Am So., 20. Okt. 2019 um 23:41 Uhr schrieb Shiro Kawai <xxxxxx@gmail.com>:
>
> On Sun, Oct 20, 2019 at 11:04 AM Marc Nieper-Wißkirchen <xxxxxx@nieper-wisskirchen.de> wrote:
>>
>>
>> > (1) Suppose 'g' wants to "peek" logger argument, that is,  'g' tries to read the
>> >     value of 'logger:' argument, but want to pass it to inner 'f'.
>> >     The caller imports two procedures, f1 and f2, both takes 'logger:' keyword argument,
>> >     from different library.  So it must rename at least one of them.
>> >       (import (rename (f1-library) (logger: f1:logger:))
>> >                   (rename (f2-library) (logger: f2:logger:))
>> >                   (g-library))
>> >
>> >       (g f1 f1:logger: foo)
>> >       (g f2 f2:logger: foo)
>> >
>> >    How can 'g' peek 'logger:' keyword argument?
>>
>> This wouldn't work. But it is not surprising, is it? If `f1' and `f2'
>> both take `logger:' keyword arguments, but both come from different
>> sources, `f1' and `f2' effectively use different, incompatible
>> protocols.
>
>
> That comes back to the point I raised initially.  With hygienic keywords, the library writers cooperate
> to share the same keywords if they want compatibility.   I don't think it works in reality.
> Having test: keywords to specify the predicate would be a natural choice, and we can expect multiple,
> independently developed libraries would use it.  Do they need to cooperate _beforehand_ to share
> the same library that exports 'test:'?  What if one library already exports 'test:', and
> another library writer want to have the same semantics of 'test:' but doesn't want to depend on the
> original library?
>
> There's one obvious solution: Create a library that effectively exports all possible keywords,
> and users of keyword arguments import them.  (It is what (gauche keyword) library in Gauche
> does).  But then, where's hygiene?

If, say, `f', `f1', `f2', and `g' all intend to implement the same
`test:' protocol, then it makes sense for their respective libraries
to import the same `test:' identifier from a common library. Passing
down any value through such a `test:' argument by, say, `g', makes
only sense if the callee understands this particular type of value. In
other words, they have to cooperate beforehand anyway.

What hygiene allows is to have two different, independently created
test protocols in the same program. This won't be possible if `test:'
is a mere identifier.

Anyway, I have also proposed on this list to allow other types than
identifiers as markers for keyword arguments. Only when you want
hygiene, you would use an identifier. Otherwise, you would use, say, a
keyword object as in SRFI 88. Macro writers can do the same.

>
> That's why I'm claiming keyword arguments isn't much about identifiers but just an extension
> of ordinary arguments.  For positional parameters, the semantics of the first parameter differs
> completely among procedures, but we don't think it's conflating the protocol.  It is ok that the first
> argument of procedure f and the first argument of procedure g has completely different meanings,
> and it's callers responsibility to pass appropriate one.  If g takes a procedure that takes filename
> as the first argument, then it's caller's responsibility to pass such procedure to g.
> The same applies to keyword argument.  If g takes a procedure that takes logger: argument of
> a specific type of object, then it's caller's responsibility to pass such procedure.
>

Keyword arguments could/would make it possible to treat some procedure
arguments in a uniform way. For position arguments, this is only
possible when we do whole-program transformations (see my CPS example
from somewhere above).

Marc