What you're looking for is called bounded polymorphism. Sadly, Typed Racket doesn't support this, so I'd try one of the options David suggests.
Sam On Apr 25, 2014 10:46 PM, "Alexander D. Knauth" <alexan...@knauth.org> wrote: > That’s what I tried at first, but my actual struct is a lot more > complicated than posn, and it kept giving be type-check errors, and trying > to enforce the types of the fields myself just ended up with completely > unreadable code and even more type errors, so I gave up on making it > polymorphic, but I still wanted to be able to specify specific cases of it > as types that would only contain that specific case of it. > > On Apr 25, 2014, at 10:25 PM, David Van Horn <dvanh...@cs.umd.edu> wrote: > > > On 4/25/14, 9:57 PM, Alexander D. Knauth wrote: > >> But then the posn constructor doesn’t enforce that it’s arguments have > to be Reals, and the posn? predicate doesn’t check it, and the accessors > don’t say that they always produce Reals. > > > > Maybe I'm not seeing the big picture, but that's what the Posn type is > > for. If you apply posn to something other than reals, you won't get a > > Posn. If you have a Posn and apply posn-x, you get a real. > > > > (define: (f [p : Posn]) : Real > > (+ (posn-x p) (posn-y p))) > > > > David > > > > > >> On Apr 25, 2014, at 9:49 PM, David Van Horn <dvanh...@cs.umd.edu> > wrote: > >> > >>> How about this? > >>> > >>> (struct: (x y) posn ([x : x] [y : y])) > >>> (define-type Posn (posn Real Real)) > >>> (define-type Origin (posn Zero Zero)) > >>> > >>> > > > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users >
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