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Pattern q"new $t(..$args)" does not work for zero argument constructors #10138

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scabug opened this issue Jan 7, 2017 · 5 comments
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Pattern q"new $t(..$args)" does not work for zero argument constructors #10138

scabug opened this issue Jan 7, 2017 · 5 comments
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@densh
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patmat quasiquotes reflection won't fix
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2.11.0

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@scabug
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scabug commented Jan 7, 2017 

Welcome to Scala 2.12.1 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_102).
Type in expressions for evaluation. Or try :help.

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> val q"new $t(..$arguments)" = q"new Foo(bar, baz)"
t: reflect.runtime.universe.Tree = Foo
arguments: List[reflect.runtime.universe.Tree] = List(bar, baz)

scala> val q"new $t(..$arguments)" = q"new Foo(bar)"
t: reflect.runtime.universe.Tree = Foo
arguments: List[reflect.runtime.universe.Tree] = List(bar)

scala> val q"new $t(..$arguments)" = q"new Foo()"
scala.MatchError: new Foo() (of class scala.reflect.internal.Trees$Apply)
  ... 29 elided
@scabug
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scabug commented Jan 7, 2017

Imported From: https://issues.scala-lang.org/browse/SI-10138?orig=1
Reporter: @Atry
Affected Versions: 2.12.1

@scabug
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scabug commented Jan 9, 2017

@densh said:
New and constructors are a bit special with respect to how they treat nullary applications (there is no way to distinguish new Foo and new Foo()). You should use ...$arguments to match cases like these.

@scabug scabug closed this as completed Jan 9, 2017
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scabug commented Jan 13, 2017

@Atry said (edited on Jan 13, 2017 7:00:43 AM UTC):
Did you mean "You should NOT use ...$arguments to match cases like these." ?

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scabug commented Jan 13, 2017

@densh said (edited on Jan 13, 2017 9:59:59 AM UTC):
Sorry if I was unclear, ...$arguments will match any arity of arguments. then you can check that it's an empty list as a pattern matching guard for example.

scala> val q"new $_(...$argss)" = q"new Foo"
argss: List[List[reflect.runtime.universe.Tree]] = List()

scala> val q"new $_(...$argss)" = q"new Foo()"
argss: List[List[reflect.runtime.universe.Tree]] = List()

scala> val q"new $_(...$argss)" = q"new Foo(x)"
argss: List[List[reflect.runtime.universe.Tree]] = List(List(x))

scala> val q"new $_(...$argss)" = q"new Foo(x, y)"
argss: List[List[reflect.runtime.universe.Tree]] = List(List(x, y))

scala> val q"new $_(...$argss)" = q"new Foo(x)(y)"
argss: List[List[reflect.runtime.universe.Tree]] = List(List(x), List(y))

@scabug
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scabug commented Jan 14, 2017

@som-snytt said:
For those of us with less than stellar vision.

Dots near unquotee annotate degree of flattening and are also called splicing rank. 
..$ expects argument to be an Iterable[Tree] and ...$ expects Iterable[Iterable[Tree]].

Also glossary.

@scabug scabug added this to the 2.11.0 milestone Apr 7, 2017
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patmat quasiquotes reflection won't fix
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