nix-shell does not come with cabal ?

[PierreR]

I am trying to give some feedback about contribution and what could be improved to make it easier.

This one comes a bit as a surprise to me:

If you go in the dhall-haskell/dhall folder and execute nix-shell then if you try cabal build it does not work:

[nix-shell:~/projects/hub/dhall-haskell/dhall]$ cabal build
cabal: command not found

Isn't this supposed to work ? nix-shell should give you an development environment with everything required to contribute.

I would like to reach a point where contributing is easy. You just do code to open vscode and you have an IDE that just works (with the help of ghcide or haskell-language-server).

See this post: https://daml.com/daml-driven/handing-over-ghcide-to-the-haskell-community/

Sorry if I miss the point. I am just a dumb user at this point ;-)

[PierreR]

Something else that would be quite useful is to have a shell that not only includes cabal but also comes with a setup that makes cabal uses the system dependencies from nix but also all the project dependencies.

IIRC this can be achieved with pkgs.haskellPackages.shellFor.

With this setup you don't have cabal downloading/resolving every haskell deps in its own local directory while they are all there already in the nix store (after a nix-build)

[Gabriella439]

@PierreR: Regarding the latter request, that is how the nix-shell already works. It provides all dependencies, including the Haskell ones.

[PierreR]

@Gabriel439 are you sure cabal uses them when doing a cabal build ? I will have a second try.

[Gabriella439]

@PierreR: As an extra precaution, try running cabal configure again when you enter the nix-shell. That will help ensure that cabal uses the ghc that Nix provides, which has all of the Nix-supplied packages in the GHC package database

[PierreR]

@Gabriel439 I am missing something ... If I don't provide a ghc I would get the following:

 ~/projects/hub/dhall-haskell  master ✓ 
→ nix-shell
Dhall core tools shell
USAGE EXAMPLES
    Default (GHC version 8.6.5, option completions shell - bash):
        $ nix-shell
    Overriding default GHC compiler (see ./nix/shared.nix for available options):
        $ nix-shell --arg args '{ compiler = ghc861; }'
    Overriding option completion shell flavor:
        $ nix-shell --argstr shell zsh
dhall-docs: command not found

[nix-shell:~/projects/hub/dhall-haskell]$ ghc --version
ghc: command not found

Maybe I need to nix-build before opening a shell with nix-shell ?

[Gabriella439]

@PierreR: Oh, the nix-shell is supposed to be run from within the project subdirectory. See the Nix section of the README for more details

[PierreR]

@Gabriel439 Thanks ! Now I have a ghc ;-) I still need to figure out why cabal is not included in the shell.

I have also found out that nix-build does not copy the same haskell package derivations than nix-shell ... (maybe because I am not using the same cabal ?). That means you end up downloading quite a lot (2x 3G)

Another surprise is that after building dhall within a shell on the dhall folder, if I go to the dhall-openapi directory and enter nix-shell then it is going to rebuild dhall again. That might make sense because I didn't actually build the dhall nix derivation (just cabal build it).

I still need to figure out how I can integrate smoothly ghcide into the mix.

[PierreR]

Passing the initial surprise about nix-shell being incomplete (cabal is missing) I have managed to find my way by installing a version of ghcide that matches the ghc version (using nix to install cabal in user space).

I am a bit surprised that each project folder seems to require a full nix rebuild of Dhall when entering the shell. I would expect the dhall derivation to be the same across dhall-docs and dhall-openapi for instance

It would be great to be able to start from the root, enter nix-shell, type 'ghcide' and be able to "jump to definition" from one dhall project to another.

A new issue could be created to enhance the first time experience if it is something of interest.

Thanks for your help @Gabriel439 !

[Gabriella439]

I'm reopening because there is still something that I can do here related to the original request of providing cabal and/or vscode with a ghcide plugin

[PierreR]

Trying to fiddle with dhall-openapi today and that has been my experience:

t -1: the setup was working nicely a week ago

1. go to the repo/ git pull/ rebase the changes on my local branches ...
2. go to dhall-openapi, open vscode, well that was really optimistic
3. I need to nix-shell -> wait 5 min because dhall 1.35.0 has to be rebuilt
   (it recompiles twice -- why I wonder )
4. ghcide -> boom -> problem with hnix (strange that is not even a dependency of dhall-openapi)
5. take a few minutes to realize I need to do cabal update
6. cabal build just to be sure - wait, it rebuilds dhall-1.35.0 again (ok let's wait)
7. success !
8. ghcide

> Building library for dhall-1.35.0..
> gcc: error: /home/vagrant/projects/hub/dhall-haskell/dist-newstyle/build/x86_64-linux/ghc-8.6.5/dhall-1.35.0/build/Dhall/Parser/Token.dyn_o: No such file or directory
> `cc' failed in phase `Linker'. (Exit code: 1)

Wait what ?
9. cabal clean
10. ghcide -> let's wait for ... dhall-1.35.0 to rebuild
11. success !! 😌
😇

edit: when entering nix-shell I believe it does it twice with/without profile enabled

[Gabriella439]

@PierreR: Yeah, Nixpkgs builds Haskell packages with profiling enabled by default (although that's configurable)

[timbertson]

I am very confused, because suddenly I'm also having the original issue - cabal not present in a nix shell (I'm inside the dhall subdirectory). It was present previously, I don't even know what I did. I have checked out a new version and done nix-channel --update, but I tried both checking out the previous version and also rolling back my nixpkgs. I believe nixpkgs is pinned anyway, so that shouldn't affect it 😕