Re: class declaration syntax

From: David Abrahams (gclbb-jamboost_at_[hidden])
Date: 2003-05-10 17:35:09

Rene Rivera <grafik666_at_[hidden]> writes:

> [2003-05-10] David Abrahams wrote:
>
>>David Abrahams <gclbb-jamboost_at_[hidden]> writes:
>>
>>> Hmm, a hybrid approach might be:
>>>
>>> rule [ class x : y z ] ( init-params )
>>> {
>>> ... init code ...
>>> rule method1 ( whatever ) { ... }
>>> }
>>>
>>> This one could work in similar ways, installing a rule-definition
>>> hook; Although the previous approach has lots of appeal, I think
>>> this one might be the most expedient.
>
> [snip]
>
>>But I don't think that's a particularly compelling case for anonymous
>>functions; they'd be more useful in constructs like:
>>
>> # filter out elements of $(y) which aren't instances of my-class
>> x = [ sequence.filter $(y) : bind ( elt ) is-instance $(elt) my-class ]
> ;
>
> Not sure that's much of a win over a locally declared rule. 

That's the old Guido-hates-lambda argument ;-)

Remember a locally-declared rule actually goes into the module scope;
there is no rule scoping on a function-by-function basis.

> It might even be a loss as it's somewhat harder to read that the
> "is-instance $(elt) my-class" is the body of function.

You can always use braces:

x = [ sequence.filter $(y)
: bind ( elt ) { return is-instance $(elt) my-class ; } 
]

>>In all, for class definition I prefer either of the following two
>>syntaxes:
>>
>> # Overwrites __init__ in the local module; do we care?
>> class myclass : base1 base2 
>> __init__ ( param1 param2 ) 
>> {
>> base.__init__( param1 )
>> rule foo_method ( bar ) { }
>> } ;
>>
>> # No lambdas if we go this route; more syntax involved
>> rule [ class myclass : base1 base2 ] ( param1 param2 )
>> {
>> base.__init__( param1 )
>> rule foo_method ( bar ) { }
>> }
>>
>>I think the 2nd one is easier to implement, and overall I view this as
>>a low-priority issue.
>>
>>Thoughts?
>
> I think I can more easily justify to my brain the second syntax. For me it
> mentally parses as:
>
> rule [ class myclass : base1 base2 ] ( ... )
> ^ ^-----------------------------^
> | |
> | +-Evaluate this to get the rule (which happens to be the class rule)
> |
> +-Normal rule declaration.

That's exactly the way it would work, too.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com