Document haskell-flake's behavoir on a missing cabal file

[gnull]

I had a long standing issue being unable to compile my cabal project with haskell-flake.

EDIT: the rest of this post is irrelevant, see this comment for the up-to date description.

Outdated description

After some trial and error, I discovered that haskell-flake requires the `fileset` option to include the project's cabal file, like in the template:

https://github.com/srid/haskell-template/blob/807aae5c6e6dc502415a60ecf9bb48ae09b0e1f8/nix/modules/flake/haskell.nix#L17

If the cabal file is not in it (or fileset is not set at all), haskell-flake seems to fail to discover the dependencies from cabal file. It also seems to ignore what I put in haskellProjects.default.packages when cabal file is not in fileset.

Given how critical fileset is to get right, could we do the following:

1. Document it in https://community.flake.parts/haskell-flake as well as feature it in the README example. Ideally, it should be somewhere one can't miss it.

   Even though it is documented by example in the template that is provided, this does not clearly define the option's semantics. Also, the current documentation does not suit users who may not be using the template, there's simply no way they can discover that this mandatory option exists.

2. Issue a warning or error when cabal file is ignored due to not being included in fileset. I don't know which of the two is more appropriate, but I feel that quietly ignoring a situation where user is likely to be misunderstanding what is going on is suboptimal.

I could draft a documentation PR for 1 if someone tells me what is the semantics of fileset and cabal file being in it (and whether my guess above is correct).

Ivan

[srid]

(I'm converting this to Q&A for now, since the action item is not clear).

unable to compile my cabal project with haskell-flake

..

quietly ignoring a situation

Could you please provide a repro for this? I wish to start from the concrete problem.

[gnull]

I apologize, what I said above about fileset option is wrong, I can't reproduce it.

I must have overlooked some detail when doing the testing before. The real reason for my problem was that cabal file was not added to Git (because I was using package.yaml). This is somewhat of an obvious source of problem because Nix is known to ignore values that are in Git. The default behavior of quietly taking it when there is no cabal file may nevertheless have some room for improvement, maybe?

gnull/haskell-template@d4ccd2c

If you start with this Git tree and do the following:

# to simulate a file that is not added to Git
cp cp haskell-template.non-cabal haskell-template.cabal 

# the following fails because dependencies are not found
nix develop --command cabal build

# the following does not show wreq even though it's added to packages in haskell.nix
nix develop --command ghc-pkg list | grep wreq

Do you think any of the following would be a reasonable way to aid users in debugging issues like this?

• fail in case no cabal files were found?
• fail in case the user has set options in haskellProjects.default , but no cabal files were found?
• add a reminder on cabal file having to be in Git to README
• feature the explicit way of pointing at the cabal file with projectRoot in README or docs

[srid]

Since haskell-flake uses callCabal2nix which in turn calls cabal2nix, I think your first question is for https://github.com/NixOS/cabal2nix

Regarding failing/warning - you can prototype a PR on this repo to show how it would look.

[gnull]

Thanks!

Both cabal2nix and callCabal2nix do fail if there is no cabal file in the given directory. Below is what I get if I run them from gnull/haskell-template@d4ccd2c.

I would expect to see the same error when I do nix develop from the same directory, but it runs with no errors and gives me a shell. Does haskell-flake ignore this error on purpose or is it accidental?

$ cabal2nix .
cabal2nix: user error (*** Found neither a .cabal file nor package.yaml. Exiting.)

$ nix eval --impure --expr 'with import <nixpkgs> {}; haskellPackages.callCabal2nix "your-package-name" ./. {}'
error:
       … in the left operand of the update (//) operator
         at /nix/store/1vcxcfl5j7pgj7bq7zclsy22h8zklls9-nixpkgs-24.11/nixpkgs/pkgs/development/haskell-modules/lib/compose.nix:48:5:
           47|     ))
           48|     // {
             |     ^
           49|       overrideScope = scope: overrideCabal f (drv.overrideScope scope);

       … in the left operand of the update (//) operator
         at /nix/store/1vcxcfl5j7pgj7bq7zclsy22h8zklls9-nixpkgs-24.11/nixpkgs/pkgs/development/haskell-modules/lib/compose.nix:48:5:
           47|     ))
           48|     // {
             |     ^
           49|       overrideScope = scope: overrideCabal f (drv.overrideScope scope);

       … while calling a functor (an attribute set with a '__functor' attribute)
         at /nix/store/1vcxcfl5j7pgj7bq7zclsy22h8zklls9-nixpkgs-24.11/nixpkgs/pkgs/development/haskell-modules/make-package-set.nix:134:5:
          133|     in
          134|     lib.makeOverridable drvScope (auto // manualArgs);
             |     ^
          135|

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: builder for '/nix/store/vnqhazqnhc974wkm7vd6x5hy26hiwpxf-cabal2nix-your-package-name.drv' failed with exit code 1;
       last 1 log lines:
       > cabal2nix: user error (*** Found neither a .cabal file nor package.yaml. Exiting.)
       For full logs, run 'nix log /nix/store/vnqhazqnhc974wkm7vd6x5hy26hiwpxf-cabal2nix-your-package-name.drv'.