Safe Haskell | None |
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We try to mix effects of two completely unrelated arrows `a`

and `b`

, where `b`

is considered pure, and `a`

— impure. Probably the most common use case would be `a = Kleisli IO`

. We perform all the pure calculations first, and do the impure ones later.

Usage example:

newtype Test input output = Test {runTest :: (Kleisli IO :+ Kleisli (State String)) input output} deriving (Category, Arrow, ArrowChoice, ArrowLoop) runStateMorphism :: s -> Kleisli (State s) :~> (->) runStateMorphism s al input = evalState (runKleisli al input) s execTest :: Test input output -> input -> IO output execTest t = runKleisli $ unPlus $ alMap (runStateMorphism "") $ runTest t rd = Test {runTest = liftImpure $ Kleisli $ const getLine} wr = Test {runTest = liftImpure $ Kleisli putStrLn} gt = Test {runTest = liftPure $ Kleisli $ const get} pt = Test {runTest = liftPure $ Kleisli put} test = proc () -> do line <- rd -< () -- effect from IO pt -< line -- effect from State line' <- gt -< () -- effect from State wr -< line' -- effect from IO

- module Control.Arrow.Mix.Category
- data (a :+ b) input output
- liftImpure :: (ArrowChoice a, ArrowLoop a, Arrow b) => a :~> (a :+ b)
- liftPure :: (Arrow a, Arrow b) => b :~> (a :+ b)
- unPlus :: Arrow a => (a :+ (->)) :~> a
- unPlus' :: Arrow a => (a :+ a) :~> a
- plusTwist :: (Arrow a, AlFunctor f, Arrow c) => (a :+ f c) :~> f (a :+ c)
- plusAssoc :: (Arrow a, Arrow b, Arrow c) => (a :+ (b :+ c)) :~> ((a :+ b) :+ c)
- plusCommute :: (Arrow a, ArrowChoice b, ArrowLoop b) => (a :+ b) :~> (b :+ a)

# Documentation

module Control.Arrow.Mix.Category

data (a :+ b) input output Source

`a :+ b`

is an arrow incapsulating both `a`

and `b`

effects. It's functorial (and even monadic) in `b`

.

liftImpure :: (ArrowChoice a, ArrowLoop a, Arrow b) => a :~> (a :+ b)Source

We can lift impure arrows

unPlus :: Arrow a => (a :+ (->)) :~> aSource

We need some way to extract the real computation from this `:+`

; fortunately, if we manage to reduce the pure arrow to a function (using `alMap`

), we can reduce the type `a :+ (->)`

to `a`

.

unPlus' :: Arrow a => (a :+ a) :~> aSource

If, for some reason, the "pure" arrow is, in fact, as impure as the "impure" one, we still can extract the real computation.

plusTwist :: (Arrow a, AlFunctor f, Arrow c) => (a :+ f c) :~> f (a :+ c)Source

(:+) is right commutative-associative: a :+ (b :+ c) and b :+ (a :+ c) can be reduced to one another with `plusTwist`

function

plusAssoc :: (Arrow a, Arrow b, Arrow c) => (a :+ (b :+ c)) :~> ((a :+ b) :+ c)Source

(:+) is also associative

plusCommute :: (Arrow a, ArrowChoice b, ArrowLoop b) => (a :+ b) :~> (b :+ a)Source

this function is experimental - no idea about how this would interact with other arrow combinators and functions defined in this module