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|
{-# OPTIONS_GHC -fglasgow-exts #-}
-----------------------------------------------------------------------------
-- |
-- Module : Control.Arrow.Transformer.Writer
-- 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)
--
-- Arrow transformer that adds accumulation of output.
module Control.Arrow.Transformer.Writer(
WriterArrow,
runWriter,
ArrowAddWriter(..),
) where
import Control.Arrow
import Control.Arrow.Internals
import Control.Arrow.Operations
import Control.Arrow.Transformer
import Data.Monoid
-- | An arrow type that augments an existing arrow with accumulating
-- output. The 'ArrowWriter' class contains the relevant operations.
newtype WriterArrow w a b c = WriterArrow (a b (c, w))
-- | Encapsulation of a writer computation, providing the accumulated output.
--
-- Typical usage in arrow notation:
--
-- > proc p -> do
-- > ...
-- > (result, output) <- (|runWriter cmd|)
runWriter :: (Arrow a, Monoid w) => WriterArrow w a e b -> a e (b,w)
runWriter (WriterArrow f) = f
rstrength :: ((a, w), b) -> ((a, b), w)
rstrength ((a, w), b) = ((a, b), w)
unit :: Monoid w => a -> (a, w)
unit a = (a, mempty)
join :: Monoid w => ((a, w), w) -> (a, w)
join ((a, w2), w1) = (a, w1 `mappend` w2)
-- arrow transformer
instance (Arrow a, Monoid w) => ArrowTransformer (WriterArrow w) a where
lift f = WriterArrow (f >>> arr unit)
-- liftings of standard classes
instance (Arrow a, Monoid w) => Arrow (WriterArrow w a) where
arr f = WriterArrow (arr (unit . f))
WriterArrow f >>> WriterArrow g =
WriterArrow (f >>> first g >>> arr join)
first (WriterArrow f) = WriterArrow (first f >>> arr rstrength)
instance (ArrowChoice a, Monoid w) => ArrowChoice (WriterArrow w a) where
left (WriterArrow f) = WriterArrow (left f >>> arr lift_monoid)
where lift_monoid (Left (x, w)) = (Left x, w)
lift_monoid (Right y) = unit (Right y)
instance (ArrowApply a, Monoid w) => ArrowApply (WriterArrow w a) where
app = WriterArrow (arr (\(WriterArrow f, x) -> (f, x)) >>> app)
instance (ArrowZero a, Monoid w) => ArrowZero (WriterArrow w a) where
zeroArrow = WriterArrow zeroArrow
instance (ArrowPlus a, Monoid w) => ArrowPlus (WriterArrow w a) where
WriterArrow f <+> WriterArrow g = WriterArrow (f <+> g)
instance (ArrowLoop a, Monoid w) => ArrowLoop (WriterArrow w a) where
loop (WriterArrow f) = WriterArrow (loop (f >>> arr swapenv))
where swapenv ~(~(x, y), w) = ((x, w), y)
-- new instances
instance (Arrow a, Monoid w) => ArrowWriter w (WriterArrow w a) where
write = WriterArrow (arr (\x -> ((), x)))
newWriter (WriterArrow f) =
WriterArrow (f >>> arr (\(x, w) -> ((x, w), w)))
instance (Arrow a, Monoid w) => ArrowAddWriter w (WriterArrow w a) a where
liftWriter = lift
elimWriter = runWriter
-- liftings of other classes
instance (ArrowCircuit a, Monoid w) => ArrowCircuit (WriterArrow w a) where
delay x = lift (delay x)
instance (ArrowError ex a, Monoid w) => ArrowError ex (WriterArrow w a) where
raise = lift raise
handle (WriterArrow f) (WriterArrow h) = WriterArrow (handle f h)
tryInUnless (WriterArrow f) (WriterArrow s) (WriterArrow h) =
WriterArrow (tryInUnless f s' h)
where s' = arr lstrength >>> first s >>> arr join
lstrength (x, (y, w)) = ((x, y), w)
newError (WriterArrow f) = WriterArrow (newError f >>> arr h)
where h (Left ex) = unit (Left ex)
h (Right (c, w)) = (Right c, w)
instance (ArrowReader r a, Monoid w) => ArrowReader r (WriterArrow w a) where
readState = lift readState
newReader (WriterArrow f) = WriterArrow (newReader f)
instance (ArrowState s a, Monoid w) => ArrowState s (WriterArrow w a) where
fetch = lift fetch
store = lift store
-- promotions of encapsulation operators
instance (ArrowAddError ex a a', Monoid w) =>
ArrowAddError ex (WriterArrow w a) (WriterArrow w a') where
liftError (WriterArrow f) = WriterArrow (liftError f)
elimError (WriterArrow f) (WriterArrow h) = WriterArrow (elimError f h)
instance (ArrowAddReader r a a', Monoid w) =>
ArrowAddReader r (WriterArrow w a) (WriterArrow w a') where
liftReader (WriterArrow f) = WriterArrow (liftReader f)
elimReader (WriterArrow f) = WriterArrow (elimReader f)
instance (ArrowAddState s a a', Monoid w) =>
ArrowAddState s (WriterArrow w a) (WriterArrow w a') where
liftState (WriterArrow f) = WriterArrow (liftState f)
elimState (WriterArrow f) = WriterArrow (elimState f >>> arr rstrength)
|
