Fwd: Closures: converting closures returning void, and interfaces vs. one-abstract-method abstract classes

Neal Gafter neal at gafter.com
Sun May 11 20:20:40 PDT 2008

Forwarded FYI

---------- Forwarded message ----------
From: Neal Gafter <neal at gafter.com>
Date: Sun, May 11, 2008 at 3:33 PM
Subject: Re: Closures: converting closures returning void, and interfaces
vs. one-abstract-method abstract classes
To: pholser at alumni.rice.edu

(1) if probably a bug.  I'll look at fixing it and get back to you.

For (2), where in the closure syntax does one put the constructor parameters
and the value of the class' enclosing instance?  A closure that does not
capture state can be hoisted to a statically allocated instance; if it were
of a class type, when are its instance variables initialized?  The scoping
rules would not allow referencing other members from the closure's body (it
is lexically scoped), so it is not as useful as you might expect even if

On Sun, May 11, 2008 at 2:54 PM, Paul Holser <pholser at gmail.com> wrote:

> Hi Neal,
> I enjoyed your "Closures Cookbook" talk at JavaOne.  I've started
> experimenting with the BGGA prototype (dated 2008-04-27), and had a
> couple of questions.  I have an existing Java library that uses
> "function types", and wanted to see whether closure conversion would
> work with it "out of the box", so to speak.
> Attached are source for a Range class, a UnaryFunction interface, and
> a Main that uses both.
> 1) When the signature of Range#forEachDo is:
>    void forEachDo( UnaryFunction<? super Integer, ?> operation )
> Main fails compilation with:
>  Main.java:3: forEachDo(UnaryFunction<java.lang.Integer,?>) in Range
> cannot be applied to ({int => void})
>        Range.fromTo( 1, 4 ).forEachDo( System.out#println(int) );
>                                    ^
> 1 error
> When I change the signature to:
>    <R> void forEachDo( UnaryFunction<? super Integer, ? extends R>
> operation )
> Main compiles and runs fine.
> What is it about the second signature that allows a void-return-type
> closure in as a parameter to forEachDo(), whereas such a closure is
> denied entry to the method with the first signature?
> The good news is that I can change the signature of forEachDo to the
> second version, and my test code, etc. compiles fine.
> 2) Would you elaborate a bit on some of the technical reasons why an
> abstract class with one abstract method cannot be the target type of a
> closure conversion?  In other words, why would the following
> definition of UnaryFunction not allow closure conversion:
>    abstract class UnaryFunction<A, R> {
>        abstract R evaluate( A argument );
>        final <T> UnaryFunction<T, R> compose( UnaryFunction<? super
> T, A> other ) {
>            // composition...
>        }
>        // ... other concrete methods...
>    }
> Thanks for your time and great work!
> --p
> --
> "To only a fraction of the human race does God give the privilege of
> earning one's bread doing what one would have gladly pursued free, for
> passion. I am very thankful."
> -- Fred Brooks, "The Mythical Man-Month"
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