Error thrown on CPP in the `base` codebase

[Kleidukos]

• Version: HLint v3.1.6, (C) Neil Mitchell 2006-2020
• OS: Fedora 32
• Command line: $ hlint -j --cpp-include="./libraries/base/include/" --cpp-include=includes --cpp-include=./_build/stage1/lib/ -h libraries/base/.hlint.yaml libraries/base
• Error message:

  hlint: # error Unknown mingw32 arch
  in libraries/base/GHC/Windows.hs  at line 81 col 1
  CallStack (from HasCallStack):
  error, called at ./Language/Preprocessor/Cpphs/CppIfdef.hs:113:21 in
  cpphs-1.20.8-DevT7HkEKU3DDQgUoHVnJY:Language.Preprocessor.Cpphs.CppIfdef

The CPP code block is :

#if defined(i386_HOST_ARCH)     
# define WINDOWS_CCONV stdcall  
#elif defined(x86_64_HOST_ARCH) 
# define WINDOWS_CCONV ccall    
#else                           
# error Unknown mingw32 arch    
#endif                          

[shayne-fletcher]

Perhaps adding --cpp-define x86_64_HOST_ARCH to the hlint invocation will help here?

[Kleidukos]

Perfect! I still get a rather peculiar message from hlint, but other than that, no problem whatsoever!

Warning: unknown directive #-}
in libraries/base/Unsafe/Coerce.hs  at line 311 col 1
No hints

This is the file in question: https://gitlab.haskell.org/ghc/ghc/-/blob/master/libraries/base/Unsafe/Coerce.hs#L303-311

[shayne-fletcher]

Perfect! I still get a rather peculiar message from hlint, but other than that, no problem whatsoever!

Awesome.

Warning: unknown directive #-}
in libraries/base/Unsafe/Coerce.hs  at line 311 col 1
No hints

This is the file in question: https://gitlab.haskell.org/ghc/ghc/-/blob/master/libraries/base/Unsafe/Coerce.hs#L303-311

Ha. That's a genuine problem. The C-preprocessor is being run over the file and it doesn't know what to make of a line that reads,

#-}

and that's where the warning is coming from. This is because the C-preprocessor interprets lines beginning with a # as being those with directives meant for it. Putting a space in front of the leading # will fix it.

If this were purely a language extension issue then potential fixes would include:

• Add {-# LANGUAGE NoCPP #-} to the extensions;
• Pass -XNoCPP to hlint so that the extension is disabled by default.

However, I'm not sure in this instance those things alone will do. I don't know if/how HLint can be told to not run CPP in general. @ndmitchell can you advise please?

[Kleidukos]

@shayne-fletcher Thanks for the fix! I'll be sure to document the hows and whys on the GHC side. Cheers!

[ndmitchell]

-XNoCpp turns off CPP entirely, --cpp-simple just strips lines with a leading # but otherwise ignores CPP markup. But in this case, with complex CPP, passing a valid set of CPP options as Shayne suggests is the right thing to do.