Unhelpful error message about "hidden" packages

[simonpj]

Anatoly Yakonenko  reported: I am trying to build ParseP on the latest ghc, and I am getting this error:

 Text/ParserCombinators/ParseP/Interface.hs:26:17:
    Could not find module `Data.ByteString.Char8':
      it is a member of package bytestring-0.9.1.4, which is hidden

Ross Mellgren responded (correctly): I assume you're using cabal, which hides all packages by default. Add "bytestring" or "bytestring >= 0.9.*" or something along those lines to Build-Depends in your .cabal file.

Suggestion: improve GHC's error message.

[simonpj]

I think this is readily fixable. Line 601 of Finder.lhs has

    pkg_hidden pkg =
        ptext (sLit "it is a member of the hidden package") <+> quotes (ppr pkg)

I suggest instead:

  "Since it is a member of package bytestring-0.9.1.4, 
   perhaps you intended to say '-package bytestring', 
   or (if using Cabal) perhaps you omitted 'bytestring' from the
   build-depends in your .cabal file."

If there are multiple hidden packages that contain the module, we probably don't want all this blurb for each one; but some simple rewording should do, and all the info is available in Finder.cantFindError.

Better wordings welcome.

Simon

[igloo]

Or perhaps Cabal should pass GHC a -cabal flag, and we could give a different error when that flag is given. Might be useful in other cases too.

[igloo]

Now fixed:

$ ./Setup build
Preprocessing library pretty-1.0.1.0...
Building pretty-1.0.1.0...

Text/PrettyPrint/HughesPJ.hs:217:7:
    Could not find module `System.Directory':
      It is a member of the hidden package `directory-1.0.0.2'.
      Perhaps you need to add `directory' to the build-depends in your .cabal file.
      Use -v to see a list of the files searched for.

You only get this extra cabal hint if -fbuilding-cabal-package is passed, and Cabal HEAD now passes this to GHC.