Expressions starting from `_` are treated as placeholders in `match` cases

[pepkin88]

Example:

match a
| _.isString => 1
| (_.isString) => 1

Result:

var ref$;
switch (ref$ = [a], false) {
case !true:
  1;
  break;
case !_.isString(ref$[0]):
  1;
}

Expected result:

var ref$;
switch (ref$ = [a], false) {
case !_.isString(ref$[0]):
  1;
  break;
case !_.isString(ref$[0]):
  1;
}

The example shows, that wrapping the expression in parentheses is enough to bypass this bug.

[rhendric]

Oh, match. The undocumented, largely untested, experimental feature that somehow people keep discovering anyway.

This sure does look like a bug, but I don't know whether this should be an error, your expected result, or something else. _ has special meaning inside match, which is complicated by the fact that _ is also used to create a partial-application function and as far as I know, every case of a match expression should be interpreted as a function (unless it isn't, like with constants).

I don't really intend to do anything to improve the state of match, unless someone feels like fully and properly specifying its behavior. The good news is, as an experimental undocumented feature, I also wouldn't feel even a tiny bit bad about ripping it out and replacing it with something else. You were all warned. 🙂

[pepkin88]

OK, fair enough.

I drew an analogy to the partial application or switch's default, where only a standalone underscore has a special meaning. In those cases using _.toString wouldn't trigger anything special.

I might fix it later. For now, let this issue be a walk-around suggestion.

[pepkin88]

Related issue with a plausible use case scenario:

This:

(_.isPlainObject || _.isArray) value

compiles to:

true || true;

instead of:

_.isPlainObject(value) || _.isArray(value);

Although I think this feature is a real gotcha and makes sense only in the context of match, it actually is documented.

[rhendric]

Ha, okay, you got me! This should be fixed.

[pepkin88]

This looks like an easy one I could tackle, but only from next week.

[rhendric]

I'll hold it for you then.

[anant-svc]

Hi, tried to get expected result @pepkin88 needed using below:

u = _
match a
| u.isString => 1
| (u.isString) => 1

Result:

var u, ref$;
u = _;
switch (ref$ = [a], false) {
case !u.isString(ref$[0]):
  1;
  break;
case !u.isString(ref$[0]):
  1;
}