Would this help you, suitable wrapped up as a macro:
#lang racket
(struct f (g default) #:property prop:procedure (struct-field-index g))
(define f* (f (lambda (x [y ((f-default f*))]) (+ x y)) (lambda () 99)))
(define (f-wrapper f-ish)
(lambda (g [d ((f-default f-ish))]) ;; all you need to know here is that
there is a default argument and it is computed from a thunk
(displayln `(wrapped ,g))
(f-ish g d)))
(define f-wrapped (f-wrapper f*))
On Feb 11, 2012, at 11:55 AM, Laurent wrote:
> Hi,
>
> Often I'd like to make a wrapper for a given function that has default values
> (possibly with keywords).
> So if I want my function to have the same default values for one particular
> optional argument, I need to redefine it myself.
>
> For example, suppose there already exists a function foo like this:
>
> (define (foo arg1 [arg2 <some-complicated-default-value>])
> ....)
>
> And I want to make a wrapper for foo on arg1, but keep foo's arg2 default
> value:
>
> (define (bar [arg2 <the-same-complicated-default-value>])
> (foo 5 arg2))
>
> But I don't feel right when I need to redefine the default argument, for a
> few reasons:
> - I feel bar should not need to know foo's arg2 default value.
> - if arg2 has a complicated default argument, it is "painful" and error-prone
> to rewrite it (and if the docs for the default value for arg2 are simplified,
> it may be problematic),
> - if the specification of foo's arg2 changes, I need to change bar too
> (though sometimes bar's arg2 needs not reflect the change, but often it does),
>
> I would prefer something like:
>
> (define (bar [arg2 (get-default-value foo arg2)])
> (foo 5 arg2))
>
> which is transparent w.r.t. foo.
>
> Does this feature exist, or does it even have a name? Or can it be easily
> implemented (maybe keywords could help)?
>
> Thanks,
> Laurent
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