What does narrow do and how should it be used?

[Paul Butcher]

"narrow" was recently added to the macro/reflection API:

https://issues.scala-lang.org/browse/SI-5951

but I'm struggling to understand what it does, exactly, and how to use it. In particular, Martin suggested that I could use it to solve a particular issue I was having with parameterised members, but I'm struggling to do so.

Specifically, given:

scala> import reflect.runtime.universe._

import reflect.runtime.universe._

scala> class Outer[T] { trait Inner[T]; def m(): Inner[T] = null }

defined class Outer

scala> val t = typeOf[Outer[Int]]

t: reflect.runtime.universe.Type = Outer[Int]

scala> val m = t.member(newTermName("m"))

m: reflect.runtime.universe.Symbol = method m

scala> m.typeSignature

res0: reflect.runtime.universe.Type = ()Outer.this.Inner[T]

I'm trying to get the type of m with T substituted for Int. If I call narrow, I get:

scala> m.typeSignature.narrow

res1: reflect.runtime.universe.Type = <none>.<refinement>.type

It's not clear to me what I can do with this type - ultimately what I need to do is get the types of the arguments to the method and its return type (in this case, ()Outer.this.Inner[Int]). How do I do this with the type that's returned from narrow?

For reference, I can do it using the following hack, but Martin indicated that narrow would allow me to avoid this :-)

scala> m.typeSignatureIn(SuperType(ThisType(t.typeSymbol), t))

res2: reflect.runtime.universe.Type = ()Outer.this.Inner[Int]

--
paul.butcher->msgCount++

Snetterton, Castle Combe, Cadwell Park...
Who says I have a one track mind?

http://www.paulbutcher.com/
LinkedIn: http://www.linkedin.com/in/paulbutcher
MSN: pa...@paulbutcher.com
AIM: paulrabutcher
Skype: paulrabutcher

[Eugene Burmako]

Hi Paul,

I'm currently swamped, so I'll answer only during the weekend or later. Okay?

Cheers,

Eugene

[Paul Butcher]

On 3 Aug 2012, at 10:45, Eugene Burmako <eugene....@epfl.ch> wrote:

I'm currently swamped, so I'll answer only during the weekend or later. Okay?

Absolutely - there's no rush associated with this - I have a working solution (even though Martin considers it to be a hack :-)

[Paul Butcher]

Just a gentle ping to see if you've had a chance to think about this yet, Eugene?

--
paul.butcher->msgCount++

Snetterton, Castle Combe, Cadwell Park...
Who says I have a one track mind?

http://www.paulbutcher.com/
LinkedIn: http://www.linkedin.com/in/paulbutcher
MSN: pa...@paulbutcher.com
AIM: paulrabutcher
Skype: paulrabutcher

[Eugene Burmako]

I don't know how this stuff is supposed to work, but will ask Martin next Thursday.

In the meanwhile, some experiments:

scala> m.typeSignatureIn(SuperType(ThisType(t.typeSymbol), t))
res0 @ 7bfbfeae: reflect.runtime.universe.Type = ()Outer.this.Inner[Int]

scala> m.typeSignatureIn(t.narrow)
res1 @ 1247de8f: reflect.runtime.universe.Type = ()this.Inner[Int]

scala> res0 =:= res1
res2 @ 2eb9e305: Boolean = false

scala> showRaw(res0)
res3 @ 6fa85031: String = MethodType(List(), TypeRef(ThisType(Outer), newTypeName("Inner"), List(Typ
eRef(ThisType(scala), scala.Int, List()))))

scala> showRaw(res1)
res4 @ 5ca32803: String = MethodType(List(), TypeRef(ThisType(newTypeName("<refinement>")), newTypeN
ame("Inner"), List(TypeRef(ThisType(scala), scala.Int, List()))))

@Paul Phillips, maybe you could chime in?

On 2 August 2012 15:13, Paul Butcher <pa...@paulbutcher.com> wrote:

[Eugene Burmako]

Sorry for the late reply, but I fail to understand how to get to ()Outer.this.Inner[Int]. 

We had a brief chat with Martin yesterday, and he also doesn't know how to implement this scenario, except for the hack you used.