Re: [boost] phoenix::bind

Subject: Re: [boost] phoenix::bind
From: Joel de Guzman (joel_at_[hidden])
Date: 2008-10-02 20:04:41

Joel de Guzman wrote:
> Steven Watanabe wrote:
>> AMDG
>>
>> Giovanni Piero Deretta wrote:
>>>> I'm not an expert on modern FP, but the above lambda doesn't seem 
>>>> like a
>>>> classic lambda (as in lambda calculus and Lisp) to me.
>>>>
>>>> Now that I've seen what Phoenix can do, I think that, were I to 
>>>> design a
>>>> Boost.Lambda2, I'd probably go with a classic lambda of the form
>>>>
>>>>   lambda( _x, _y )[ _x + _y ]
>>>>
>>>> where the inner _x + _y is not a function object, and it is the 
>>>> lambda[]
>>>> that turns it into one. (That is, the evaluation of an inner expression
>>>> would be done with eval(expr, args...) and not with expr(args...).)
>>>>     
>>>
>>> Oh yes please! Unfortunately I think that this would require major
>>> surgery of phoenix...
>>>   
>>
>> +1 from me for that syntax.
>>
>> I don't think that it will actually require major surgery after all.  It
>> really just requires a small adjustment of the environment.
> 
> Hmmm... I don't think Phoenix requires a major surgery for this.

Ok, Easy. It's just a matter of filtering the /bare/ variables
in the lambda declaration (using fusion! hurray) and transforming
the list of declarations (again fusion!) such that each unassigned
variable takes an argument.

This:

     lambda( _x, _y, _z = 123 )[ ... ]

will be transformed to this:

     lambda( _x = _1, _y = _2, _z = 123 )[ ... ]

I wonder though if it's a good idea to separate the signature
from the locals:

     lambda( _x, _y )[ let( _z = 123 )[ ... ] ]

seems to be more "idiomatic"? No?

In the last case, lambda just takes in the signature. Let "let" do
the locals.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net