[racket] a question of style, and one of performance

I suppose it could prevent certain optimizations in parameter passing, forcing space for arguments to be allocated on the heap. 

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On Jan 8, 2012, at 12:42 PM, Danny Yoo <dyoo at cs.wpi.edu> wrote:

> On Sun, Jan 8, 2012 at 3:23 PM, Jordan Schatz <jordan at noionlabs.com> wrote:
>> This code runs, but I'm guessing that its not the "right way" to do it.
>> 
>> (define (js-date [i (current-date)])
>>  (let ([original-format (date-display-format)]
>>        [return ((λ ()
>>                     (date-display-format 'rfc2822)
>>                     (date->string i #t)))])
>>    (date-display-format original-format)
>>    return))
>> 
>> 1) In "some other language" using a function as the default value for an
>> argument is inefficient and frowned upon. Is that the case in racket?
> 
> Hi Jordan,
> 
> Can you give an example of such a language?  I'm curious.
> 
> I'm not sure where the inefficiency would come from, unless computing
> the default value expression's value is costly.
> 
> According to the documentation in:
> 
> http://docs.racket-lang.org/reference/lambda.html#(form._((lib._racket/private/base..rkt)._lambda))
> 
> with regards to "default-expr": "... if no such argument is provided,
> the default-expr is evaluated to produce a value associated with id."
> 
> From the reference docs, it sounds like that, unlike a language like
> Python, the default value is evaluated for every use of the function,
> rather than just once when the function's defined.  We can experiment
> with this: