Just got started with Arrows yesterday. As an exercise, I decided to try to build a "Reader Arrow" analogous to Control.Monad.Reader.Reader. I've called it EA ("Environment Arrow"). Question follows after code.
import Prelude hiding ((.), id)
import Control.Category
import Control.Arrow
import qualified Data.Bifunctor as BiF
newtype EA r a b = EA { runEA :: r -> a -> b }
instance Arrow (EA r) where
arr f = EA $ const f
first (EA f) = EA $ BiF.first . f
second (EA f) = EA $ BiF.second . f
instance Category (EA r) where
id = arr id
(EA b) . (EA a) = EA $ \r -> b r . a r
ask :: EA r a r
ask = EA const
test :: EA Int Int Int
test = proc i -> do
f <- ask -< i -- How can I build an "ask" that doesn't need a parameter?
returnA -< f
How can I build ask so that it works with proc notation but does not need a parameter? In other words, I want to say f <- ask and not f <- ask -< i, since the parameter is never used.
It can't be done. f <- ask isn't even valid syntax in arrow notation. Every arrow must be "applied" to some argument in a proc block.