Use foldr to define filter and map

January 12, 2020•122 words

Filter

Without foldr

_filter :: (a -> Bool) -> [a] -> [a]
_filter p (x:xs) | p x == True = x : _filter xs
                 | otherwise   = _filter xs

With folder

_filter :: (a -> Bool) -> [a] -> [a]
_filter p = foldr (\x xs -> if p x then x : xs else xs) []

Map

Without foldr

_map :: (a -> b) -> [a] -> [a]
_map f [] = []
_map f (x:xs) = f x : _map f xs

With foldr

_map :: (a -> b) -> [a] -> [a]
_map f = foldr (\x xs -> f x : xs) []

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Use foldr to define filter and map

Filter

Without foldr

With folder

Map

Without foldr

With foldr

More from TAll posts

Else statements and early returns

Convert csv to json

More from T
All posts