Array Predicates

The array operations any, all and find-index test the elements of an array against a predicate expression.

Note: Array predicates are experimental and subject to change. Tracking issue: slint-ui/slint#12777 ↗.

Syntax

A predicate expression has the form (name) => condition. It binds name to a value and evaluates condition, which must be a bool. The name is in scope only inside condition.

Predicates are passed to the array operations any, all and find-index, which supply each element in turn as name:

• array.any((name) => condition) reads true if condition holds for at least one element.
• array.all((name) => condition) reads true if condition holds for every element.
• array.find-index((name) => condition) reads the index of the first element for which condition holds, or -1 if no element matches.

The argument name is available only inside the predicate expression, and its type is inferred from the array element type.

any returns false for an empty array, all returns true for an empty array, and find-index returns -1 for an empty array.

For a plain value-equality lookup, prefer the stable array.index-of(value) over find-index((name) => name == value): it needs no predicate and isn’t experimental. Reach for find-index when the match condition is more than equality against a single value.

Example

export component Example {
    in-out property<[int]> list-of-int: [1, 2, 3];

    out property <bool> contains-two: list-of-int.any((value) => value == 2);   // true
    out property <bool> all-positive: list-of-int.all((value) => value > 0);    // true
    out property <int> index-of-first-even: list-of-int.find-index((value) => mod(value, 2) == 0); // 1
}

slint

Current Limitations

Predicates are the only form of closure in the Slint language, and they are only accepted as the argument of any, all and find-index. A predicate cannot be stored in a property, passed to a function or callback, or called directly. The predicate must be written inline in the call to any, all or find-index; passing a closure stored in a local variable is rejected with an error.