Proposal: Improved Wildcard Syntax for Java

[Joseph D. Darcy]

Hello.

While the existing "? extends" and "? super" syntax is certainly a bit 
obtuse (but better than "+" and "-"!), I don't find introducing 
redundant ways of declaring those semantics attractive at this point.

-Joe

Neal Gafter wrote:
> Improved Wildcard Syntax for Java
> http://docs.google.com/Doc?id=ddb3zt39_79g2mtpccz
>
> AUTHOR(S):
>
> Neal Gafter
>
> OVERVIEW
>
> The current syntax for Java's wildcards is confusing enough that
> mneumonics are required to read code.  We suggest adding an
> alternative syntax that is much more readable, and then migrating code
> to the more readable syntax over time.
>
> FEATURE SUMMARY:
>
> [Note: since this is an additional syntax for a feature already in the
> language, most existing tutorial material can be used.]
>
> A covariant wildcard generic argument can be written either "? extends
> Y" or "out Y".
>
> A contravariant wildcard generic argument can be written either "?
> super Y" or "in Y".
>
> MAJOR ADVANTAGE:
>
> While the existing syntax is mneumonic from the point of view of
> describing its effect on the type system, it is not mneumonic in its
> use.  The new syntax is self-mneumonic for users.
>
> MAJOR BENEFIT:
>
> Jave code that uses wildcards is more readable, as are diagnostics
> involving them.
>
> MAJOR DISADVANTAGE:
>
> Two ways of doing the same thing, though we hope code will migrate
> over time toward the new more readable syntax.
>
> ALTERNATIVES:
>
> None.
>
> EXAMPLES
>
> SIMPLE EXAMPLE:
>
> interface Collection<E> {
>     boolean addAll(Collection<out E> c);
>     ...
> }
>
> ADVANCED EXAMPLE:
>
> class Collections {
>     public static <T extends Comparable<in T>> void sort(List<T> list) {...}
>     public static <T> void sort(List<T> list, Comparator<in T> c) {...}
>     int binarySearch(List<out Comparable<in T>> list, T key) {...}
>     public static <T> void fill(List<in T> list, T obj) {...}
>     public static <T> void copy(List<in T> dest, List<out T> src) {
>     ...
> }
>
> DETAILS
>
> SPECIFICATION:
>
> "in" and "out" are added as context-sensitive keywords with precisely
> the same meaning of "? super" and "? extends".  As context-sensitive
> keywords, they continue to be usable as identifiers in all contexts.
>
> COMPILATION:
>
> As today.
>
> TESTING:
>
> As today, though a bit of additional work to ensure that the context
> sensitive keyword is handled properly by the compiler.  Specifically,
> that they continue to be fully usable as identifiers in other
> contexts.
>
> LIBRARY SUPPORT:
>
> None.
>
> REFLECTIVE APIS:
>
> None.
>
> OTHER CHANGES:
>
> None.
>
> MIGRATION:
>
> A trivial textual substitution can be used to translate old code to
> the new syntax.
>
> COMPATIBILITY
>
> BREAKING CHANGES:
>
> None.
>
> EXISTING PROGRAMS:
>
> No impact.
>
> REFERENCES
>
> EXISTING BUGS:
>
> None
>
> URL FOR PROTOTYPE (optional):
>
> None
>
>