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

[Maxime Devos]

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 (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 ...))



Greetings,
Maxime.

(define slot-machine "todo: use define-syntax + identifier-syntax + syntax-error etc.") 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!).

OpenPGP_0x49E3EE22191725EE.asc
Description: OpenPGP public key

OpenPGP_signature
Description: OpenPGP digital signature