GHC's CPP and Cabal's unlit

Duncan Coutts duncan.coutts at
Thu Nov 29 05:33:10 EST 2007

On Wed, 2007-11-28 at 22:37 +0000, Alistair Bayley wrote:
> > More puzzling is that the files that Cabal runs through ghc's CPP
> > don't get the # n lines, so we end up with something like this:
> (Answering my own message)
> Having done some more testing with ghc-6.8.1 and ghc-6.6.1 and cabal's
> and 1.3, I've realised that the cpp optP-P option in Cabal-1.3
> is suppressing the # n lines, so that means the {-# LINE 1 "Test.lhs"
> #-} comment does indeed end up immediately preceding the first real
> line of the program (thus causing unlit to spit the dummy).
> I've also noticed that the options passed from cabal- to the
> ghc cpp phase do NOT include -x hs, so ghc unlits the file before
> cabal then tries to unlit it. Surely this cannot work, and indeed it
> does not, because the resulting .hs file contains no code.
> I'm of a mind to fix two things in cabal:
>  - the haddock command runs unlit first, THEN cpp
>  - the unlit module preserves comments, for the benefit of haddock
> I already have these done in my local Cabal-1.3, so creating patches
> ought to be straightforward. I've only tested with ghc on Windows
> though.
> Comments?

Sounds great. Send your patches to cabal-devel at You may also
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