Re: Advice sought for 6.9 and Arrow/Category

15 Jul 2008

      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

On Tue, Jul 15, 2008 at 4:43 PM, Conal Elliott  wrote:
...
All code that defines Arrow instance breaks, because (>>>) is not a method
of Arrow.  I have a lot of such instances, so I'm now adding #if directives
to test for ghc-6.9 or later.  - Conal
On Mon, Jul 14, 2008 at 8:38 PM, Don Stewart  wrote:
...
conal:
...
I want to use ghc-6.9 for improved support of type families, but I
see
   that the change to the Arrow interface breaks some of my libraries
(since
   (>>>) is no longer a method of Arrow).  Will this change really be in
(>>>) has been reexported from Control.Category through Arrow for a few
months.  Does that fix things, or is your code still broken?
-- Don