Tuple unpacking should also be applicable for nested tuples

[asarkar]

Compiler version

3.3.1

Minimized example

def f(x: (Int, Int)) = x(0) + x(1)
List((1, 2), (3, 4)).map(f) // ok
List((1, (2, 3)), (3, (4, 5))).map((a, (b, c)) => a + f(b, c)) // not ok

Output

) expected but => found
not a legal formal parameter for a function literal
Not found: b
Not found: c

Expectation

Tuple unpacking should also be applicable for nested tuples.

[nicolasstucki]

Note that this works with

List((1, (2, 3)), (3, (4, 5))).map { case (a, (b, c)) => a + f(b, c) }

[nicolasstucki]

4 |  List((1, (2, 3)), (3, (4, 5))).map((a, (b, c)) => a + f(b, c)) // not ok
  |                                         ^^^^^^
  |                       not a legal formal parameter for a function literal

We might be able to improve the error message to guide the user to the version with case.

[bishabosha]

Tuple unpacking should also be applicable for nested tuples.

Note that all tuple unpacking is doing is adapting a tuple to a standard function literal, which does not have patterns

[asarkar]

@nicolasstucki

guide the user to the version with case

Scala 3 added the feature for tuple unpacking, and a user would expect it to work for all sorts of tuples. It’d be pretty confusing to have to use case for some. Either it works, or it doesn’t, it works for one and not for another is not a good user experience.

[ritschwumm]

seconding @asarkar - purely from a user perspective:

when i migrated my projects to scala 3 i found it quite annoying i couldn't just mechanically remove all case modifiers that were actually irrefutable tuple unpacking but had to leave some of them in place and therefore still risk match errors.

[nicolasstucki]

The current spec for tupled functions is in
https://github.com/lampepfl/dotty/blob/58b81089b9f37e30709bb78f516e411820927b60/compiler/src/dotty/tools/dotc/ast/Desugar.scala#L1557-L1577

If we want to change this to support more complex patterns we would need a SIP. The first step would be to open a discussion on https://contributors.scala-lang.org/