First attempt at getting haskell.nix to work with nixpkgs provided compiler

[andreabedini]

Addresses #1565, #1531

Test with

$ nix build -f test/with-nixpkgs-ghc/default.nix

You can compare with haskell.nix provided ghc doing

$ nix build -f test/with-nixpkgs-ghc/default.nix --arg with-nixpkgs false

It doesn't work yet.

error: builder for '/nix/store/kc6s5i90sg8f9vcc84bmba724v98rhzc-Cabal-lib-Cabal-3.6.3.0.drv' failed with exit code 1;
       last 10 log lines:
       > filepath >=1.3.0.1 && <1.5,
       > mtl >=2.1 && <2.3,
       > parsec >=3.1.13.0 && <3.2,
       > pretty >=1.1.1 && <1.2,
       > process >=1.1.0.2 && <1.7,
       > text >=1.2.3.0 && <1.3 || ==2.0.*,
       > time >=1.4.0.1 && <1.13,
       > transformers ==0.3.* || >=0.4.1.0 && <0.6,
       > unix >=2.6.0.0 && <2.8
       >
       For full logs, run 'nix log /nix/store/kc6s5i90sg8f9vcc84bmba724v98rhzc-Cabal-lib-Cabal-3.6.3.0.drv'.
error: 1 dependencies of derivation '/nix/store/cyj07lpxk87ild1lzppg3861bk1my5kh-hpack-exe-hpack-0.35.0.drv' failed to build
error: 1 dependencies of derivation '/nix/store/0dfk0qj15vml9b8x07lw80a0lb1h4wb5-hpack-lib-hpack-0.35.0-config.drv' failed to build
error: 1 dependencies of derivation '/nix/store/sddb67idq4lhkkk51z5nagjz3wp8qqcd-hpack-lib-hpack-0.35.0.drv' failed to build
error: 1 dependencies of derivation '/nix/store/yq8vnjaw5lwqsgj16bpwrq63lrwzx4qk-nix-tools-lib-nix-tools-0.1.0.0-config.drv' failed to build
error: 1 dependencies of derivation '/nix/store/k0di5bkr6zk309r361zrfhvzzw9ffzpa-nix-tools-lib-nix-tools-0.1.0.0.drv' failed to build
error: 1 dependencies of derivation '/nix/store/zsxmd6rbh0jjfjxvnjy5p0r7nfwlz5j1-nix-tools.drv' failed to build
error: 1 dependencies of derivation '/nix/store/dk9842nlgawms6a57ydnsxfkq29zmb6v-haskell-project-plan-to-nix-pkgs.drv' failed to build

[andreabedini]

Ok, ghc is missing the evalDeps and exactDeps folders, which we rely on to create the configure flags in the *-config/configure-flags derivations. Haskell.nix creates those folders during ghc bootstrap.

[angerman]

Question: what do we do if the compiler does not exist in the nixpkgs used with Haskell.nix? As they are orthogonal, I don't think there is any guarantee about the GHC we can expect form nixpkgs?

Edit: to be clear, I'm not against this. I just want to know how we expect to deal with this in a way that the user can make some sense of.

[junjihashimoto]

If not, it will probably fail with an error.
Now when there is no cache, it forces to build ghc, then we wait for a long time.

[andreabedini]

Well the short answer is simply

❯ nix build -f test/with-nixpkgs-ghc/default.nix
trace: No index state specified for haskell-project, using the latest index state that we know about (2023-02-01T00:00:00Z)!
error: attribute 'ghc8107xxx' missing

       at /home/andrea/work/haskell.nix/test/with-nixpkgs-ghc/default.nix:9:25:

            8|     let
            9|       haskell-nix-ghc = prev.haskell-nix.compiler.${compiler-nix-name};
             |                         ^
           10|       nixpkgs-ghc = prev.haskell.compiler.${compiler-nix-name};
(use '--show-trace' to show detailed location information)

but from an API pov your raise a good point because by referencing the compiler version by its name, haskell.nix has always replaced the compiler in nixpkgs. Perhaps we could (in some future) migrate to passing a compiler derivation attribute rather than compiler-nix-name?

[michaelpj]

Perhaps we could (in some future) migrate to passing a compiler derivation attribute rather than compiler-nix-name?

There's actually a reason why we do this... cross-compilation 🙈 We want to be able to select both the cross and normal version of the compiler, and the way we do this at the moment is by using the name as the key to look up both. An example of where this matters is when you have a cross-built project but you want to get a dev shell... then you need to use the non-cross compiler.

See #610 for when it was introduced.

but from an API pov your raise a good point because by referencing the compiler version by its name, haskell.nix has always replaced the compiler in nixpkgs.

A slightly more indirect way of supporting what you want here would be to provide a way to override haskell.nix's CompilerName -> Compiler mapping, so you would just pass { "ghc8107" = nixpkgs.ghc8107 } or something.

[andreabedini]

Thanks for the explanation @michaelpj!

[hamishmack]

I'm closing this in favour of #1854 as I am borrowing the ideas here to use there.