Syntax proposal `if let`

[jaredly]

In swift, there's this nice construct for simple destructuring

if case .someEnumVariant(let value) {
 ...
} else {
 ...
}

It's basically a one-armed match.
It would be super handy to have that for reason, and I imagine it would look like

if let [%pattern] = [%value] {
  // x
} else {
  // y
}

which we'd translate to

switch [%value] {
| [%pattern] => // x
| _ => // y
}

[pjhenning]

Yes its occasionally nice being able to do (a bit redundant here but this is the specific use case I had been thinking about):

if let aValue = anOptional {
    doSomethingWith(aValue)
}

[jaredly]

Other discussion from the ocaml folks
ocaml/ocaml#194
https://caml.inria.fr/mantis/view.php?id=6685

I'm not very moved by their counterarguments. I think if let is much more accessible to folks coming from javascript etc., to whom Option.iter would be foreign.

[jaredly]

Another thing that swift's if let does might be quite useful (found myself wanting it in code I'm writing just now)

if
  let Some(x) = optx,
  let Some(y) = opty,
  let Some(z) = optz
{
 // all your things are here
}

is much nicer imo than

switch (optx, opty, optx) {
| (Some x, Some y, Some z) => ...

(especially if optx is some non-trivial expression)

[ckknight]

I think using and instead of , let as a connector for multiple guards could work well particularly because it's the same syntax used with let rec x = ... and y = ...;

e.g.

if let Some(x) = optx
   and Some(y) = opty
   and Some(z) = optz
{
 // all your things are here
}

[jaredly]

It's a cool idea! One thing that might be confusing is that I want successive cases to be able to use values from previous cases.
e.g.

if let Some(x) = optx,
   let Awesome(y) = doSomethingWith(x) {
}

and syntax in ocaml requires them to not use each other, and let rec and requires them to be functions. So something like

let x = 2
and y = x

is invalid.

[ckknight]

Fair, this pattern seems like a pretty great way to guard:

if let Some(x) = getValue(),
   let Some(y) = doSomething(x),
   let Some(z) = doSomethingElse(z) {
  doSomethingCool(z);
} else {
  failwith("Couldn't do the thing!");
}

The ability to use the inner values makes it superior to

switch (a, b, c) {
  | (Some(a), Some(b), Some(c)) => ...,
  _ => ...
}

It'd have to be expressed as a series of nested switches or ifs, which would be less clear and add unnecessary nesting.

[gilbert]

For reference, this is similar to Elixir's with statement:

opts = %{width: 10, height: 15}
with {:ok, width} <- Map.fetch(opts, :width),
     {:ok, height} <- Map.fetch(opts, :height),
     do: {:ok, width * height}
#=> {:ok, 150}