liftとか

Program Haskell

liftで遊んでみる。

> :t map
map :: (a -> b) -> [a] -> [b]
> :t fmap
fmap :: (Functor f) => (a -> b) -> f a -> f b
> :t liftM
liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
> :t liftM2
liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r
> :t ap
ap :: (Monad m) => m (a -> b) -> m a -> m b

> :t liftM (+)
liftM (+) :: (Num a1, Monad m) => m a1 -> m (a1 -> a1)
> :t liftM2 (+)
liftM2 (+) :: (Num a1, Monad m) => m a1 -> m a1 -> m a1
> :t liftM2 id
liftM2 id :: (Monad m) => m (a2 -> r) -> m a2 -> m r

> liftM2 (+) [1..4] [3,4]
[4,5,5,6,6,7,7,8]
> liftM (+) [1..4] `ap` [3,4]
[4,5,5,6,6,7,7,8]
> ap [succ,(*2)] [1..4]
[2,3,4,5,2,4,6,8]

> :t liftM (+) [1..4]
liftM (+) [1..4] :: (Num a1, Enum a1) => [a1 -> a1]

> map (\f -> f 10) (liftM (+) [1..4])
[11,12,13,14]
> liftM (+) [1..4] >>= return . (\f -> f 10)
[11,12,13,14]