gistfile1.md

When installing Haskell Platform fails on Mac OS X (with quick fix below)

I was fighting with Haskell last weekend. At first, I couldn't install some missing libraries with Cabal, and then, when trying to find out what's wrong, I ended up removing the whole Haskell installation — only to find out I could no longer install neither the Haskell Platform nor even just Cabal Install! The warnings I would see were more or less about the use of the single quote in source code:

Preprocessing library text-0.11.2.3...

Data/Text.hs:6:52:
     warning: missing terminating ' character [-Winvalid-pp-token]
-- Copyright   : (c) 2009, 2010, 2011, 2012 Bryan O'Sullivan,
                                                   ^

The errors that finally prevented the installation, were about having non-preprocessor-directive lines start with # in Haskell files that should be preprocessed with the C preprocessor (CPP):

Data/Text.hs:442:3:
     error: invalid preprocessing directive
     #-}

(I should mention here, by the way, that Clang is all correct in rejecting these source files: it's trying to protect poor coders from writing e.g. #elsif where #elif would be needed! It's just that Haskellers are used to a less strict version of the C preprocessor, one that's been supported by the GNU compiler since ages ago.)

I copied both the above warnings and the error from the Haskell text package bug report haskell/text#53, but similar problems have been previously cited in the GHC bug tracker as well as Homebrew bug tracker about cabal-install.

I asked about the problem in the #haskell IRC channel but I'm unsure where the source of this problem should be actually fixed:

• a) in broken packages (i.e. by moving the closing pragma markers, #-}, to the end of the preceding line),
• b) in the Homebrew formulas (in that case, I guess it would be best to fix the GHC formula), or
• c) in GHC itself, to make it not use the current C compiler for preprocessing Haskell source files (there's a Haskell implementation of the CPP in http://hackage.haskell.org/package/cpphs-1.16 but some commenters in IRC were concerned about the LGPL-mode licensing.)

So before we'll find a satisfactory permanent solution, here's my quick interim fix for the problem.

QUICKFIX: Steps to install Haskell Platform on Mac OS X

…with Homebrew and a very recent version of Clang.

1. Install Glasgow Haskell Compiler 7.6.3. (This would be installed by brew install haskell-platform anyway, but we need it before that.)

   brew install ghc
   

2. Enable installation of GCC versions, and install e.g. gcc-4.7.

   brew tap homebrew/versions
   brew install gcc47
   

3. GHC uses /usr/bin/gcc by default, which is actually Clang. And apparently, new Clang versions don't support the -traditional GCC option that makes the C preprocessor work in a friendly manner to Haskell files. So, instead of using Clang as the preprocessor, reconfigure the GHC compiler script to compile with /usr/local/bin/gcc-4.7 instead.

   nano /usr/local/Cellar/ghc/7.6.3/lib/ghc-7.6.3/settings
   

   (In the above file, replace the second line:

   ("C compiler command", "/usr/bin/gcc"),
   

   with:

   ("C compiler command", "/usr/local/bin/gcc-4.8"),
   

4. Now you should be good to install Haskell Platform – or just cabal-install if that's what you like.

   brew install haskell-platform
   

This should now work out with reasonably few problems, warnings or errors.

(At least, I'm keeping my fingers crossed!)

— @pyrtsa, 2013-08-12.