[racket] Typed racket and hash table

On Fri, Jun 27, 2014 at 4:08 PM, Manfred Lotz <manfred.lotz at arcor.de> wrote:
> On Fri, 27 Jun 2014 15:59:47 -0400
> "Alexander D. Knauth"
> <alexander at knauth.org> wrote:
>
>> Are you sure you meant "hash-ref!" and not just "hash-ref”?
>>
>
> Well I thought hash-ref! is the way to go because I just wanted #f in
> case of not found.
>
>> Because even with (hash-ref! my-hash size (lambda () #f)), it would
>> expect the #f to be of type (Listof String), which it isn’t.
>>
>
> Oops, that makes the difference. Using hash-ref works fine now.
> It seems that in untyped racket hash-ref! is fine but it is not suited
> for typed racket.


When you use `hash-ref!`, you're actually changing the hash table, not
just returning #f in the case that the key isn't found. Typed Racket
doesn't let you change the hash table to have the wrong kind of data
in it. So using `hash-ref!` is wrong both with typed or untyped Racket
-- the type checker is just catching the bug.

Sam