Email list hosting service & mailing list manager

Re: Alternative formulations of keywords John Cowan 12 Apr 2006 16:26 UTC

Marc Feeley scripsit:

> For your purposes, wouldn't it be sufficient to use a rest parameter
> in the definition of foo and parse that in foo?
>    (define (foo a . rest)
>      (let ((optionals (alistify rest)))
>        ...))
> Note that this is what the reference implementation of SRFI 89 does.
> Then there is no need for the compiler to treat keywords specially
> (except they need to be self evaluating).

Well, that amounts to using a disembodied property list rather than an
a-list to represent the keys and values.  P-lists require fewer pairs,
but all R5RS systems come with a-list manipulating functions as standard
equipment.  I'm okay with either provided one is standardized.

> Also, consider the function definition:
>    (define (f a b #!key (y 11) (z 22)) ...)
> and the call:
>    (f x: 33 z: 44 y: 55)
> In this case, I want parameter a to be bound to x: and b to be bound
> to 33.  I don't see how this would work with your proposals.

I consider that to be a Wrong, namely, a mismatched number of arguments.
If you wish to do that, write (f 'x: 33 z: 44 y: 55).  Keyword syntax
is reserved for keyword-argument construction.

> In summary, I fail to understand which problem you are trying to
> solve with proposal #2.

It eliminates keywords as a run-time object in favor of symbols, and
provides a standard run-time representation of a sequence of keywords
and values as a list of some sort.

> I also don't understand proposal #3.

In proposal #3, the use of keywords in procedure calls means that you
are calling a procedure of a different *name*, and the use of them in
DEFINE-definitions (as opposed to general lambda lists) means that you
are simultaneously defining 2^k variants with separate names.  Since the
mapping to the name is documented, you are free to define overriding
individual variants, and free to use the special name directly.

Thus (from x delete: y test: z) is syntax sugar for (from:delete:test:
x y z), and (define (from x #!key delete test) ... ) defines the names
last of these, variable(s) are locally bound to the default value.

A mosquito cried out in his pain,               John Cowan
"A chemist has poisoned my brain!"    
        The cause of his sorrow       
        Was para-dichloro-
Diphenyltrichloroethane.                                (aka DDT)