Stream: ideas

Topic: Should tuples be extendable in patterns

Ayaz Hafiz (Feb 06 2023 at 14:07):

Today you can pattern match records with a subset of the record's fields, e.g.

{foo} = {bar: 1, foo: 2}

or

when {bar: 1, foo: 2} is
  {foo} -> ...

moreover you can have functions that generalize over the shape of a record, e.g. \r -> r.foo indexes any record with a field "foo".

I think the second is useful for tuples too, but I don't think it's a good semantics for tuples. For example, neither of these seems intentional:

(x, y) = (1, 2, 3)

when (x, y, z) is
  (x, y) -> ..

and is probably more indicative of a bug. The same might sometimes be true for records, but in tuples it's more pronounced since the fields of a tuple are not named.

The suggestion is to remove such semantics for tuples.

What do folks think?

Folkert de Vries (Feb 06 2023 at 14:09):

I agree that looks weird. in pattern matches I think tuples should behave more like a tag Triple a b c

Folkert de Vries (Feb 06 2023 at 14:10):

where you cannot pattern match on just a subset of the fields of the tag

Georges Boris (Feb 06 2023 at 15:14):

iirc the idea was to build "extensible tuples" so we could get more useful functions Tuple.first matching any tuple size, right? If that doesn't apply to pattern matching then how would the user create extensible-tuple functions themselves?

Georges Boris (Feb 06 2023 at 15:16):

maybe this could be made explicit in the pattern matching?

when tuple is
  (a,b) -> "I have two items"
  (a,b)* -> "I have two or more items"

Georges Boris (Feb 06 2023 at 15:16):

(wouldn't the same be valuable for records too?)

Folkert de Vries (Feb 06 2023 at 15:19):

we can re-use the .. syntax from lists?

Folkert de Vries (Feb 06 2023 at 15:19):

so first = \(x, .. ) -> x

Joshua Warner (Feb 06 2023 at 16:18):

I believe under the hood this maps exactly to the ext parameter on the tuple - so (a,b)* would make the most sense to me.

Folkert de Vries (Feb 06 2023 at 16:19):

in the type, sure, but for pattern syntax .. has precedent

Notification Bot (Feb 06 2023 at 17:56):

37 messages were moved from this topic to #ideas > Tuple rest patterns by Ayaz Hafiz.

Joshua Warner (Feb 06 2023 at 17:57):

:thumbs_up: I'm on board with the (x, ..) syntax

Last updated: Jun 16 2026 at 16:19 UTC