[Haskell-cafe] Conditional code for Haddock

I want to generate documentation for the following code snippet:

import qualified Data.ByteString as BS ( ByteString, length )

-- | Foo 'BS.empty' equals 0 foo ∷ BS.ByteString → Int foo = BS.length

Because I do not explicitly import BS.empty haddock will not generate a nice link. I can fix it by adding:

import qualified Data.ByteString as BS ( empty )

But now the compiler rightly warns me:

Warning: Imported from `Data.ByteString' but not used: `BS.empty'

There is a ticket related to this problem [1], but I do not think making haddock 'magically' find BS.empty is the right answer. What I would like to do is:

#ifdef __DOC__ import qualified Data.ByteString as BS ( empty ) #endif

Because I use additional symbols in my comments it feels natural to also have special imports for them. I know that cabal used to define the __HADDOCK__ macro. Is it possible to manually define a preprocessor macro when haddock is run? Perhaps an equivalent to ghc-options: haddock-options? Ideally I want only have to add the following to my .cabal file:

haddock-options: -D__DOC__

Regards, Roel van Dijk 1 - http://trac.haskell.org/haddock/ticket/78