Warning 8: You forgot to handle a possible case here

1

Is there a way to tell the compiler that really, it is OK?

I wrote this code:

    let result = isTitle(line)
    if result->Array.length == 2 {
      let [_, title] = result // TODO: silence warning
      Js.log("TITLE: " ++ title)
    }

The compiler replies:

Warning 8: You forgot to handle a possible case here, for example:
[]

And the generated code contains:

      var result$1 = isTitle(line);
      if (result$1.length === 2) {
        if (result$1.length !== 2) {
          throw {
                RE_EXN_ID: "Match_failure",

I tried using @@warning but that is not recognised.

2

There’s a way to work around the error:

switch isTitle(line) {
| [_, title] => Js.log("TITLE: " ++ title)
| _ => ()
}

And the generated code will be:

var match = isTitle(line);
if (match.length === 2) {
  var title = match[1];
  console.log("TITLE: " + title);
}
1 Like
3

Deactivating the warning should work fine, but you set it for a range of lines, not a single line.

@@warning("-8")
let result = isTitle(line)
if result->Array.length == 2 {
  let [_, title] = result
  Js.log("TITLE: " ++ title)
}
@@warning("+8")

If you omit the @@warning("+8"), the warning is disabled for the rest of the file.

Playground link

But you rather should write something similar to DZahk’s suggestion. While you can easily pattern-match on tuples or records just with a let-binding, it does not work without warnings where there are other possible permutations that your match misses, like arrays or lists. For such data structures you should prefer using switch.