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Conal Elliott | 14 Jul 13:56

Advice sought for 6.9 and Arrow/Category

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 ghc-6.9?  Does anyone have coping strategies for keeping libraries working in 6.8 *and* 6.9, particularly ones that define instances of Arrow?

Thanks,  - Conal 

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Don Stewart | 14 Jul 20:38

Re: Advice sought for 6.9 and Arrow/Category

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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Conal Elliott | 15 Jul 16:43

Re: Advice sought for 6.9 and Arrow/Category

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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Conal Elliott | 15 Jul 17:45

Re: Advice sought for 6.9 and Arrow/Category

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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Spencer Janssen | 22 Jul 11:57

Re: Advice sought for 6.9 and Arrow/Category

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
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Duncan Coutts | 22 Jul 14:05

Re: Advice sought for 6.9 and Arrow/Category


On Tue, 2008-07-22 at 04:57 -0500, Spencer Janssen wrote:

> > 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

This is the sort of example where our Cabal syntactic sugar proposal
helps a bit:

library
 if package(base >= 2.0)
   cpp-options: -DAPPLICATIVE_IN_BASE

So two lines rather than six and you don't need to understand the
relational semantics so clearly to make sense of it. Now if only we'd
get round to implementing it.

Duncan
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Ian Lynagh | 15 Jul 14:38

Re: Advice sought for 6.9 and Arrow/Category

On Mon, Jul 14, 2008 at 01:58:00PM +0200, Conal Elliott wrote:
> 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 ghc-6.9?

Yes. It was changed with a library proposal:
    http://hackage.haskell.org/trac/ghc/ticket/1773
    http://www.haskell.org/pipermail/libraries/2007-October/008291.html

Thanks
Ian
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