:: (c -> d) -> (a -> b -> c) -> a -> b -> d -package:pointless-fun package:composition is:exact

Compose two functions. f .: g is similar to f . g except that g will be fed two arguments instead of one before handing its result to f. This function is defined as
(f .: g) x y = f (g x y)
Example usage:
concatMap :: (a -> [b]) -> [a] -> [b]
concatMap = concat .: map
Notice how two arguments (the function and the list) will be given to map before the result is passed to concat. This is equivalent to:
concatMap f xs = concat (map f xs)
Equivalent to .: The pattern of appending asterisks is straightforward to extend to similar functions: (compose2 = .*, compose3 = .**, etc). However, .: has been commonly adopted amongst Haskellers, and the need for compose3 and beyond is rare in practice.