Your second suggestion is described in this (hard to find) section of
the TR Guide on Type Generalization:
I agree it should be much more prominent. At the very least, any docs
for invariant constructors should link to that section I think?
On Fri, Feb 14, 2020 at 2:41 PM 'John Clements' via users-redirect
> I think I may understand what’s going on here, but a student and I worked on
> this for quite a while today before I found the problem.
> Here’s a program:
> #lang typed/racket
> (define-type Store (Mutable-HashTable Integer Value))
> (define-type Value (U Real Boolean String))
> (define top-store (cast (make-hash (list
> (cons -1 14)
> (cons 1 #t)
> (cons 2 #f)))
> (hash-set! top-store 5 1234)
> It fails with this error:
> contract violation
> expected: (or/c (and/c byte? positive?) #t #f)
> given: 1234
> in: the values of
> the 3rd conjunct of
> (or/c (and/c byte? positive?) #t #f)
> contract from: typed-world
> blaming: cast
>(assuming the contract is correct)
> at: unsaved-editor:6.18
> If I understand what’s going on here, the basic issue is that the mutable
> hash table’s type is being inferred as (Immutable-HashTable Exact-Integer (U
> Boolean Positive-Byte)), and then as part of the cast, a contract is being
> inserted, which checks that all added values match the expected value type.
> The outer cast allows type checking to proceed, but then at runtime it fails
> because the given value doesn’t match the inferred value type.
> This error doesn’t occur with immutable hash tables, because it’s fine to
> extend an immutable hash table to a larger one that contains it; the original
> one’s contract isn’t violated.
> In this case, one easy error is to change the ‘cast’ into an ‘ann’, which
> works fine.
> This is the first time I’ve encouraged my students to use a mutable hash,
> which is presumably why I haven’t encountered this before.
> I’m trying to formulate a solution that I can put in a Hints for TR file, and
> I think the answer is probably this:
> - When you use “make-hash” in TR, you should always specify the types
> explicitly using an “inst”.
> or maybe
> - When you call “make-hash” in TR, the call should be immediately wrapped
> with an “ann” type annotation.
> In a perfect world, I think I would ask for a warning when casting a mutable
> hash table. It could go in that nice “warnings” box. Oh, wait…
> … joking aside, actually I just did turn on the “log” window and I don’t see
> any warning about this, which is not too surprising. Oh, wait, I see another
> Thanks for reading, let me know if there’s any more obvious solution.
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