Haskell Language Foldable Counting the elements of a Foldable structure
Example
length counts the occurences of elements a in a foldable structure t a.
ghci> length [7, 2, 9] -- t ~ []
3
ghci> length (Right 'a') -- t ~ Either e
1 -- 'Either e a' may contain zero or one 'a'
ghci> length (Left "foo") -- t ~ Either String
0
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.
