It’s good practice to make sure your code is warning-free before committing or releasing. In the throes of editing, though, it can be extremely annoying to have to fix a swathe of trivial warnings when you really just want to see if things basically make sense (especially if you’re fond of undefined-driven development, and are using a prelude which marks undefined as deprecated).

I used to just edit the cabal file to add and delete “-Werror” according to what mode I was in, but this was both annoying to do, and potentially incorrect: if you add -Werror in when your code is already built, cabal will not automatically rebuild local modules.

Thankfully, cabal flags do force a rebuild. This means you just need this at the top level in mycoolproject.cabal:

flag lib-Werror
  default: True
  manual: True

and this in your executable & library stanzas:

  if flag(lib-Werror)
    ghc-options: -Werror

Now, when you want to run cajun style, you can edit your command line to something like

stack build --flag mycoolproject:-lib-Werror

turning this into a convenient makefile action is left as an exercise to the reader - extending to hpack-style is similar.

(This tiny but pleasant tip is due to Michael Xavier.)