Re: [Haskell-cafe] Re: Arrow without `>>>'

Valery V. Vorotyntsev wrote:

On 1/23/08, David Menendez wrote:

...

On Jan 23, 2008 12:20 PM, Valery V. Vorotyntsev wrote:

...

I've built GHC from darcs, and... Could anybody tell me, what's the purpose of Arrow[1] not having `>>>' method? It's derived from the Category superclass.

Yes, it is.

The right question: how to build `arrows' in such circumstances?

Here go 2 changes I made to `CoState.hs' accompanied by the error messages. :) Unfortunately, I'm not arrow-capable enough to make _proper_ changes to the code and satisfy GHC... Any help?

Well, without looking at your code, generally all you have to do is 1) move the definition of (>>>) to Category and rename it to (.) after flipping the arguments. 2) define the id method of Category which is just (arr id) or returnA. So essentially instance Arrow (Foo a) where a >>> b = compose a b pure f = ... first a = ... becomes instance Arrow (Foo a) where pure f = ... first a = ... instance Category (Foo a) where id = arr id a . b = compose b a That's it. It's too bad there's no way to do this automatically in the libraries, but it could be noted in the API docs. Pete

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Change #1:

$ darcs w Control/Arrow/Transformer/CoState.hs What's new in "Control/Arrow/Transformer/CoState.hs":

{ hunk ./Control/Arrow/Transformer/CoState.hs 23 +import Control.Category ((>>>)) }

-------------------------------------------------- Error #1:

Control/Arrow/Transformer/CoState.hs:29:7: `>>>' is not a (visible) method of class `Arrow' Failed, modules loaded: Control.Arrow.Operations.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Change #2:

$ darcs diff -u Control/Arrow/Transformer/CoState.hs --- old-arrows/Control/Arrow/Transformer/CoState.hs 2008-01-24 14:54:29.852296559 +0200 +++ new-arrows/Control/Arrow/Transformer/CoState.hs 2008-01-24 14:54:29.852296559 +0200 @@ -20,12 +20,13 @@

import Control.Arrow import Control.Arrow.Operations +import Control.Category ((>>>))

newtype CoStateArrow s a b c = CST (a (s -> b) (s -> c))

instance Arrow a => Arrow (CoStateArrow s a) where arr f = CST (arr (f .)) - CST f >>> CST g = CST (f >>> g) +-- CST f >>> CST g = CST (f >>> g) first (CST f) = CST (arr unzipMap >>> first f >>> arr zipMap)

zipMap :: (s -> a, s -> b) -> (s -> (a,b))

-------------------------------------------------- Error#2:

Control/Arrow/Transformer/CoState.hs:27:0: Could not deduce (Control.Category.Category (CoStateArrow s a)) from the context (Arrow a) arising from the superclasses of an instance declaration at Control/Arrow/Transformer/CoState.hs:27:0 Possible fix: add (Control.Category.Category (CoStateArrow s a)) to the context of the instance declaration or add an instance declaration for (Control.Category.Category (CoStateArrow s a)) In the instance declaration for `Arrow (CoStateArrow s a)' Failed, modules loaded: Control.Arrow.Operations.

Thank you.