99 questions/Solutions/4

(*) Find the number of elements of a list.

myLength           :: [a] -> Int
myLength []        =  0
myLength (_:xs)    =  1 + myLength xs

myLength' :: [a] -> Int
myLength' list = myLength_acc list 0 -- same, with accumulator
	where
		myLength_acc [] n = n
		myLength_acc (_:xs) n = myLength_acc xs (n + 1)

myLength'      =  foldl (\n _ -> n + 1) 0
myLength''     =  foldr (\_ n -> n + 1) 0
myLength'''    =  foldr (\_ -> (+1)) 0
myLength''''   =  foldr ((+) . (const 1)) 0
myLength'''''  =  foldr (const (+1)) 0
myLength'''''' = foldl (const . (+1)) 0

myLength' xs = snd $ last $ zip xs [1..] -- Just for fun
myLength''   = snd . last . (flip zip [1..]) -- Because point-free is also fun
myLength'''  = fst . last . zip [1..] -- same, but easier

myLength = sum . map (\_->1)

This is length in Prelude.

-- length returns the length of a finite list as an Int. length  :: [a] -> Int length [] = 0 length (_:l) = 1 + length l

The prelude for haskell 2010 can be found here.