[Haskell-cafe] Cabal HackORama -- how to handle par and pseq on stable platforms

John D. Ramsdell ramsdell0 at gmail.com
Tue Nov 10 18:06:31 EST 2009

I am writing a Cabal file for an application that uses par and pseq.
I want to support both modern distributions of GHC and the version
that comes with Ubuntu Hardy Heron, a Long Term Support version of
Ubuntu that will be retired in April 2011.  It provides GHC 6.8.2
with Cabal and no parallel package.  You might be interested
in my solution to supporting both releases.

I did the obvious in my Cabal file.  A defined a flag as follows:

-- Disable with -f-par option during configuration.

Flag Par
  Description:		Enable use of the parallel construct par
  Default:		True

The description of the executable includes:

  if flag(par)
    Cpp-Options: -DHAVE_PAR
    GHC-Options: -threaded
    Build-Depends: parallel

The C preprocessor is used to toggle between a parallel version of map
and the usual one.

#if defined HAVE_PAR

parMap :: (a -> b) -> [a] -> [b]
parMap _ [] = []
parMap f (x:xs) =
    par y (pseq ys (y:ys))
      y = f x
      ys = parMap f xs


parMap :: (a -> b) -> [a] -> [b]
parMap = map


Here is what makes this cabal file sweet.  Cabal 1,2,3.0 supports
flags, but ignores the Default specification.  It assumes the default
value of every flag is False.  Thus, for both modern versions of GHC
and the one on Hardy Heron, no user supplied flags are required during


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