Alexander, I am bit confused, but is the following what you want: #lang typed/racket
(struct: (α β) posn ((x : α) (y : β))) (define-type (Posn x y) (posn x y)) (define-type Origin (Posn Zero Zero)) ; (: O Origin) ;; works with or without commenting out this line (define O (posn 0 0)) On Apr 25, 2014, at 11:06 PM, Alexander D. Knauth wrote: > Then is there something like and/c for types? (like U is like or/c for types) > That could probably solve it. > > It seems like typed racket is already doing this in some cases, for example: > #lang typed/racket > > (define positive-real? (make-predicate Positive-Real)) > > (define (f x) > (if (positive-real? x) > (if (exact-integer? x) > x > (error "error")) > (error "error"))) > > > f > - : (Any -> Positive-Integer) > #<procedure:f> > In this, typed racket figures out that f produces a Positive-Integer, when it > was only given that it produces something that is both a Positive-Real and an > Integer. > > But I couldn’t find it in the docs. > > On Apr 25, 2014, at 10:48 PM, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> > wrote: > >> 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 > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users
____________________ Racket Users list: http://lists.racket-lang.org/users
