On 11/6/18 11:31 AM, Alexis King wrote:
On Nov 5, 2018, at 20:01, Ryan Culpepper <ry...@ccs.neu.edu> wrote:

You could use a chaperone to prohibit `struct-info`

Good point! I had forgotten that `struct-info` is a chaperoneable operation. 
This isn’t ideal, though, since I don’t think `struct-info` is ever actually 
supposed to raise an error, it’s just supposed to return #f, instead. It’s 
possible to call `struct-info` on absolutely anything, even things like 
numbers, and get a result:

    > (struct-info 3)

If I understand correctly, this is because all values in Racket are “morally 
structures”, and reflective operations on values that are not actually 
implemented with structures should behave identically to reflective operations 
on values for which you do not have a sufficiently powerful inspector to 

Unfortunately, it isn’t possible to regain this behavior with 
`impersonate-struct`, since `impersonate-struct` does not allow you to 
impersonate `struct-info`.

One solution is to delay the error by having `struct-info` return a chaperoned struct-type (using `chaperone-struct-type`) that makes `struct-type-make-constructor` raise an error. The struct-type chaperone should also prevent the creation of substruct instances (say, by adding a guard that always raises an error).

It seems like it should also be reasonable to let the `struct-info` interposition replace a struct type with #f, since that represents less information at the level of the *interpretation* of the operation. So it's morally a projection wrt the information ordering for the `struct-info` operation, even though it's not a projection wrt the generic information ordering for Racket values.


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