Return 'this' proposal

Reinier Zwitserloot reinier at
Tue Mar 17 17:07:10 PDT 2009


What's the coin viability for a proposal that does NOT change the type  
system, but just introduces syntax sugar that translates:



TypeOfExpression $unique = expression;

where the setBar() method has a void return type, but has also been  
ktited out with a flag that indicates its legal for call sites to do  
the syntax sugar transformation above? The way this would work is very  
similar to varargs: The method with the 'varargs flag' really doesn't  
do anything special other than have a flag. It's the callers that do  
all the work.

There are backwards compatibility issues, because most of the code  
that wants to 'return this' currently does not return void, it instead  
returns its own type, but I have one strategy for dealing with that  
issue that is simple and doesn't introduce any new keywords -  
basically, you get to add the flag to any method, and any callers will  
check if the type of the expression is tighter than the return type.  
If so, then the type of the returned value of the call is similarly  
tightened. In trade, setting the flag requires the java code to always  
"return this" for all return statements; anything else is an instant  
compiler error. The flag is set by adding 'this' as a keyword to the  
method's modifier section. This modifier is strictly inherited. So:

public abstract class MyBuilder {
     public this MyBuilder setFoo(int foo) {
         return this;

  --Reinier Zwitserloot

On Mar 17, 2009, at 23:48, Joseph D. Darcy wrote:

> Marek Kozieł wrote:
>> 2009/3/17 Joseph D. Darcy <Joe.Darcy at <mailto:Joe.Darcy at 
>> >>
>>    This 'this' proposal remains vague and imprecise.
>>    Including this type/self type in a language is a continuing area
>>    of study; for example, see the recent paper
>>    "Matching ThisType to Subtyping," Chieri Saito and Atsushi
>>    Igarashi, Kyoto University, Japan, ACM SAC 2009.
>>    There are open bugs requesting this capability. For example typing
>>    " <> this type" into a popular
>>    search engine quickly yields, amongst other hits,
>>    6479372 Add self types (type of 'this' aka ThisClass) to the  
>> language
>>    This bug discusses the size of the type system impact of this
>>    change, a magnitude far too large for Project Coin.
>>    There is no need to submit further refinements of this idea; any
>>    proposal along the lines of adding a this type will be out of
>>    scope for Project Coin.
>>    -Joe
>> I'll check it, but I'm afraid that introducing 'This' type will be
>> impossible for Java and for all other languages with Inheritance,  
>> or I
>> would say it's possible but conditions would be huge.
>> return 'this':
>> - Idea is quite simple: you use object from left side of dot as
>> returned from method, it's the same quite idea with converting void  
>> ->
>> this, but allowed only when it's written directly.
>> - Byte-code for that is other story and I'm not sure how much
>> limitation this contains.
>> Maybe you cold while what problems you see (that could help)?
> As the author and submitter of a proposal, it is your responsibility  
> to
> research, understand, and explain the consequences and implications of
> your proposal.
> -Joe

More information about the coin-dev mailing list