Flix supports a special foreach-yield construct which is used to build collections from other collections. The foreach-yield construct is related to the foreach construct, but should not be confused with it. The foreach construct is for effectful iteration through collections whereas the foreach-yield construct is for building new collections.

We can use the foreach-yield construct to build a new collection from existing collections. For example:

def main(): Unit \ IO = 
    let fruits = List#{"Apple", "Pear", "Mango"};
    let creams = List#{"Vanilla", "Stracciatella"};
    let result: Set[(String, String)] =
        foreach(fruit <- fruits; cream <- creams)
            yield (fruit, cream);

Here we construct a Set[(String, String)] from two List[String]. The type of the resulting collection is typically inferred, but in this case we have placed an explicit type annotation on the result local variable to specify it.

We can use the foreach-yield construct with several different types of collections. For example:

def main(): Unit \ IO = 
    let c1 = Some(1);
    let c2 = Set#{1, 2, 3};
    let c3 = List#{1, 2, 3};
    let result: Set[Int32] =
        foreach(x <- c1; y <- c2; z <- c3)
            yield x + y + z;

Here we iterate through three collections c1, c2, and c3 and return a set of the sums of their pairwise combinations.

The Collectable Trait

The workhorse behind the foreach-yield construct is the Iterable trait (discussed in the previous section) and the Collectable trait.

pub trait Collectable[t: Type] {
    /// The element type of the Collectable.
    type Elm[t]: Type

    /// Run an Iterator collecting the results.
    pub def collect(iter: Iterator[Collectable.Elm[t], ef, r]): t \ (ef + Collectable.Aef[t] +  r)

Without going into details, the Collectable trait is implemented by all kinds of collections that can be constructed from an iterator. Notably, this includes the List, Chain, and Set data types.

Note: We cannot collect into a non-empty chain (Nec) or non-empty list (Nel) since we cannot guarantee that an Iterator is non-empty.

Note: The foreach-yield construct can only collect into immutable collections. If we want to build a mutable collection, we should explicitly introduce its region, allocate the mutable collection, and add each element using a regular foreach-loop.