Given a structure with elements whose type is a
Monoid, combine
them via the monoid's
(<>) operator. This fold
is right-associative and lazy in the accumulator. When you need a
strict left-associative fold, use
foldMap' instead, with
id as the map.
Examples
Basic usage:
>>> fold [[1, 2, 3], [4, 5], [6], []]
[1,2,3,4,5,6]
>>> fold $ Node (Leaf (Sum 1)) (Sum 3) (Leaf (Sum 5))
Sum {getSum = 9}
Folds of unbounded structures do not terminate when the monoid's
(<>) operator is strict:
>>> fold (repeat Nothing)
* Hangs forever *
Lazy corecursive folds of unbounded structures are fine:
>>> take 12 $ fold $ map (\i -> [i..i+2]) [0..]
[0,1,2,1,2,3,2,3,4,3,4,5]
>>> sum $ take 4000000 $ fold $ map (\i -> [i..i+2]) [0..]
2666668666666