Match quoted tuple prepend

[Adam-Vandervorst]

Compiler version

3.1.2-RC1-bin-20211025-968dd1b-NIGHTLY

Minimized code

import scala.quoted.*

def printallImpl[T <: NonEmptyTuple](xs: Expr[T])(using Quotes): Expr[Unit] =
  xs match
    case '{ $head *: (${tail}: Tuple) } =>
      '{ print($head); print(" *: "); println($tail) }
    case '{ ($fst, $snd) } =>
      '{ print($fst); print(", "); println($snd) }

inline def printall[T <: NonEmptyTuple](inline xs: T) = ${ printallImpl('{xs}) }

@main def example = printall((1, 2))

Output

1, 2

Expectation

1 *: Tuple(2)

I expect '{ $head *: (${tail}: Tuple) } to catch something matching '{ ($fst, $snd) }, but I'm new to this area.

[nicolasstucki]

The natural assumption you probably made is that (1, 2) is equivalent to 1 *: 2 *: EmptyTuple. This is unfortunately not the case on the term level. Thought on types it is true (1, 2) is of type Int *: Int *: EmptyTuple.

The reason behind this is that to keep compatibility with Scala 2 we need to keep the old TupleN (from 1 to 22) types and constructors. This implies that (1, 2) is actually Tuple2(1, 2). To make old tuples work with the new tuples we make Tuple2[T1, T2] a subtype of T1 *: T2 *: EmptyTuple. We also have to add the operations of Tuple using inline definitions which makes things a bit more complex.

At some point in the future, when we drop Scala 2 backward compatibility we will be able to simplify this.

An alternative to match against the type instead of the constructor.

    case '{ $tuple : Int *: Tuple } =>

or

    case '{ type tail <: Tuple; $tuple : fst *: `tail` } =>

Though then you cannot extract the elements and would need to call head to get them at runtime.

As long as we have TupleN we will need to match offer its constructors and aliases. See https://github.com/lampepfl/dotty/blob/master/library/src/scala/quoted/FromExpr.scala#L174-L181

[nicolasstucki]

I will convert this to a discussion because even if it is not an the current and future discussions can help other users to understand what to do in these situations.