linting GHC

30 Apr 2021

      Hi devs,

I see in CI that we're now linting the GHC source code. In order to avoid CI failure, I would like to lint locally, and I somehow know (though I forget where I learned it) that hadrian/build lint:compiler is the way to do this. After building hlint, I am able to do this. But the output is surprising; here is the tail:

# hlint (for lint:compiler)
compiler/GHC/Tc/Utils/Unify.hs:2:1-36: Warning: Unused LANGUAGE pragma
Found:
  {-# LANGUAGE MultiWayIf #-}
Perhaps you should remove it.

compiler/stage1/build/GHC/Parser.hs:6:1-29: Warning: Unused LANGUAGE pragma
Found:
  {-# LANGUAGE ViewPatterns #-}
Perhaps you should remove it.

compiler/stage1/build/GHC/Parser.hs:8:1-27: Warning: Unused LANGUAGE pragma
Found:
  {-# LANGUAGE LambdaCase #-}
Perhaps you should remove it.

compiler/stage1/build/GHC/Parser.hs:38:1-54: Warning: Use fewer imports
Found:
  import Control.Monad ( unless, liftM, when, (<=<) )
  import Control.Monad ( ap )

Perhaps:
  import Control.Monad ( unless, liftM, when, (<=<), ap )

compiler/stage1/build/GHC/Parser.hs:43:1-18: Warning: Use fewer imports
Found:
  import GHC.Prelude
  import GHC.Prelude

Perhaps:
  import GHC.Prelude

compiler/stage1/build/GHC/Cmm/Parser.hs:17:1-24: Warning: Use fewer imports
Found:
  import GHC.StgToCmm.Prof
  import GHC.StgToCmm.Prof

Perhaps:
  import GHC.StgToCmm.Prof

compiler/stage1/build/GHC/Cmm/Parser.hs:71:1-20: Warning: Use fewer imports
Found:
  import Control.Monad
  import Control.Monad ( ap )

Perhaps:
  import Control.Monad

compiler/stage2/build/GHC/Parser.hs:6:1-29: Warning: Unused LANGUAGE pragma
Found:
  {-# LANGUAGE ViewPatterns #-}
Perhaps you should remove it.

compiler/stage2/build/GHC/Parser.hs:8:1-27: Warning: Unused LANGUAGE pragma
Found:
  {-# LANGUAGE LambdaCase #-}
Perhaps you should remove it.

compiler/stage2/build/GHC/Parser.hs:38:1-54: Warning: Use fewer imports
Found:
  import Control.Monad ( unless, liftM, when, (<=<) )
  import Control.Monad ( ap )

Perhaps:
  import Control.Monad ( unless, liftM, when, (<=<), ap )

compiler/stage2/build/GHC/Parser.hs:43:1-18: Warning: Use fewer imports
Found:
  import GHC.Prelude
  import GHC.Prelude

Perhaps:
  import GHC.Prelude

compiler/stage2/build/GHC/Cmm/Parser.hs:17:1-24: Warning: Use fewer imports
Found:
  import GHC.StgToCmm.Prof
  import GHC.StgToCmm.Prof

Perhaps:
  import GHC.StgToCmm.Prof

compiler/stage2/build/GHC/Cmm/Parser.hs:71:1-20: Warning: Use fewer imports
Found:
  import Control.Monad
  import Control.Monad ( ap )

Perhaps:
  import Control.Monad

13 hints
Error when running Shake build system:
  at want, called at src/Main.hs:104:30 in main:Main
* Depends on: lint:compiler
  at cmd_, called at src/Rules/Lint.hs:70:3 in main:Rules.Lint
* Raised the exception:
Development.Shake.cmd, system command failed
Command line: hlint -j --cpp-define x86_64_HOST_ARCH --cpp-include=includes --cpp-include=_build/stage1/lib --cpp-include=compiler --cpp-include=_build/stage1/lib/ghcplatform.h --cpp-include=_build/stage1/compiler/build -h compiler/.hlint.yaml compiler
Exit code: 1
Stderr:

There are several curiosities here:

* I introduced one of these errors, in GHC.Tc.Utils.Unify; the other errors are in files I have not modified. Will these affect CI?
* Hadrian crashes at the end. Is that expected? It says "system command failed" with a bunch of gobbledegook. Is this an error within Hadrian? Or is this just Hadrian's way of saying that it encountered an error when linting? If it's the latter, the current message is hard to read.
* As you may be able to see, I use a dark-background terminal window. The colors output by the hlint process are nigh impossible to read when describing redundant LANGUAGE pragmas. Are these colors configurable?

Thanks!
Richard

linting GHC

Richard Eisenberg