2011/12/2 Sam Tobin-Hochstadt <sa...@ccs.neu.edu> > On Fri, Dec 2, 2011 at 6:26 PM, Ismael Figueroa Palet > <ifiguer...@gmail.com> wrote: > > Sam, first I want to thank you for all your answers, you've helped me a > lot. > > Thanks! > > > >> > I'm trying to implement a function wrapper, that given any function > >> > returns > >> > a function of the same type, but that maybe applies a different > >> > argument. So > >> > far I haven't been successful, and based on previous questions to the > >> > mail > >> > list it seemed that I needed to go to an untyped module because I > >> > couldn't > >> > constraint values to a given type. > >> > >> In general, I don't think that any solution is going to work here. > >> You can't take arbitrary functions and apply them to specific > >> arguments (such as the string "blabla") in a type-safe way. > > > > > > For what I know, the design goal of Typed Racket is to avoid all runtime > > errors. Maybe that is too strict for programs like the one I intend to > make. > > The idea of proposing a cast operation is to be able to establish a > tradeoff > > between type safety and expressive power, according to the programmer > needs. > > > >> > >> In you particular example, the function `f' is a constant function. > >> But what if it expected `Integer' as its argument? Then your untyped > >> code would be violating the type invariants. > > > > > > I think in that case a contract violation error should happen (?) > > > >> > >> > >> However, you can make your program work just by removing the > >> parameterization over `A', and replacing that with `Any'. Then your > >> program is safe, and your untyped module can safely call the function > >> with any value it wants. > > > > > > In that case, my program can only work with values of type Any, and I'd > need > > to make all functions Any -> (T Any) since I can't downcast to any other > > type, but since I'm passing functions as arguments I will have the > problem > > of subtyping between functions... > > > >> As an additional note, the `CAST' operation you describe below, can't > >> do exactly what you expect, because it's runtime semantics depends on > >> how you instantiate `B'. But that would mean that you'd have to > >> generate code for the `CAST' without knowing what `B' is. > > > > > > do you think it can work if I do (CAST f (Integer -> Boolean)) instead? > > Yes, this could work. I haven't added `cast' yet, but I'll try to do that > soon. > > > Maybe the flaw in my reasoning is that I think that when you evaluate the > > polymorphic function, then you know what A and B are... so you should be > > able to cast a value to B, is not that so? > > > > is it that you statically prove that the body is OK for all A and B and > then > > you don't get that information when applying?? > > Yes, this latter description is how Typed Racket works. > > >> > >> Typed > >> Racket, like ML or Haskell, doesn't pass type parameters around as > >> values, so you can't do this. > > > > > > I don't know much about how the type system of ML or Haskell works. > > > > What I'm trying to do, so you get the full picture, is to write a > variation > > of Aspect Scheme in Typed Racket, so you can statically check that > advice is > > well-typed. I couldn't type correctly the weaver function, so I tried > with > > an untyped module. Maybe it is not possible at all?? > > That sounds neat. However, I'm suspicious that you won't be able to > type check everything that people currently write in Aspect Scheme > without changing some part of Typed Racket to understand your system > better. >
It seems so... the biggest problem for now is that either I can't construct the woven function with the same type as the original one, or I can't apply it :-p (the problem at the origin of the thread) I think the weaver should have type (All (A B) ((A -> B) jp aspect-env -> (A -> B))) and that's what I've been trying to do, unsuccesfully. Any ideas for a reduced version that might work? I was thinking of using a union type with a certain subset of types, and then manually check one-by-one to produce the expected result. Thanks -- Ismael
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