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:

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conal:

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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

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

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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