rhai/doc/src/language/iterator.md

Iterators for Custom Types

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If a [custom type] is iterable, the for loop can be used to iterate through its items in sequence.

In order to use a for statement, a type iterator must be registered for the [custom type] in question.

Engine::register_iterator<T> allows registration of a type iterator for any type that implements IntoIterator:

// Custom type
#[derive(Debug, Clone)]
struct TestStruct { ... }

// Implement 'IntoIterator' trait
impl IntoIterator<Item = ...> for TestStruct {
    type Item = ...;
    type IntoIter = SomeIterType<Self::Item>;

    fn into_iter(self) -> Self::IntoIter {
        ...
    }
}

engine
    .register_type_with_name::<TestStruct>("TestStruct")
    .register_fn("new_ts", || TestStruct { ... })
    .register_iterator::<TestStruct>();           // register type iterator

With a type iterator registered, the [custom type] can be iterated through:

let ts = new_ts();

// Use 'for' statement to loop through items in 'ts'
for item in ts {
    ...
}