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A functional iterator is a function that traverses a data structure and calls in turn a given function over the elements of that structure to compute some value. Another approach is sometimes possible: loops.

There are three kinds of functional iterations over maps: the fold, the map (not to be confused with the map data structure) and the iteration.

Let us consider first here the fold, which is the most general form of functional iteration. The folded function takes two arguments: an accumulator and the structure element at hand, with which it then produces a new accumulator. This enables having a partial result that becomes complete when the traversal of the data structure is over.

The function Map.fold performs a fold over the binding of a map, in increasing order of its keys.

type player = string
type abscissa = int
type ordinate = int
type move = [abscissa, ordinate]
type game = map<player, move>
const horizontal_offset = (g: game): int => {
let folded = ([acc, j]: [int, [player, move]]) => acc + j[1][0];
return Map.fold(folded, g, 0);

Note: See the predefined namespace Map