

oh good idea, didn't realise we could do that...