Haskell Language Foldable Counting the elements of a Foldable structure


length counts the occurences of elements a in a foldable structure t a.

ghci> length [7, 2, 9]  -- t ~ []
ghci> length (Right 'a')  -- t ~ Either e
1  -- 'Either e a' may contain zero or one 'a'
ghci> length (Left "foo")  -- t ~ Either String
ghci> length (3, True)  -- t ~ (,) Int
1  -- '(c, a)' always contains exactly one 'a'

length is defined as being equivalent to:

class Foldable t where
    -- ...
    length :: t a -> Int
    length = foldl' (\c _ -> c+1) 0

Note that this return type Int restricts the operations that can be performed on values obtained by calls to the length function. fromIntegralis a useful function that allows us to deal with this problem.