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Template std.algorithm.iteration.fold

Implements the homonym function (also known as accumulate, compress, inject, or foldl) present in various programming languages of functional flavor. The call fold!(fun)(range, seed) first assigns seed to an internal variable result, also called the accumulator. Then, for each element x in range, result = fun(result, x) gets evaluated. Finally, result is returned. The one-argument version fold!(fun)(range) works similarly, but it uses the first element of the range as the seed (the range must be non-empty).

template fold(fun...) ;

Contained Functions



fun the predicate function(s) to apply to the elements

See Also

Fold (higher-order function)

sum is similar to fold!((a, b) => a + b) that offers precise summing of floating point numbers.

This is functionally equivalent to reduce with the argument order reversed, and without the need to use tuple for multiple seeds.


immutable arr = [1, 2, 3, 4, 5];

// Sum all elements
writeln(arr.fold!((a, b) => a + b)); // 15

// Sum all elements with explicit seed
writeln(arr.fold!((a, b) => a + b)(6)); // 21

import std.algorithm.comparison : min, max;
import std.typecons : tuple;

// Compute minimum and maximum at the same time
writeln(arr.fold!(min, max)); // tuple(1, 5)

// Compute minimum and maximum at the same time with seeds
writeln(arr.fold!(min, max)(0, 7)); // tuple(0, 7)

// Can be used in a UFCS chain
writeln(!(a => a + 1).fold!((a, b) => a + b)); // 20

// Return the last element of any range
writeln(arr.fold!((a, b) => b)); // 5


Andrei Alexandrescu


Boost License 1.0.