Sam, hello. On 2011 Aug 16, at 23:08, Sam Tobin-Hochstadt wrote:
> 1. The error message you're getting about `apply' is wrong -- it > should instantiate just fine. I'll fix that. > 2. `apply' should work, without instantiation, in this case. Right > now, it doesn't handle fixed-arity functions applied to fixed length > lists, but I'll fix that, too. Ah, right -- thanks. I'm trying to convert a module to typed/racket, partly out of curiousity, and doing a bit of head-scratching. > But third, you're right that (String String -> String) is not a > subtype of (String * -> String), so if your instantiation had worked, > the program wouldn't typecheck. The way to see why this is is as > follows: > > (: test : (String * -> String) -> Any) > (define (test f) > (f "a" "b" "b")) > > (test (lambda: ([x : String] [y : String]) x)) > > Now, if this program type checked, which is what that subtyping would > allow, we'd get an error when we applied our function to the wrong > number of arguments. So we can't allow that subtyping. OoooKay. I was thinking of subtyping in terms of a naive mixture of the (set) extensions of types (the set of things which are instances of (List String String) is a subset of the things which are instances of (Listof String)), and assignment-compatibility. That understanding clearly fails in your example, because while (List String String) can be assigned to a (Listof String), the function test is declared to accept -- in the sense of have bound to its arguments -- (Listof String) which in this case isn't true. Gotcha (I think). I think I do need to have another go at the Little MLer or Haskell, unless there's a nice compact introduction to the formalities of types someone could point to (and I want to see alphas and betas in there!). Thanks again. Best wishes, Norman -- Norman Gray : http://nxg.me.uk School of Physics and Astronomy, University of Glasgow, UK _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
