FWIW, reasons like this is why most of the standard libraries have #f or some other simple value for a default. Then, the "real" default is computed in the body.
Robby On Saturday, February 11, 2012, Laurent <laurent.ors...@gmail.com> 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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