> On Mar 3, 2020, at 8:19 AM, Lassi Kortela <xxxxxx@lassi.io> wrote:
>
>> I think we should make a distinction between the API thar R7RS-large will provide and the extensibility of the language by the users.
>> While for the R7RS-large API, the default value of #f will just be fine and will ease programming with the provided libraries, this does not necessarily mean that we have to prevent users from providing non-"#f" defaults in their own libraries.
>
> You can always give your own "real default value" using let:
>
> (define/kw (download url (scheme port))
>  (let* ((scheme (or scheme (url-scheme url) "http"))
>         (port   (or port (default-port scheme))))
>    ...))
>
> and indeed, people do it all the time in systems like Emacs.
>
> The only real difference is when you need to define a procedure where passing in `#f` means something distinct from an absent argument. In my opinion, it's a design mistake in the procedure if it needs to make that distinction.

Perhaps in general, but there’s at least one case where it is useful… knowing which parameters were passed to give error messages that correspond to what the programmer wrote.  For example if you have this definition

(define/kw (f str1 str2 (foo)) …)

and str1 and str2 must be strings and foo can be anything, then you want the call

(f "a-string" 'sym)

to give the error message

ERROR -- argument 2 of (f "a-string" 'sym) is not a string

not the message

ERROR -- argument 2 of (f "a-string" 'sym foo: #f) is not a string

So in the case you want to give precise error messages you want to know if the caller supplied a value for a keyword parameter or relied on the default.  The issue is the same for optional positional parameters.

Marc