Proposal: Improved Wildcard Syntax for Java

Reinier Zwitserloot reinier at
Sat Feb 28 23:08:41 PST 2009

This proposal is lacking in the 'disadvantages' section, in two ways.

The listed disadvantage is unfairly sugared over; because the 'out/in'  
terminology does not allow you to bind the generics parameter, the  
'extends'/'super' syntax will never become outdated and the need to  
remember that 'out' is analogous to 'extends' and 'in' is analogous to  
'super' does not go away.

There's another disadvantage that isn't listed at all: Neither 'in'  
nor 'out' are keywords in java currently. Therefore, you introduce the  
notion of a context-sensitive keyword, but as far as I understand it,  
java does not currently have context-sensitive keywords. Adding them  
is a big can of worms that should probably warrant some discussion (in  
the sense that it may complicate java parsers, and how the idea of  
context-sensitive keywords makes it a lot harder for any parser to use  
a keyword as an anchor point); at any rate it is a disadvantage that  
should be listed.

There's also a typo in 'mneumonic' :)

NB: For what it's worth, because the need to remember super-produces  
extends-consumes does not go away, I don't think this is going to help  
much. Perhaps make 'out' and 'in' valid context-sensitive keywords  
that can be used as a substitution for super and extends, and add that  
a missing generics variable on the LHS implies a wildcard?

  --Reinier Zwitserloot

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