QoL: Sound binding in pattern alternatives

(This is about pattern alternatives, not union types !)

enum Tree:
  case Wrap(t: Tree)
  case Node(t1: Tree, t2: Tree)
  case Leaf(value: Int)
import Tree.*

def rightmostValue(tree: Tree): Int =
  tree match
    case Wrap(subtree) | Node(_, subtree) => rightmostValue(subtree)
    case Leaf(value) => value

The code above is unambiguous, right ?
We extract a value subtree of type Tree, and recurse on it

But this is not allowed:

Illegal variable subtree in pattern alternative

It is said as much in the Scala 2 spec:
“They may not bind variables other than wildcards.”

My somewhat crude proposal is the following:

Binding identifier id is allowed in pattern alternatives iff all the following conditions are true:

  • each alternative i extracts id with type T_i
  • effect: id has type T = T_1 | ... | T_n
  • if an alternative or one of its sub-patterns is id: U_i, then T must be =:= U_i
  • id cannot be bound in a “pattern binder” id@Extractor(...)

The last condition could also be replaced by “if id appears in a pattern binder, then the pattern must be the same in all aternatives”


tree match
  case Wrap(subtree) | Leaf(subtree) =>
    // body where subtree has type Tree | Int
tree match
  case Wrap(subtree) | Leaf(subtree: Int) =>
    // error: subtree is bound to type Tree | Int, not Int
tree match
  case Wrap(subtree@Wrap(_)) | Leaf(subtree) =>
    // error: multi-bound variables cannot be part of pattern binders
tree match
  case Wrap(subtree@Wrap(_)) | Node(_, subtree@Wrap(_)) =>
    // Might be okay ?

This has the impact that P1 | P2 is not always equivalent to P2 | P1:

tree match
  case Wrap(t) | t: Tree =>
    // not equivalent to
  case t: Tree | Wrap(t) =>

Questions for you:

  1. Is this sound ?
  2. Does this have good UX ?
  3. Would this be useful ?
  4. Suggestions ?

It can be implemented as syntactic sugar (am I right?), and thus will not introduce any new unsoundness.

It was discussed a little here: QoL: Sound binding in pattern alternatives

I’m sorry I don’t understand what you want to say, this is a link to the current discussion

See also this archived feature request: Support Variables in pattern alternatives · Issue #12 · lampepfl/dotty-feature-requests · GitHub.

And Allow variables in pattern alternatives using union types · Issue #175 · lampepfl/dotty-feature-requests · GitHub
(Which was just flagged as a duplicate of yours)

So it seems this is already “common sense”

I think the most straightforward way to implement this is to desugar

tree match
  case Wrap(subtree: Int | Tree) | Leaf(subtree: Int | Tree) =>


tree match
  case Wrap(subtree: Int | Tree) =>
  case Leaf(subtree: Int | Tree) =>

(Avoids having to deal with symbols being defined twice)

For this to be predictable, the proposal needs to be restricted like so:
Binding identifier id is allowed in pattern alternatives iff there is a unique T such that:
each alternative extracts id with type T (through the extractor’s return type and/or through narrowing (id: T))
I believe this is acceptable

Downside to this simplicity is it can lead to code explosion:

treePair match
  case ( 
      (Wrap(subtree1: Int | Tree) | Leaf(subtree1: Int | Tree)), 
      (Wrap(subtree2: Int | Tree) | Leaf(subtree2: Int | Tree)) 
  ) =>

will become

tree match
  case (Wrap(subtree1: Int | Tree), Wrap(subtree2: Int | Tree)) =>
  case (Wrap(subtree1: Int | Tree), Leaf(subtree2: Int | Tree)) =>
  case (Leaf(subtree1: Int | Tree), Wrap(subtree2: Int | Tree)) =>
  case (Leaf(subtree1: Int | Tree), Leaf(subtree2: Int | Tree)) =>

Duplicating arbitrary user code is a big no-no. :wink: So to be supported, it needs to go deep into actual semantics, not just a desugaring.

There’s nothing fundamentally against this support. My understanding is that the cost/benefit ratio has always been considered too high.

1 Like

(Except with inline parameters)

1 Like

This idea was also mentionned here:

But the goal there would be for things like

pair match
  case (a, a) => 
  case _ =>

to work

Which is similar but completely orthogonal

My bad, I’ve posted the wrong link from mobile. What I meant to post is
Could it be possible to allow variable binging in patmat alternatives for Scala 3.x?

There is also a link from @LPTK to some prototype for this feature.

Thank you !
I had searched if someone had already opened something here, but didn’t find it