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An alternative proposal felix winkelmann (15 Apr 2006 21:20 UTC)
Re: An alternative proposal Alex Shinn (16 Apr 2006 07:27 UTC)

An alternative proposal felix winkelmann 15 Apr 2006 21:20 UTC

I (well, Alex was a bit quicker, actually) propose a binding form,
similar to Shiver's let-optionals, that gives you everything needed to
provide keyword-style interfaces to a procedure while not limiting them
to one particular behaviour:

[syntax] (let-keywords* <ARGLIST> ((<VARIABLE> <KEYWORD> [<DEFAULT>])
             ...) BODY ...)

The semantics should be obvious, for example:

(define (message-box title . args)
  (let-keywords* args ((title 'title "<untitled>") (buttons 'buttons '(ok)))

Default values are optional and default to #f (or something else). One
could also imagine an optional trailing "rest" argument after the last
binding to keep remaining arguments, but I think that would have to
large an impact of performance, so I personally tend to leave
much unspecified.

- the <KEYWORD> expressions are evaluated and don't necessarily
  have to be keyword objects
- the semantics are not as confusing as with DSSSL lambda-lists
  (#!key before or after #!rest ?, in what context are which variables
  visible?, etc.)
- You have more control over the actual argument processing
  (how do you implement
  (message-box "Hello!" button: "ok" button: "cancel")
  with DSSSL keyword args? Seperating out the binding form gives
  the user the chance to implement custom behaviour)
- This can be implemented on every R5RS Scheme
- You don't muck with lambda-list syntax, which I think is the
  major problem with this SRFI (and writing definition functions for
  something like generic functions, or implementing alternative
  macro systems are just a few examples where you really
  appreciate this)
- You can still provide keyword-based interfaces to the outside,
  so you get the functionality of DSSSL keywords without the
  warts (funny lambda list markers, fixed and complicated behaviour,
  unnecessary coupling to keyword objects).