How to easily set -Werror only in the Nix build of things?

[michaelpj]

A common desire is to set -Werror in CI/production builds but not in development builds.

It's not clear how to do this best in haskell.nix. Options that I can see:

1. Put package a: ghc-options: -Werror in your cabal.project for all your local packages. (Can't do it globally because that affects dependencies: ghc-options applies to all dependencies, not just local packages haskell/cabal#3883). Provide a cabal.project.local that overrides this and gets rid of the flag.
   • Note: I haven't tried this, it might not work.
2. Put the same in cabal.project.somethingelse, somehow get haskell.nix to include it?
3. Put ghc-options: -Werror in all your .cabal files, guarded by a flag. Set this flag for all packages with module options.
   • Doesn't seem to be a way to do this globally? So I think you still have to set an option for every package in turn.
4. Set the ghcOptions module option to -Werror for all your packages.

At the moment I'm using option 4., but it's somewhat painful due to having to manually list all your packages. Both 3. and 4. would be eased if there was an easy way to set a module option for all the project packages in one go. Maybe there's a clever way to use the module system to do this?

[cdepillabout]

Both 3. and 4. would be eased if there was an easy way to set a module option for all the project packages in one go

We'd like this ability as well.

This was sort of discussed in the following issue: #298

[NorfairKing]

+1 I also want this as a stack user because I use the non --pedantic build while developing so I don't need to fix all my redundant imports during development.