Confusing type errors / warning when using optional record fields

[ryyppy]

Here's an example on different confusing errors / warnings:

type props = {count: int, username?: string}

let make = props => {
  // This is a pattern match, so it warns me with
  // "You forgot to handle a possible case here, for example: {count: _, username: None, _}"
  // As a user I would have expected `username` just to be a `option<string>` and that's it
  let {count, username} = props 

  // This typechecks, even though this shouldn't.
  // `username` should be an option<string>
  Js.String2.toLowerCase(username)->ignore

  // This pattern match warns me on missing case, since it thinks it's matching on `props`, not
  // on `props.username`:
  // "You forgot to handle a possible case here, for example: {count: _, username: None, _}"
  let _ = switch props.username {
  | Some("test") => "test"
  | _ => "whatever"
  }

  <div />
}

I think it all boils down to the fact that the initial props destructing doesn't really do what I actually expected.

Is this expected behavior?

[cknitt]

This is indeed expected behavior. You would need to do

let {count, ?username} = props

This is documented here: https://rescript-lang.org/docs/manual/latest/record#pattern-matching-on-optional-fields

I agree that it feels quite unexpected when destructuring. It tripped me up, too, when I first encountered it.

[zth]

Maybe we can help out here and automatically insert ? via the editor tooling now that we have autocomplete in destructuring...?

[zth]

We should also probably document an actual destructure, the docs has switches only.

[zth]

Fixed in the editor tooling in rescript-lang/rescript-vscode#715

[ryyppy]

IMO if this is the only way an optional attr should be destructured, then it should be enforced on the type checker level. Everything else seems hacky to me.

[cristianoc]

This is not about optional args. You can as well write let 3 = x with similar outcome.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.