Hi everyone,
I'm writing a macro-generating macro where the generated macro uses
`syntax-parse`, and I can't figure out how to work with attributes using
the dot syntax where the base identifier is generated by the outer macro.
For example, consider the following macro:
(define-syntax example-macro
(syntax-parser
[(_ (~alt (~optional (~seq #:a (~var a
(expr/c #'symbol?)))
#:defaults ([a.c #''default-a]))
(~optional (~seq #:b (~var b
(expr/c #'string?)))
#:defaults ([b.c #'"default-b"])))
...)
#`(list a.c b.c)]))
I have a macro like `example-macro`, but more complicated and with many,
many more potential keyword arguments, so I wanted to write a macro that
would let me define `example-macro` with a more declarative syntax, like
this:
(define-example-macro/working generated-example-macro
[#:a 'default-a symbol?]
[#:b "default-b" string?])
My initial attempt looked something like this:
(define-syntax (define-example-macro/buggy stx)
(define-syntax-class arg-clause
(pattern [kw:keyword default:expr contract:expr]))
(syntax-parse stx
[(_ name clause:arg-clause ...+)
;#:with (arg ...) (generate-temporaries #'(clause.kw ...))
;#:with (arg.c ...) (generate-temporaries #'(clause.kw ...))
#`(define-syntax name
(syntax-parser
[(_ (~alt (~optional
(~seq clause.kw (~var arg (expr/c #'clause.contract)))
#:defaults ([arg.c #'clause.default]))
...)
(... ...))
#`(list arg.c ...)]))]))
which raises a syntax error "syntax: no pattern variables before ellipsis
in template".
The problem seems to be that `arg.c` is not recognized as a pattern
variable derived from `arg`. I tried several variants on this, none of
which allowed me to refer to `arg.c`. For example, if I uncomment the two
`#:with` lines, I get the error "~optional: attribute bound in defaults but
not in pattern".
I eventually figured out a tricky way to avoid using dotted attributes
altogether (see below), but it's rather less elegant than the non-working
way. I am hoping there is a way to generate the right identifier for
`arg.c` somehow.
Thanks,
Philip
(define-syntax (define-example-macro/working stx)
(define-syntax-class arg-clause
(pattern [kw:keyword default:expr contract:expr]
#:with arg (generate-temporary #'kw)
))
(syntax-parse stx
[(_ name clause:arg-clause ...+)
#:with add-contract (generate-temporary 'add-contract)
#:with (unsyntax-arg.c ...)
#`(#,@(for/list ([arg-stx (in-list (syntax-e #'(clause.arg ...)))]
[contract-stx (in-list (syntax-e #'(clause.contract
...)))])
(with-syntax ([arg arg-stx]
[contract contract-stx])
#'#,(add-contract #'arg #'contract))))
#`(define-syntax (name inner-stx)
(define (add-contract arg-stx contract-stx [err-name "name goes
here"])
(syntax-parse arg-stx
[the-arg
#:declare the-arg (expr/c contract-stx
#:name err-name
#:macro #'name)
#'the-arg.c]))
(syntax-parse inner-stx
[(_ (~alt (~optional
(~seq clause.kw clause.arg)
#:defaults ([clause.arg #'clause.default]))
...)
(... ...))
#`(list unsyntax-arg.c ...)]))]))
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