Hey there, I’m not sure if this has been asked before (I’ve searched around and haven’t seen it), but has there been any conversation about the idea of pattern matching on record field values? For example: case myRecord of { a = 1 } -> ... _ -> ... or even with nested pattern matching: c…

I’ve definitely also wanted this from time to time. Usually it’s a sub-pattern, such as: case mState of Just { isDisabled = True } -> .... It’s similar to quite a few language additions that I can surely live without, but don’t see the harm in adding. However, that is ‘quite a few’ so …

I have found case to have an odd set of capabilities to be honest. It can do some very sophisticated things but then it doesn’t have other things which I would consider more “obvious”. For instance list destructuring strikes me as very sophisticated and also very non-obvious: case myList of [] …

[image] kmurph1271: Pending | Partial _ -> I agree that collapsing multiple branches in pattern matching would be really convenient in some cases! I wouldn’t call list pattern matching very sophisticated though. As previously said, “it depends on you background”.

The challenge on matching multiple cases at once is what named values they bring into scope. It would take a rule that said something like, for a collection of cases to be treated together, those cases need to introduce the same named values and those named values all must have the same types.

[image] MarkHamburg: for a collection of cases to be treated together, those cases need to introduce the same named values and those named values all must have the same types. That’s exactly how F# works. (Its error messages aren’t super good, it’s always just “The two sides of this ‘or’ patt…

For reference, here’s an example of what Rust’s errors for these situations look like. I think people coming from Rust will likely have the same intuition as kmurph1271, since it doesn’t support list destructuring but does support matching multiple patterns with |, I did at least. enum MyEnum { …

[image] kmurph1271: I have found case to have an odd set of capabilities to be honest. It can do some very sophisticated things but then it doesn’t have other things which I would consider more “obvious”. For instance list destructuring strikes me as very sophisticated and also very non-obvious: …

Pattern match on record field values

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allanderek September 3, 2021, 8:15am 5

I’ve wanted ‘or-patterns’ for a long time, it’s one of the “quite a few” that I mentioned above.

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Pattern match on record field values

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