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should pure-value cases narrow?

See original GitHub issue

Currently, a case with only values, such as case ("foo"|"bar") doesn’t narrow the type of the switched variable. After #4402 that’s inconsistent and special-casey. Also it would be useful to narrow the type when you have multiple values listed in a single case.

I think we should change this. However, it would be a source code breaking change, since in 1.x we allowed you to have a variable switch expression if all the cases were value cases. We would have to close that loophole.

Issue Analytics

State:
Created 6 years ago
Comments:8 (8 by maintainers)

Top GitHub Comments

1reaction

gavinkingcommented, Apr 28, 2018

One could decide to narrow only if it is not variable.

Alright, @ePaul, so I figured out a way to implement this. It’s kinda nasty but in a way, but not actually fragile. It’s on the 7248 branch.

Still, I will probably add a deprecation warning to the case of switching a variable, since this behavior is pretty inconsistent with how is and exists conditions treat variables.

0reactions

gavinkingcommented, Apr 28, 2018

Spec updated, closing.