Advice sought for 6.9 and Arrow/Category

[Conal Elliott]

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 <conal at conal.net> 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 <dons at galois.com> 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
>>
>
>
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