Collect

collect is a control-flow operator unique to mimas. It turns any loop into an array builder: each value you collect is appended to a running list, and when the loop finishes – whether it runs out of iterations or hits a break – the whole loop evaluates to that array.

Think of it as continue that carries a value: like continue, it moves on to the next iteration.

let numbers = [1, 5, 9, 2];

let doubled = for x in numbers collect x * 2;
// -> [2, 10, 18, 4]

Loop bodies don’t have to be blocks, so that last example fits on one line – close to a Python list comprehension:

// python:  doubled = [x * 2 for x in numbers]
let doubled = for x in numbers collect x * 2;

Add an if to filter which values get collected:

let evens = for x in numbers {
    if x % 2 == 0 {
        collect x;
    }
};
// -> [2]

It composes with every loop, including while let, which is handy for gathering results from a source until it’s exhausted:

let ages: [int] = while let person = next_person() {
    collect person.age;
};