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|
{-# LANGUAGE NoImplicitPrelude #-}
module Control.Applicative where
import Data.Functor
import Data.Function
import Data.Maybe
infixl 4 <*>, <*, *>
class Functor f => Applicative f where
pure :: a -> f a
(<*>) :: f (a -> b) -> f a -> f b
liftA2 :: (a -> b -> c) -> f a -> f b -> f c
(*>) :: f a -> f b -> f b
(<*) :: f a -> f b -> f a
(<*>) = liftA2 id
liftA2 f x y = f <$> x <*> y
as *> bs = (\x y -> y) <$> as <*> bs
as <* bs = (\x y -> x) <$> as <*> bs
infixl 3 <|>
class Applicative f => Alternative f where
empty :: f a
(<|>) :: f a -> f a -> f a
some :: f a -> f [a]
many :: f a -> f [a]
many v = some v <|> pure []
-- Not working! some v = (:) <$> v <*> many v
-- These are defined by Parse.hs
-- <|>
-- some
-- many
instance Applicative [] where
pure x = [x]
fs <*> xs = [f x | f <- fs, x <- xs]
instance Applicative Maybe where
pure x = Just x
(Just f) <*> (Just x) = Just (f x)
_ <*> _ = Nothing
|
