Subject: Re: typechecker wish
From: Craig Chambers (chambers@cs.washington.edu)
Date: Tue Jan 16 2001 - 14:50:41 PST
Here is more from Vass, with my responses.
Vassily Litvinov wrote:
>
> Craig,
>
> Here are a couple of random remarks that came to my mind.
>
> 1) It sounds like a situation similar to the one where you want to be able
> to access type variables of an upper bound of a type variable has already
> occured before, namely in the library object "collector." Somehow the
> solution that we discovered today is the same as the one used there.
Interesting.
>
> 2) Consider a parameterized declaration with implicit type parameters,
> e.g., forall A,B,C where A<=Graph[B,C]: object Graph1[A].
> Then, when a method's formal has type Graph1[`A], we now know that the
> typechecker creates fresh type variables for that method that correspond
> to B and C of the "object Graph1[A]" declaration.
>
> Soooo in this case there obviously is a desire to allow the programmer to
> refer to those fresh type variables - instead of having the programmer
> introduce his own versions of those. Are you interested in coming up with
> some syntax for that?
I think that just writing down the upper bounds explicitly is fine enough. I.e.,
instead of just Graph1[`Z], the user would write Graph1[`Z <= Graph[`X, `Y]] (I
changed the parameter names, to emphasize that they're different type variables
than the implicit internal ones used by the type checker itself). As we
discussed, I don't see any advantage for the user in trying to merge the
internal and external type variables.
>
> There's a caveat here, though. While it is clear which type variables are
> being referred to in case of type/object decls, it is not so for message
> sends: different signatures/implementations of the same message may have
> different sets of implicit type variables (and I cannot think of any way
> for the programmer to specify which of those sets he is referring to). So
> the trick may only work for types, not messages. (This is probably the
> right thing for message implementations, as their implicit type variables
> should not be visible to clients. But you cannot say that about message
> signatures.)
I didn't understand this. Maybe you could offer an example or two?
>
> 3) Coming back to your example where the typechecker knows the result type
> of a message but the programmer cannot write it. It is perhaps obvious,
> but one can at least force the typechecker to display this type. Simply
> assign the value returned by the message to a variable with type "none".
> The type error will include the type of the expression being assigned to
> the variable.
But in the case of an internal type, it will print out in a form that isn't a
legal type that the user could write down, so it doesn't solve the problem I was
pointing out.
>
> Vass
-- Craig
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