[Haskell-cafe] Trouble defining `instance Arrow SF'

[Silvio Frischknecht]

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> SF.hs:11:10: `>>>' is not a (visible) method of class `Arrow' 
> Failed, modules loaded: none.

In the base package Arrows are defined a bit different from what you
some times see in the literature. I also stumbled over this once. The
(>>>) operator is just a synonym for (.) from Control.Category. Note
that (.) is not the same (.) as in Prelude.

class Category cat where
  Control.Category.id :: cat a a
  (Control.Category..) :: cat b c -> cat a b -> cat a c

class Category a => Arrow a where
  arr :: (b -> c) -> a b c
  first :: a b c -> a (b, d) (c, d)

f >>> g = g . f

your implementation would probably look something like this.

module Main where

import Control.Arrow
import Control.Category as Cat

newtype SF a b = SF {runSF :: [a] -> [b]}

instance Category SF where
    id = SF Cat.id
    SF f . SF g = SF (f Cat.. g)

instance Arrow SF where
    arr f = SF (map f)
    first (SF f) = SF (unzip >>> first f >>> uncurry zip)

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