[plt-scheme] struct copy with optional arguments

On Apr  2, Jon Rafkind wrote:
> 
> ..BTW it would have been cool if I could have done
>   (define/kw (copy obj #:key (field (field-accessor obj)) ...)
> 
> instead of adding that ugly if. Can kw support something like that?

It does, both the 372 kw library:

  > (require (lib "kw.ss"))
  > (define/kw (foo a #:key [b (add1 a)] [c (* b 2)]) (list a b c))
  > (foo 1)
  (1 2 4)
  > (foo 1 #:b 10)
  (1 10 20)

and the built-in support in 3.99:

  > (define (foo a #:b [b (add1 a)] #:c [c (* b 2)]) (list a b c))
  > (foo 1)
  (1 2 4)
  > (foo 1 #:b 10)
  (1 10 20)

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