[racket] TR: scope of universally quantified type variables is ... very strange?

From: John Clements (clements at brinckerhoff.org)
Date: Fri Jan 6 20:24:29 EST 2012

On Jan 5, 2012, at 5:32 PM, Sam Tobin-Hochstadt wrote:

> On Thu, Jan 5, 2012 at 11:56 PM, John Clements
> <clements at brinckerhoff.org> wrote:
>> I wrote a piece of code like this without thinking:
>> #lang typed/racket
>> (: f (All (T) (Number -> T -> T)))
>> (define ((f x) y)
>>  (ann y T))
>> ... and then, after a second, was sort of flabbergasted that it worked.  What's the scope of the type variable T? Apparently I can use it anywhere in the definition of the identifier that it names? That seems really fragile. Am I misunderstanding this?
> You're correct about the scope, and the fact that you wrote it without
> thinking, and it just did the right thing, is precisely what I was
> going for.  What's fragile here?

Okay, yes, it makes sense.  The scope is not defined by a pair of parentheses, but I guess I can get past that; after all, our internal defines are headed in that direction, too.


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