On Sat, Oct 19, 2019 at 10:39 PM Marc Nieper-Wißkirchen <xxxxxx@nieper-wisskirchen.de> wrote:

Think of a procedure `loggable' on may want to implement with keyword
arguments. `loggable' shall take a procedure `f' and return a new
procedure `g', which shall have the same interface as `f' except that
`g' takes an extra keyword argument, say `logger:'. When `g' is called
without the `logger:' argument, it shall foward the call to `f''.
However, when the `logger.' argument is provided, `g' shall log to
this logger that it is calling `f' before it is doing the actual call.

As `f' may already be a procedure taking keyword arguments, it is here
where hygiene is helpful because the symbol (!) `logger:' may clash
with an already existing argument to `f'.

Using hygiene only solves part of the problem.

These are the cases hygiene can't help:

(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 f1 f1:logger: foo)
      (g f2 f2:logger: foo)

   How can 'g' peek 'logger:' keyword argument?

(2) 'g' want to call inner 'f' adding 'test:' keyword argument; it only knows 'f' accepts 'test:'.
     The caller imports f1 and f2 from different libraries.  How 'g' can pass both f1 and f2
     the 'test:' arguments, not knowing which library they are imported?

(3) You want to read a list of arguments from file and pass them to 'z'.  'z' takes keyword
     arugments.  If keywords should be identifiers, how you can convert symbols read from
     the file to appropriate identifiers?

Besides, in your example, the implementor of 'g' *knows* it can shadow 'logger:', so it is
a part of API.  Either the API states you can't pass 'logger:' to 'f', or it can provide
an alternative way (e.g. 'extra-arguments:' keyword that takes a list of arguments to be passed
to 'f'). 

I claim hygiene isn't a right tool to solve the given problem.

Scheme is not CL and has hygienic macros. Its keyword-system shouldn't
be less elegant and beautiful than its `syntax-rules' system.

You don't need to persuade me the beauty of hygiene here.  I already agreed with that.