Re: Advice sought for 6.9 and Arrow/Category

22 Jul 2008


      On Tue, Jul 15, 2008 at 05:45:55PM +0200, Conal Elliott wrote:
...
By the way, here's how I'm changing my code to work with the new and old
arrow interface.  I'd appreciate any suggested improvements.
The old code (example):
import Control.Arrow
[...]
instance Arrow (~>) => Arrow (Bijection (~>)) where
      Bi ab ba >>> Bi bc cb = Bi (ab >>> bc) (cb >>> ba)
      [...]
The new code:
#if __GLASGOW_HASKELL__ >= 609
    import Control.Category
    import Prelude hiding ((.), id)
    #endif
    import Control.Arrow
[...]
#if __GLASGOW_HASKELL__ >= 609
    instance Category (~>) => Category (Bijection (~>)) where
      id = Bi id id
      Bi bc cb . Bi ab ba = Bi (bc . ab) (ba . cb)
    #endif
instance Arrow (~>) => Arrow (Bijection (~>)) where
    #if __GLASGOW_HASKELL__ < 609
      Bi ab ba >>> Bi bc cb = Bi (ab >>> bc) (cb >>> ba)
    #endif
    [...]
I'm testing for ghc version.  Could I somehow test for the base-library
version instead?   - Conal
Yes.  Here is a snippet from binary.cabal:

    flag applicative-in-base

    library
      if flag(applicative-in-base)
        build-depends: base >= 2.0
        cpp-options: -DAPPLICATIVE_IN_BASE
      else
        build-depends: base < 2.0


Cheers,
Spencer Janssen