[racket-dev] Motivation for polymorphic opaque types

I wouldn't say you'd do it for cons cells (they are special in the
runtime). This is for structs that are declared in TR. You would add the
field everywhere (or places where the programmer asks only).

The boundaries have the information; that's how the contracts got inserted
in the first place.

Robby


On Sun, Jan 6, 2013 at 2:03 PM, Sam Tobin-Hochstadt <samth at ccs.neu.edu>wrote:

> On Sun, Jan 6, 2013 at 2:55 PM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
> > What if you added an extra field to immutable values (ie all TR structs
> > would have this hidden field) such that, when they passed across a
> boundary,
> > the field got mutated to say what type it passed at. Then, when the value
> > passed back into TR, TR could check to see if has this secret field and
> what
> > the type was and then avoid more checking.
>
> The problem is that this info isn't available anywhere (and I'm not
> sure it's really sensible).  Consider the `cons` function (on lists
> only).  If you write `(cons 'hello empty)` in untyped code, what type
> should get inserted, and where should that be calculated?  This
> calculation can't really be done both automatically and correctly
> ahead of time, and would add a bunch more expense.
>
> Sam
>
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