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Fix #3861: Spec changes to extractors implemented in #3747 #4024

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Mar 15, 2018
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Fix #3861: Spec changes to extractors implemented in #3747 #4024

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26 changes: 26 additions & 0 deletions docs/docs/reference/changed/vararg-patterns.md
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Original file line number Diff line number Diff line change
Expand Up @@ -18,4 +18,30 @@ supported:
/*!*/ case List(1, 2, xs @ _*) // syntax error
/*!*/ case List(1, 2, _*) => ... // syntax error

Another change relates to extractors producing values with `T*` types, which can then be
matched by vararg patterns. Previously, such extractors used an `unapplySeq` whereas now they use an `unapply` (in the long term, we plan to get rid of `unapplySeq` altogether, replacing all its usages with `unapply`).

Example: Previously, this was correct:

class Person(val name: String, val children: Person *)
object Person {
def unapplySeq(p: Person) = Some((p.name, p.children))
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Maybe add a paragraph explaining the rules that make an unapply qualify for vararg extraction. I am not even sure what are the rules myself. The return type must be of the form Option[Seq[T]] or Option[(..., Seq[T])]?

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I can't context switch into that, sorry. Constant context switching is what kills my productivity. So either somebody else digs out the details and adds them or we leave it as is. Again, my plea stands that somebody will have to take over the pattern matching problematic.

}

def childCount(p: Person) = p match {
case Person(_, ns @ _*) => ns.length
}

Now, the equivalent program is written as follows:

class Person(val name: String, val children: Person *)
object Person {
def unapply(p: Person) = Some((p.name, p.children))
}

def childCount(p: Person) = p match {
case Person(_, ns : _*) => ns.length
}