no implicit conversions for member selections like this.x?

[Adriaan Moors-2]

Hi,

I think I may have encountered a bug or at least a limitation in  
implicit conversions and I was wondering why this is so. It seems  
implicit conversions are only inferred for member selections with an  
explicit target object, different from |this|, such as |a.foo| and  
not when the target object is |this| or when it is left implicit.

I've tried to construct a small example that exhibits the same  
behaviour as in my actual code (which uses implicit conversions to  
insert the unrolling and rolling operations that witness the  
isomorphism between a recursive type and its unrolling).


implicit def foo2bar(foo :Foo) :Bar = foo.bar

class Foo(val bar :Bar) {
        def testCoercion ={ val a = this; a.baz /* here, foo2bar is inferred  
by the compiler, as expected */}
        def testCoercionThis = baz  // --> error: not found: value baz
        def testCoercionThis = this.baz  // --> error: value baz is not a  
member of Foo
}

class Bar { def baz = System.out.println("baz")}


Obviously, this is a contrived example, but it would really be useful  
in my actual code (see my earlier post http://article.gmane.org/ 
gmane.comp.lang.scala/1956/match=hodgp and http://www.cs.kuleuven.be/ 
~adriaan/?q=hodgp_scala
I'm trying to get rid of |Fix()| and the indirection via the |out|-
member, which would largely make up for the lack of recursive type  
synonyms).

I guess this could also be used to implement a kind of poor man's  
forwarding.

regards,
adriaan

[Martin Odersky]

Adriaan Moors wrote:
> Hi,
>
> I think I may have encountered a bug or at least a limitation in  
> implicit conversions and I was wondering why this is so. It seems  
> implicit conversions are only inferred for member selections with an  
> explicit target object, different from |this|, such as |a.foo| and  not
> when the target object is |this| or when it is left implicit.
>
Thanks for this observation. Indeed a view is not searched when the
method name is a simple identifier. I still have to find out why no view
is found when the prefix is `this'. In any case, I think you are right
that we should correct this, so I classify these things as bugs.

Cheers

-- Martin

> I've tried to construct a small example that exhibits the same  
> behaviour as in my actual code (which uses implicit conversions to  
> insert the unrolling and rolling operations that witness the  
> isomorphism between a recursive type and its unrolling).
>
>
> implicit def foo2bar(foo :Foo) :Bar = foo.bar
>
> class Foo(val bar :Bar) {
>     def testCoercion ={ val a = this; a.baz /* here, foo2bar is
> inferred  by the compiler, as expected */}
>     def testCoercionThis = baz  // --> error: not found: value baz
>     def testCoercionThis = this.baz  // --> error: value baz is not a  
> member of Foo
> }
>
> class Bar { def baz = System.out.println("baz")}
>
>
> Obviously, this is a contrived example, but it would really be useful  
> in my actual code (see my earlier post http://article.gmane.org/ 
> gmane.comp.lang.scala/1956/match=hodgp and http://www.cs.kuleuven.be/ 
> ~adriaan/?q=hodgp_scala
> I'm trying to get rid of |Fix()| and the indirection via the |out|-
> member, which would largely make up for the lack of recursive type  
> synonyms).
>
> I guess this could also be used to implement a kind of poor man's  
> forwarding.
>
> regards,
> adriaan
>