Looping

Looping allows an expression to run repeatedly until an exit condition is met.

Basic use

A loop is formed of a few parts:

• The loop keyword.
• A capture to use on every iteration.
• The assignment operator =.
• The initial value to be captured.
• The block to be run repeatedly.

-- Start at 1, and add 1 each time.
let count_up_forever = loop x = 1 (x + 1)

Throwing

By default, a loop will never terminate. This means you will need to set an exit condition in order for the loop to be useful.

So, loops can be thrown values, just like catch blocks can. When this happens, the loop terminates, and evaluates to the thrown value.

-- Find the largest multiple of two that's less than `limit`.
let largest_mult = fn [.limit ~ num] loop n = 2 (
	if 2 * n > limit then throw n
	else n * 2
)

let sixteen = largest_mult [.limit 25]

Standard throwing rules apply; throws must be local (non-viral) and type-safe.

Multiple data

Using tuple captures, a loop can keep track of multiple named data with every iteration.

let sum_of_consecutive = loop [index, total] = [10, 0] (
	if index == 0 then throw total
	else [index - 1, total + index]
)