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? 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?? > 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?? Thanks again -- Ismael
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