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|
{-# OPTIONS_GHC -fglasgow-exts #-}
-----------------------------------------------------------------------------
-- |
-- Module : Control.Arrow.Transformer.CoState
-- Copyright : (c) Ross Paterson 2003
-- License : BSD-style (see the LICENSE file in the distribution)
--
-- Maintainer : ross@soi.city.ac.uk
-- Stability : experimental
-- Portability : non-portable (multi-parameter type classes)
--
-- Transformation of state readers.
--
-- /TODO:/ define operations for this arrow.
module Control.Arrow.Transformer.CoState(
CoStateArrow,
) where
import Control.Arrow
import Control.Arrow.Operations
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)
first (CST f) = CST (arr unzipMap >>> first f >>> arr zipMap)
zipMap :: (s -> a, s -> b) -> (s -> (a,b))
zipMap h s = (fst h s, snd h s)
unzipMap :: (s -> (a,b)) -> (s -> a, s -> b)
unzipMap h = (fst . h, snd . h)
-- there is no transformer
-- promotions of standard classes
instance ArrowLoop a => ArrowLoop (CoStateArrow s a) where
loop (CST f) = CST (loop (arr zipMap >>> f >>> arr unzipMap))
instance ArrowZero a => ArrowZero (CoStateArrow s a) where
zeroArrow = CST zeroArrow
instance ArrowPlus a => ArrowPlus (CoStateArrow s a) where
CST f <+> CST g = CST (f <+> g)
|
