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
