Modifying Emacs Inferior Haskell processes to enable CPP processing - haskell

Question

If we look at the source of The random package we have a file Random.hs. Because of CPP extensions one has to invoke ghci via the following command :
ghci -cpp Random.hs
Alternatively one can do :
ghci -cpp
and then from within ghci :
Prelude GOA> :load Random
[1 of 1] Compiling System.Random ( Random.hs, interpreted )
Ok, modules loaded: System.Random.
If I use Emacs Inferior Haskell mode (Emacs/Inferior Haskell processes) and I have
the source :
module Main where
import System.Random
gen = (random (mkStdGen 0)) :: (Bool,StdGen)
mymult :: Int -> Int
mymult x = 2 * x
main = do
print $ mymult 5
then upon typing the emacs command :
C-c C-l
which is inferior-haskell-load-file, ghci is opened in a subwindow in emacs. However if from within this window I type load Random.hs then I get the error message :
*Main GOA> :load Random.hs
Random.hs:1:2: lexical error at character 'i'
Failed, modules loaded: none.
How can I load Random.hs taking into account cpp extensions? Or alternatively how do I modify haskell-mode/inf-haskell.el such that ghci is invoked with the -cpp option upon typing C-c C-l, so that the command :load Random.hs can be executed without error?

Answer 1

The most reliable way is certainly to request CPP, along with other extensions, in the file header:
{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 701
{-# LANGUAGE Trustworthy #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module : System.Random
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file LICENSE in the 'random' repository)
The random package only does this in the .cabal file.
The easiest way might be to simply turn CCP on permanently in all ghci sessions, by adding
:set -XCPP
to your ~/.ghci file.