Backpassing

Function based

• Gleam's use expressions
• Koka's with statement
• Roc's "backpassing" (which unfortunately appears to be a removed feature)

Monad based

• Haskell's do notation
• Iris's ! notation

Uses With Containers

func maybe_sum(x: Option<U32>, y: Option<U32>) -> Option<U32> {
    x.flat_map do(i) {
        y.flat_map do(j) {
            Some(i + j)
        }
    }
}

func maybe_sum(x: Option<U32>, y: Option<U32>) -> Option<U32> {
    let i = x.flat_map?;
    let j = y.flat_map?;

    Some(i + j)
}

Backpassing match Special Form

• Rust's let else construct

match(foo) {
  True?,
  False -> print("nope"),
};

print("yep");

Expression returns a tuple.

With values

match(foo) {
  Pass(value)?,
  Fail(error) -> print("oh no: \(error)"),
};

print("yippee: \(value)");