Re: boiler plate class generation, writing fresh variables with macros

[Maxime Devos <maximedevos@telenet.be>]

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On 22-07-2022 11:43, Blake Shaw wrote:

> But if I try to use `slot-machine` inside a class definition i'm out of
> luck:
>
> (define-class <dummy> ()
>    (slot-machine 'inner 'color "green"))
> => While compiling expression:
> Syntax error:
> socket:7257:0: source expression failed to match any pattern in form
> (define-class-pre-definition ((quote inner) (quote color) "green"))
> This is actually a recurring theme with my experience with Guile, working
> on a project, needing to generate boilerplate, and then being unable to
> find a result, so I figured its time I reach out to figure out what I'm
> doing wrong in this situation.
Syntax transformations in Scheme work from the outside to the inside, 
not the other way around, so you can't do things like this (define-class 
doesn't know what to do with this 'slot-machine' thing, it will reject 
it for not being a valid slot definition). However, you can define a 
syntax that generates the surrounding define-class and interprets things 
to insert the result of slot-matchine into a proper define-class form.

Something like (untested):

(define-syntax easy-define-class
    (lambda (s)
     (syntax-case s ()
        ((_ <class-name> <parents> slot ...)
         #`(define-class <class-name> <parents> #,(fixup #'slot) ...)))))
(define slot-machine "todo: use define-syntax + identifier-syntax + 
syntax-error etc.")
(define (fixup slot)
   (syntax-case slot (slot-machine)
     ((slot-machine category quality value)
      (let ((sym ...)
              ...)
        #`(#:getter #,get-sym [etcetera])))
     (boring #'boring))) ; old-style slot definitions

(easy-define-class <dummy> () (slot-machine ...))

Important: fixup is _not_ a macro, just a helper procedure of the 
easy-define-class macro that happens to transform syntax!  As such, it 
needs to use 'define', not 'define-syntax'. Also, you probably can't do 
symbol-append like you are doing currently as Guile's hygiene code won't 
know the context of the generated symbol, so you'll need some 
datum->syntax + syntax->datum + symbol-append for your technically 
unhygienic 'fixup' (though a very mild form of inhygiene!).

Greetings,
Maxime.


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