Hi Jérôme, `syntax-parse` was in my list to explore! Thanks for the
example! I did a new version following your example
(require
(for-syntax
racket/base
racket/syntax
syntax/parse))
(begin-for-syntax
(define-syntax-class (element-exp)
(pattern (element:id value)
#:with element? (format-id #'element "~a?" #'element)
#:with var-decl #'(define element value)
#:with pred-decl #'(define (element? v) (eq? element v)))))
(define-syntax (define-enum stx)
(syntax-parse stx
[(_ name (~var e element-exp) ...)
(with-syntax ([name? (format-id #'name "~a?" #'name)])
#'(begin
e.var-decl ...
e.pred-decl ...
(define (name? v) (or (e.element? v) ...))))]))
(module+ test
(require rackunit)
(define-enum animal (dog 1) (cat "2"))
(check-true (dog? dog))
(check-false (cat? dog))
(check-true (animal? dog))
(check-false (animal? 3)))
I removed the prefix and added a function to check if a value is a
valid enum value. When trying to write the later, I stuck on the
expression
(define (name? v) (or e.element ...))
which I knew was not what I wanted. Then I though "You know, I
really liked to write (or (e.element v) ...). You know what .. why
not give a try?". I did and it worked as a charm! I am amused by the
power of `...` in templates.
On Mon, Apr 29, 2019 at 12:37 AM Jérôme Martin
<jerome.martin....@gmail.com> wrote:
>
> Hello there!
>
> I'm really glad you're picking up Racket and enjoying it!
>
> I've been through the same paths and struggles before, so thank you very much
> for posting that piece of
> experience here :)
>
> For your information, here is a syntax-parse version of your macro, which I
> find simpler to write and understand:
> (it's basically syntactic sugar over syntax-case, but it's really useful!)
>
> #lang racket/base
>
> (provide
> define-enum)
>
> (require
> (for-syntax
> racket/base
> racket/syntax
> syntax/parse))
>
> (define-syntax (define-enum stx)
> (syntax-parse stx
> [(_ name (element value) ...)
> #:with (name-element? ...)
> (datum->syntax
> #'(element ...)
> (map (lambda (elem)
> (format-id elem "~a-~a?" #'name elem))
> (syntax->list #'(element ...))))
> #'(begin
> (define element value) ...
> (define (name-element? v)
> (eq? element v)) ...)]))
>
> (module+ test
> (require rackunit)
>
> (define-enum unit
> (dry #\d)
> (empty #\.)
> (rock #\#)
> (water #\~))
>
> (check-true (unit-dry? #\d))
> (check-false (unit-empty? #\#))
> (check-false (unit-rock? #\~))
> (check-true (unit-water? #\~)))
>
> To remove that ugly map, which requires transforming syntaxes to list then
> back to syntaxes, you can use a syntax-class:
>
> (begin-for-syntax
> (define-syntax-class (element-exp unit)
> (pattern (element value)
> #:with pred-name (format-id #'element "~a-~a?" unit #'element)
> #:with variable #'(define element value)
> #:with predicate #'(define (pred-name v) (eq? element v)))))
>
> (define-syntax (define-enum stx)
> (syntax-parse stx
> [(_ name (~var e (element-exp #'name)) ...)
> #'(begin
> e.variable ...
> e.predicate ...)]))
>
> It's a bit more complicated at first, but it becomes really powerful once you
> understand how syntax-parse and syntax-classes work.
>
> (the tricky part here is that I'm using a syntax-class that takes the unit
> name as an argument, which is not so common)
>
> Have fun ;)
>
>
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--
Abraço,
Wanderley Guimarães
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