Docs / Language Manual / ImperativeLoops

Imperative Loops

For Loops

For loops iterate from a starting value up to (and including) the ending value.

for (myBinding in startValue to endValue) { /* use myBinding here */ };

The parenthesis around startValue and endValue may be omitted if they are unnecessary.

let xStart = 1; let xEnd = 3; /* prints: 1 2 3 */ for (x in xStart to xEnd) { print_int(x); print_string(" ") };

You can make the for loop count in the opposite direction by using downto.

for (myBinding in startValue downto endValue) { statements };
let xStart = 3; let xEnd = 1; /* prints: 3 2 1 */ for (x in xStart downto xEnd) { print_int(x); print_string(" ") };

While Loops

While loops execute a code block while some condition is true. The form of a while loop includes a single expression, the condition to test.

while (testCondition) { statements };

Tips & Tricks

There's no loop-breaking break keyword (nor early return from functions, for that matter) in Reason. However, we can break out of a while loop easily through using a mutable binding.

Random.self_init(); let break = ref(false); while (! break^) { if ( === 3) { break := true } else { print_endline("hello") } };

Recursive functions

Another technique to accomplish an early return is by using recursive functions. If we look at our example from the Recursive functions section:

let rec listHas = (list, item) => switch (list) { | [] => false | [a,] => a == item || listHas(rest, item) };

The listHas function will return true as soon as a == item is true. Otherwise, it calls itself again and continues the search.

We don't have a preference on whether you should use a loop over map/filter/reduce. Some algorithms are better expressed with the former, and some with the latter.