I got to thinking about this, and I know that they can’t work as purely first-class values, because syntax-parse needs to know the attributes of a syntax-class at compile time, but would it be possible to have a system sort of like in racket/unit where units can be first-class values but you can attach the compile-time information like attributes to them with a form like define-unit-binding? Or would that not work for some other reason?
On Oct 21, 2014, at 3:04 PM, Jack Firth <[email protected]> wrote: > With syntax/parse, would it be possible to make syntax classes first class > values instead of just a macro extension on top of syntax parse? That way you > could parameterize syntax classes over arbitrary values, including other > syntax classes, to create composable syntax classes. For instance, I find it > very common when writing complex macros that I need to represent a series of > optional keyword clauses like so: > > (define-syntax foo > (syntax-parser > [(_ req1 req2 > (~optional (~or (~seq #:kw1 op1) > (~seq #:kw2 op2) > (~seq #:kw3 op3))) > ...) > ; do something ])) > > But this allows for items to be repeated and can be very verbose if the > clauses are complex. I can define a syntax class for each of the optional > clauses: > > (define-splicing-syntax-class op1 > (pattern (~seq #:kw1 op1-pattern))) > > (define-splicing-syntax-class op2 > ... > ... > > I'd like to also be able to define a function that takes those syntax classes > and returns a new syntax class representing an optional series of any of > those classes where each can be given once: > > (define (optional-sequence-of class-a class-b class-c) > (splicing-syntax-class > (pattern (~optional (~or (~once (~var a class-a)) > (~once (~var b class-b)) > (~once (~var c class-c)))) ...))) > > And then my original macro would just be: > > (define-syntax foo > (syntax-parser > [(_ req1 req2 (~var optionals optional-sequence-of op1 op2 op3)) > ; do something ])) > > The reason I think this would be useful is that you could re-use the > optional-sequence-of class in other macros since it's generalized. You can > make some general "template" syntax classes for common macro tasks, such as a > sequence of pairs of unbound unique identifiers with expressions (for > let-like macros) or for a definition of a function's arguments (so Racket's > definition of a function ((f a b) g) and Typed Racket's definition of a > function ((f [a : Number] [b : Number]) [g : Number Number -> Number) could > re-use a lot of the same macro plumbing). > > I don't know how possible this is, or what issues would need to be ironed out > to make it particularly useful, but I welcome any thoughts on the matter. > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
