I believe your intermediate expression there, the one with two "ann"otations in it, is a Typed Racket bug. You should definitely not be able to take something with the type (Listof Symbol) and coerce it to (List Symbol Symbol). For instance:
> (: f ((Listof Symbol) -> (List Symbol Symbol))) > (define (f x) x) stdin::124: Type Checker: Expected (List Symbol Symbol), but got (Listof Symbol) in: x Carl Eastlund On Wed, Dec 22, 2010 at 3:33 PM, Neil Van Dyke <[email protected]> wrote: > Carl Eastlund wrote at 12/22/2010 03:06 PM: >> >> So each time you apply cons, if you don't want a Listof type you have to >> "convince" it to build something else. Do this by either annotating each >> step, or binding cons with a simpler type. >> > > Thanks for the explanation. I found that annotating each step in the > building of lists with "cons" works, but I was a surprised that was > necessary. > > More importantly, I don't understand why the following works without > signaling an error... > >> (list 'a 'b) > - : (List 'a 'b) > (a b) >> (ann (list 'a 'b) (Listof Symbol)) > - : (Listof Symbol) > (a b) >> (ann (ann (list 'a 'b) (Listof Symbol)) (List Symbol Symbol)) > - : (Listof Symbol) > (a b) >> (ann (list 'a 'b) (List Symbol Symbol)) > - : (List Symbol Symbol) > (a b) > > ...yet the following does get errors when trying to do the same coercion > from "(Listof Symbol)" to "(List Symbol Symbol)": > >> (cons 'a (cons 'b '())) > - : (Listof (U 'a 'b)) > (a b) >> (ann (cons 'a (cons 'b '())) (Listof Symbol)) > - : (Listof Symbol) > (a b) >> (ann (cons 'a (cons 'b '())) (List Symbol Symbol)) > . . Type Checker: Polymorphic function cons could not be applied to > arguments: > Types: a (Listof a) -> (Listof a) > a b -> (Pairof a b) > Arguments: 'a (Listof 'b) > Expected result: (List Symbol Symbol) > in: (cons (quote a) (cons (quote b) (quote ()))) >> (ann (ann (cons 'a (cons 'b '())) (Listof Symbol)) (List Symbol Symbol)) > . . Type Checker: Polymorphic function cons could not be applied to > arguments: > Types: a (Listof a) -> (Listof a) > a b -> (Pairof a b) > Arguments: 'a (Listof 'b) > Expected result: (List Symbol Symbol) > in: (cons (quote a) (cons (quote b) (quote ()))) > > > -- > http://www.neilvandyke.org/ _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
