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 <div class="gmail_quote">On Apr 25, 2014 10:46 PM, "Alexander D. Knauth" <<a href="mailto:alexander@knauth.org">alexander@knauth.org</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> 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 <<a href="mailto:dvanhorn@cs.umd.edu">dvanhorn@cs.umd.edu</a>> 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 <<a href="mailto:dvanhorn@cs.umd.edu">dvanhorn@cs.umd.edu</a>> 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: <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </blockquote></div>