Applying a list of functions in Haskell

Question

I wrote a function that applies a list of functions to an item.

applyAll :: [a -> b] -> a -> [b]
applyAll [] _ = []
applyAll (f:fs) x = (f x) : (a         


        

Answer 1

Lee's solution is what I'd recommend, but this reads perhaps even nicer:

import Control.Applicative

applyAll' fs v = fs <*> pure v

or

applyAll'' fs v = fs <*> [v]

This sort of makes stuff more complicated than necessary, though: we really only need the Functor instance of lists, whereas applyAll' injects and immediately extracts from the Applicative instance.

Answer 2

This function actually already exists as a special case of a monadic function:

applyAll :: [a -> b] -> a -> [b]
applyAll = sequence

Answer 3

You could use:

 applyAll l v = fmap ($ v) l

fmap lifts a function over the input list (actually any Functor). $ has type (a -> b) -> a -> b so it applies a function to a given value. ($ v) is a section which applies the given function to v.