Effect Casts

An effect cast instructs the compiler that an expression has a specific effect.

Warning️️: Effect casts are by nature extremely dangerous and should be used with utmost caution!

A Flix programmer should not normally use effect casts except in two cases:

  • To cast an pure function to an effect polymorphic function.
  • To cast a pure function to an impure function.

Both cases are legitimate and safe.

Example: Safe Cast of Pure Function to Effect Polymorphic

Flix does not (yet) have sub-effecting which means that in certain rare cases it can be necessary to manually insert a cast. For example:

def findRight(f: a -> Bool & ef, l: List[a]): Option[a] & ef =
    def loop(ll, k) = match ll {
        case Nil     => k()
        case x :: xs => loop(xs, () -> if (f(x)) Some(x) else k())
    };
    loop(l, () -> None as & ef)

Here the cast () -> None as & ef is required because otherwise the function () -> None would be pure and not effect polymorphic as required.

Warning: Never cast effectful expression to pure. You have been warned.