Syntax proposal guard let

[jaredly]

A follow-up to #1301, swift has this other nice piece of sugar, guard let.
Here's what it might look like in reason

guard let Some(val) = something else { None };
// ...rest of the code

which translates to

switch something {
| Some(val) => {
  // ...rest of the code
}
| _ => None
}

More generally

guard let %pattern = %value else { %if_not_matched_expression };
%if_matched_expression

becomes

switch %value {
| %pattern => %if_matched_expression
| _ => %if_not_matched_expression
}

early returns can be super helpful for readability imo.

ppx_let does a similar kind of thing, albeit with somewhat more magic https://github.com/janestreet/ppx_let

[anmonteiro]

unlikely we'll do this