Try expressions

[bvssvni]

Sometimes you want to continue running after a function call fails:

// Get first item from current `arr`
foo() ~ arr = clone(arr[0])

fn main() {
    ~ arr := [] // `arr` is empty, so there is no first item.
    a := foo() // ERROR: Out of bounds `0`
    println(a)
}

A try expression lets you convert runtime failures into err and success into ok:

foo() ~ arr = clone(arr[0])

fn main() {
    ~ arr := []
    a := try foo()
    println(a) // prints `err(...)`
}

Try expressions are convenient together with the ? operator:

fn bar() -> res { ... }

fn main() {
    a := try bar()?
    if is_err(a) {
        // Calling `bar` failed.
        ...
    }
}

This maps err to early returns, and runtime failures to err:

ok(X), err(Y), fail(Z) => ok(X), err(Z), return(err(Y))