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Compiler type inference problem?

4 replies

Fri, 2012-02-03, 11:39

edmondo1984

Joined: 2011-09-14,

Dear all,
look at the following case and why it doesn't compile:
trait Generic[T]trait Space { def snapshot[T](item:java.lang.Object):Generic[T] } class TypeInferenceProblem { def example(space:Space){ space.snapshot(new java.lang.Double(20d)) space.snapshot(new java.lang.Integer(30)) }
}
error: polymorphic expression cannot be instantiated to expected type;found : [T]com.gottex.scala.types.Generic[T]required: Unit space.snapshot(new java.lang.Integer(30))

Whatever the return type of space.snapshot is, since the method will return unit, why does the compiler care?
Best
Edmondo

Fri, 2012-02-03, 12:01

#1

Hubert Plociniczak

Joined: 2009-09-12,

Re: Compiler type inference problem?

Looks like #4336 that was fixed recently.

hubert

On 02/03/2012 11:39 AM, Edmondo Porcu wrote:

CAM-vFRzffjk0z655zGtUZSTZ4HPbB9QFgjfCViHvYjw-aAKM5A [at] mail [dot] gmail [dot] com" type="cite"> Dear all,
look at the following case and why it doesn't compile:
trait Generic[T] trait Space { def snapshot[T](item:java.lang.Object):Generic[T] } class TypeInferenceProblem { def example(space:Space){ space.snapshot(new java.lang.Double(20d)) space.snapshot(new java.lang.Integer(30)) }
}
error: polymorphic expression cannot be instantiated to expected type; found : [T]com.gottex.scala.types.Generic[T] required: Unit space.snapshot(new java.lang.Integer(30))

Whatever the return type of space.snapshot is, since the method will return unit, why does the compiler care?
Best
Edmondo

Fri, 2012-02-03, 15:11

#2

H-star Development

Joined: 2010-04-14,

Re: Compiler type inference problem?

unit is not a subclass of everything, so this error makes perfect sense.
to fix it:

def example(space: Space) {
space.snapshot(new java.lang.Double(20d))
space.snapshot(new java.lang.Integer(30))
return
}

-------- Original-Nachricht --------
> Datum: Fri, 3 Feb 2012 11:39:12 +0100
> Von: Edmondo Porcu
> An: scala-user
> Betreff: [scala-user] Compiler type inference problem?

> Dear all,
>
> look at the following case and why it doesn't compile:
>
> trait Generic[T]
> trait Space {
> def snapshot[T](item:java.lang.Object):Generic[T]
> }
> class TypeInferenceProblem {
>
> def example(space:Space){
> space.snapshot(new java.lang.Double(20d))
> space.snapshot(new java.lang.Integer(30))
> }
>
> }
>
> error: polymorphic expression cannot be instantiated to expected type;
> found : [T]com.gottex.scala.types.Generic[T]
> required: Unit
> space.snapshot(new java.lang.Integer(30))
>
>
> Whatever the return type of space.snapshot is, since the method will
> return
> unit, why does the compiler care?
>
> Best
>
> Edmondo

Fri, 2012-02-03, 15:21

#3

Viktor Klang

Joined: 2008-12-17,

Re: Compiler type inference problem?

On Fri, Feb 3, 2012 at 3:01 PM, Dennis Haupt <h-star@gmx.de> wrote:

unit is not a subclass of everything, so this error makes perfect sense.
to fix it:

def example(space: Space) {
space.snapshot(new java.lang.Double(20d))
space.snapshot(new java.lang.Integer(30))
return

or simply: ()

}

-------- Original-Nachricht --------
> Datum: Fri, 3 Feb 2012 11:39:12 +0100
> Von: Edmondo Porcu <edmondo.porcu@gmail.com>
> An: scala-user <scala-user@googlegroups.com>
> Betreff: [scala-user] Compiler type inference problem?

> Dear all,
>
> look at the following case and why it doesn't compile:
>
> trait Generic[T]
> trait Space {
> def snapshot[T](item:java.lang.Object):Generic[T]
> }
> class TypeInferenceProblem {
>
> def example(space:Space){
> space.snapshot(new java.lang.Double(20d))
> space.snapshot(new java.lang.Integer(30))
> }
>
> }
>
> error: polymorphic expression cannot be instantiated to expected type;
> found : [T]com.gottex.scala.types.Generic[T]
> required: Unit
> space.snapshot(new java.lang.Integer(30))
>
>
> Whatever the return type of space.snapshot is, since the method will
> return
> unit, why does the compiler care?
>
> Best
>
> Edmondo

--
Viktor Klang

Akka Tech LeadTypesafe - The software stack for applications that scale

Twitter: @viktorklang

Fri, 2012-02-03, 16:01

#4

Hubert Plociniczak

Joined: 2009-09-12,

Re: Compiler type inference problem?

Actually that one was #4853 and all the credit goes to Paul.
Anyway, fixed in trunk.

hubert

On 02/03/2012 11:55 AM, Hubert Plociniczak wrote:

4F2BBD3D [dot] 6010004 [at] epfl [dot] ch" type="cite"> Looks like #4336 that was fixed recently.

hubert

On 02/03/2012 11:39 AM, Edmondo Porcu wrote:
CAM-vFRzffjk0z655zGtUZSTZ4HPbB9QFgjfCViHvYjw-aAKM5A [at] mail [dot] gmail [dot] com" type="cite"> Dear all,
look at the following case and why it doesn't compile:
trait Generic[T] trait Space { def snapshot[T](item:java.lang.Object):Generic[T] } class TypeInferenceProblem { def example(space:Space){ space.snapshot(new java.lang.Double(20d)) space.snapshot(new java.lang.Integer(30)) }
}
error: polymorphic expression cannot be instantiated to expected type; found : [T]com.gottex.scala.types.Generic[T] required: Unit space.snapshot(new java.lang.Integer(30))

Whatever the return type of space.snapshot is, since the method will return unit, why does the compiler care?
Best
Edmondo

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