Whether support `*:` as a valid return type of `unapply` in extractors

[liufengyun]

The following code is currently not supported by the compiler:

object U:
  def unapply(s:String): String *: EmptyTuple = Tuple1(s)

"hello" match
case U(x) => 

If we change String *: EmptyTuple to Tuple1[String], it will compile without problem.

The problem is that according to the current specification, String *: EmptyTuple does not qualify as a product pattern, as it does not contain selectors _1, ..., _N.

The question is:

• Whether we should support String *: EmptyTuple?
• How to change the specification?

One particular concern is that making *: a special case seems to cancel the past efforts to make pattern match more generic.

Related PR: #10987

[nicolasstucki]

We could use the NonEmptyTuple.apply(N) instead of _N. This gets optimized to _N when the tuple is known to be implemented with a TupleN for N <= 22.

One particular concern is that making *: a special case seems to cancel the past efforts to make pattern match more generic.

This is part of a different dimension of genericity that we need to be able to handle as well.

[liufengyun]

This is part of a different dimension of genericity that we need to be able to handle as well.

I agree, if we can find a way to change the specification without a special case for *:, that would be ideal.

[odersky]

Is there a strong reason why *: should be a valid return type? If not, I think we have now seen enough arguments why it should not.

[nicolasstucki]

• Support unapply that returns a match type that operates on generic tuples. These types might be propagated from operations in Tuple (take, map, ...).
• Fix MatchError in PatternMatcher$Translator.matchArgsPatternPlan #10958
• Support tuples with more than 22 arguments

[liufengyun]

• Fix MatchError in PatternMatcher$Translator.matchArgsPatternPlan #10958
• Support tuples with more than 22 arguments

The last two items seem to be not strong reasons. The first can be fixed by changing the return type. For the second, I don't think realistic code would write pattern match to deal with > 22 arguments.

[joroKr21]

I'd add - support extractors for generic code.
For example I might want to derive an extractor the same way I can derive a type class.

[liufengyun]

I'd add - support extractors for generic code.

@joroKr21 To check whether it affects the specification, could you be more specific about this with a simple example?

[joroKr21]

@liufengyun I can give you a complex example of what I would like to try in Scala 3: recursion-schemes.scala

The idea is that for a recursive type (Expr) we can derive it's pattern functor ExprF[_] automatically. In ExprF[A] recursive occurrences of Expr are replaced by the type parameter A. We can also derive automatically conversions between Expr and the fixpoint ExprF[Fix[ExprF]]. Based on that we can build a library of recursion schemes (cata-, ana-, hylo-, para-, etc. morphisms).

But to use them we need to pattern match on ExprF[x] (for example ExprF[Int] in eval, ExprF[String] in show, etc.) which is not an ADT by itself - it's just a raw sum of products. In Scala 2 we can achieve that in a somewhat hacky way but it would be so much nicer if we can match directly on generic tuples (products) in Scala 3. We can already match directly on union types which can be used to represent sums.

I will think a bit about a simpler example.

[dwijnand]

How does Scala 3 support case List(a)? In Scala 2 it's supported by special-casing List in the match analysis to be rewritten as a :: Nil and then :: is already a 2-element case class.

So if Scala 3 generalised that into rewriting any case U(a) with an unapply with that return type into a *: EmptyTuple (and case U(a, b) => case a *: b *: EmptyTuple..) then it would be using *:.unapply:

object *: {
  def unapply[H, T <: Tuple](x: H *: T): (H, T) = (x.head, x.tail)
}

[liufengyun]

How does Scala 3 support case List(a)? In Scala 2 it's supported by special-casing List in the match analysis to be rewritten as a :: Nil and then :: is already a 2-element case class.

Dotty has some similar logic in exhaustivity check:

https://github.com/lampepfl/dotty/blob/437d02ad7d2f582d25e57d86e70fda96b6e0a330/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala#L501-L503

However, this is just a trick in exhaustivity check: type checking and semantic translation don't have such things.

The following semantic rewrite seems to be problematic, as it throws U away:

case U(a, b) => case a *: b *: EmptyTuple