Example
A fold (from the left) is a higher-order relation between:
- a predicate with 3 arguments
- a list of elements
- an initial state
- a final state, which is the result of applying the predicate to successive elements while carrying through intermediate states.
For example: Use foldl/4 to express the sum of all elements in a list, using a predicate as a building block to define the sum of two elements:
?- foldl(plus, [2,3,4], 0, S).
S = 9.
