loop

Summary

Generates a stack-safe recursive function.

Description

Colloquially known as "trampoline" this function allows you to implement stack-safe recursive functions. You must use recur to bootstrap the next recursive call and cease to signal the exit condition and the return value.

Examples

Without tail call optimization recursive functions can exhaust the stack:

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const nums =
 (x, y, ret = []) =>
 (x === y
 ? ret
 : nums(x+1, y, (ret.push(x), ret)));
nums(1, 1000);
//=> [
//=> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
//=> 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
//=> 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
//=> 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
//=> 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
//=> 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72,
//=> 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84,
//=> 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
//=> 97, 98, 99, 100,
//=> ... 899 more items
//=> ]
nums(1, 10000);
//=> Uncaught RangeError: Maximum call stack size exceeded

With loop/recur you can build the same recursive function without fear:

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const nums =
 loop((x, y, ret = []) =>
 (x === y
 ? cease(ret)
 : recur(x + 1, y, (ret.push(x), ret))));
nums(1, 100000); // ×ばつ 10 more!
//=> [
//=> 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
//=> 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
//=> 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,
//=> 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48,
//=> 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
//=> 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72,
//=> 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84,
//=> 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
//=> 97, 98, 99, 100,
//=> ... 99899 more items
//=> ]

Parameters

Name	Type	Description
fn	function

Return

function