Expressions starting from _ are treated as placeholders in match cases

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.

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. 🙂

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.

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.

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

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

I'll hold it for you then.

@pepkin88

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;
}

fix gkz#1025

fix #1025

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Expressions starting from `_` are treated as placeholders in `match` cases #1025

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Expressions starting from _ are treated as placeholders in match cases #1025

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rhendric commented on Mar 26, 2018

pepkin88 commented on Mar 26, 2018

pepkin88 commented on Apr 2, 2018

rhendric commented on Apr 2, 2018

pepkin88 commented on Apr 2, 2018

rhendric commented on Apr 2, 2018

anant-svc commented on Jun 11, 2018

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Expressions starting from `_` are treated as placeholders in `match` cases #1025