Feature request: Super fast output of all warnings

[nh2]

For SublimeHaskell (and potentially all other editors and similar tools) we need to quickly be able to list all warnings of a cabal project.

cabal build does not show warnings on already compiled modules, so we currently use

cabal build --ghc-options="-fforce-recomp -Wall -fno-code"

to quickly get all warnings of the project. This is obviously slow since all code has to be re-compiled (even though code generation can be skipped).

It would be nice if cabal could use its knowledge of which code has no changes to maintain a cache of warnings and error messages that can be queried super quickly.

We could then add an option cabal build --check that only typechecks the project and prints out all errors/warnings of unchanged files from the cache, so that a no-op typecheck with warnings is instant and can give feedback very quickly or even as you type.

[nh2]

Related to https://ghc.haskell.org/trac/ghc/ticket/9243

[ezyang]

This is actually annoying to do in the current architecture, because ghc --make spews out all of the warnings into one stderr, so you'd have to parse and separate them out by hand.

I actually implemented warning caching in ghc-shake https://github.com/ezyang/ghc-shake which is a drop-in replacement for ghc. Warnings are put in *.log files so if you use find dist-newstyle/build/ -name "*.log" -exec cat {} \; you'll see all the warnings. Works pretty well for me.

This won't get fixed unless someone steps up to do it.

[jneira]

Not sure if that behaviour should be included in cabal itselfor in a external tool.