[racket] Bouncing default value

From: Laurent (laurent.orseau at gmail.com)
Date: Sat Feb 11 11:55:13 EST 2012


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

(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

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)?

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