After long discussions we finally decided on an unwrap syntax for both the option and result types that we are happy with and that still matches the explicitness of ReScript we all like.

What is it exactly?

let? or let-unwrap is a tiny syntax that unwraps result/option values and early-returns on Error/None. It’s explicitly experimental and disabled by default behind a new “experimental features” gate.

Example

RESCRIPT
let getUser = async (id) => {
  let? Ok(user)  = await fetchUser(id)
  let? Ok(decodedUser) = decodeUser(user)
  Console.log(`Got user ${decodedUser.name}!`)
  let? Ok() = await ensureUserActive(decodedUser)
  Ok(decodedUser)
}

This desugars to a sequence of switch/early-returns that you’d otherwise write by hand, so there’s no extra runtime cost and it plays nicely with async/await. Same idea works for option with Some(...) (and the PR also extends support so the left pattern can be Error(...)/None, not just Ok(...)/Some(...)).

Beware it targets built-ins only: result and option. (Custom variants still need switch.) And it is for block or local bindings only; top-level usage is rejected. Compiled JS code is the straightforward if/return form (i.e., “zero cost”).

How to enable it (experimental)

We have added an experimental-features infrastructure to the toolchain. The corresponding compiler flag is -enable-experimental. This means you can enable let? in your rescript.jsons compiler-flags and it forwards the feature to the compiler.

This is purely a syntactical change so performance is not affected.