Comments on: Cozipping http://comonad.com/reader/2008/cozipping/ types, (co)monads, substructural logic Wed, 28 Jul 2010 13:34:03 -0400 http://wordpress.org/?v=2.8.4 hourly 1 By: Edward Kmett http://comonad.com/reader/2008/cozipping/comment-page-1/#comment-1171 Edward Kmett Tue, 06 May 2008 01:00:58 +0000 http://comonad.com/reader/2008/cozipping/#comment-1171 You're right, I flipped the names when I was slapping together the code over lunch. I had left my notes at home. I'll fix it up. Hrmm, of course now you need an extra 'fair' acounzip as well. You’re right, I flipped the names when I was slapping together the code over lunch. I had left my notes at home. I’ll fix it up.

Hrmm, of course now you need an extra ‘fair’ acounzip as well.

]]> By: Twan van Laarhoven http://comonad.com/reader/2008/cozipping/comment-page-1/#comment-1168 Twan van Laarhoven Tue, 06 May 2008 00:23:36 +0000 http://comonad.com/reader/2008/cozipping/#comment-1168 An interesting post, I was thinking along a slightly different (and wrong) route myself. First of all, shouldn't cozip be called counzip and vice-versa? fzip has type "(f a * f b) -> f (a*b)" doesn't taking the dual give "(f a + f b) Either (f a) (f b)"? I was thinking about the type > almostcozip :: f (Either a b) -> (f a, f b) This function can be implemented unambiguously for [] and Maybe, and is in quite useful in practice. It also has a right inverse, > acounzip :: (f a, f b) -> f (Either a b) If (f x) is a Monoid then > acounzip (fa, fb) = fmap Left fa `mappend` fmap Right fb Thinking about it some more, there is also the dual type > azip :: Either (f a) (f b) -> f (a,b) > aunzip :: f (a,b) -> Either (f a) (f b) But I can't think of any nice function with this type. An interesting post, I was thinking along a slightly different (and wrong) route myself.

First of all, shouldn’t cozip be called counzip and vice-versa? fzip has type “(f a * f b) -> f (a*b)” doesn’t taking the dual give “(f a + f b) Either (f a) (f b)”?

I was thinking about the type

> almostcozip :: f (Either a b) -> (f a, f b)

This function can be implemented unambiguously for [] and Maybe, and is in quite useful in practice. It also has a right inverse,

> acounzip :: (f a, f b) -> f (Either a b)

If (f x) is a Monoid then
> acounzip (fa, fb) = fmap Left fa `mappend` fmap Right fb

Thinking about it some more, there is also the dual type

> azip :: Either (f a) (f b) -> f (a,b)
> aunzip :: f (a,b) -> Either (f a) (f b)

But I can’t think of any nice function with this type.

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