> This has tanked my performance to the point where the tests for
> non-scalar types (vectors and hashes) time out on the package
> server, and thus fail.
> https://gitlab.com/HiPhish/MsgPack.rkt/commit/0b6cdc7115389db97a8de2a5175c1feb4c939f8f

The performance is probably because HashTable and Vector values can be
mutable, so Typed Racket needs to do extra work to protect them.

To test this, I cloned msgpack, removed `Vector` from the `Packable`
type, and changed `HashTable` to `Immutable-Hashtable`. Change here:

When I run the tests in test/pack/map.rkt, I see:
- 40 seconds with the types on master
- 13 seconds with immutable types
- 9 seconds on commit ac2b005 (before changing the types?)

I'm working on a pull request to add Immutable-Vector to Typed Racket.
That should be ready for the 6.12 release, and then msgpack can make
the `Packable` type immutable.

If that PR doesn't solve the performance problem, then I'd be happy to
keep looking for something that does.

> Another thing I noticed is that the predicate
> does not provide any proposition, if it succeeds the type checker is not
> informed that the object is of type `Packable`.

It *should* be possible to make the predicate a proposition by
changing the type of `packable?` to `(-> Any Boolean : Packable)`. But
I couldn't get this to type check.
Here's a simpler case that did work: http://pasterack.org/pastes/92542

But it's probably easier to just use `(define-predicate packable? Packable)`

> Another is that Typed Racket
> seems unable to narrow down the type of the result. Take a look at this
> example:
>     > (require msgpack)
>     > (define bs (call-with-output-bytes (λ (out) (pack 3 out))))
>     > bs
>     - : Bytes
>     #"\3"
>     > (define v (call-with-input-bytes bs (λ (in) (unpack in))))
>     > v
>     - : Packable
>     3
>     > (+ v 3)
>     ; readline-input:8:3: Type Checker: type mismatch
>     ;   expected: Number
>     ;   given: Packable
>     ;   in: v
>     ; [,bt for context]
>     > (exact-integer? v)
>     - : Boolean
>     #t
> So the type checker can figure out that v is an integer, but it cannot treat
> it as one for addition.

The type checker doesn't know that `v` is an integer. It just knows
`v` is Packable.

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