_5.A where val _5: Out.this.In

trait Out {   import Out._  ...} object Out {  trait In {    ...   }}

I am trying to figure out why exec doesn't compile.  If I move type A into trait Out it compiles.  
trait Out {  trait In {    type A    def get() : A     def doit(a:A) : Int  }    def in : In    def exec : Any = in doit in.get}
The error message I get is:
console>:14: error: type mismatch; found   : Out.this.In#A required: _5.A where val _5: Out.this.In          def exec : Any = in.doit (in.get)   
Is there a way to make it compile without having to move the type A declaration into the outer class?
Jesse

Hey Jesse,

On Monday 26 April 2010 10:21:56 am Jesse Eichar wrote:
> I am trying to figure out why exec doesn't compile. If I move type A into
> trait Out it compiles.
>
> trait Out {
> trait In {
> type A
> def get() : A
> def doit(a:A) : Int
> }
>
> def in : In
>
> def exec : Any = in doit in.get
> }
>
> The error message I get is:
>
> console>:14: error: type mismatch;
> found : Out.this.In#A
> required: _5.A where val _5: Out.this.In
> def exec : Any = in.doit (in.get)

The issue is that 'in' is not a stable reference. In 'exec', the calls to
'in' could return different instances. The unqualified A in 'get' and 'doit'
means A according to this instance of In, not A for any instance of In.
Expanding the A in those methods:

def get() : this.A
def doit(a: this.A) : Int

The body of 'exec':

in doit in.get

translates to something like:

val in1 = in
val in2 = in
val got: in2.A = in2.get
in1.doit(got)

The problem is that 'got' is of type in2.A but in1.doit expects an instance of
type in1.A. If you use the above expansion as the body of 'exec', the error
message is much clearer:

error: type mismatch;
found : in2.A
required: in1.A
in1.doit(got)

> Is there a way to make it compile without having to move the type A
> declaration into the outer class?

Yes, here are some solutions:

// widen the type to be A for any instance of In, not just this one
def doit(a: In#A): Int

// or, define Out.in to be a stable reference
val in : In

// or, create a stable reference in 'exec'
def exec : Any = {
val i = in
i doit i.get
}

-Mark

I am trying to figure out why exec doesn't compile.  If I move type A into trait Out it compiles.  
trait Out {  trait In {    type A    def get() : A     def doit(a:A) : Int  }    def in : In    def exec : Any = in doit in.get}
The error message I get is:
console>:14: error: type mismatch; found   : Out.this.In#A required: _5.A where val _5: Out.this.In          def exec : Any = in.doit (in.get)   
Is there a way to make it compile without having to move the type A declaration into the outer class?
Jesse