[racket] Determining type from a syntax object

On Jun 16, 2014, at 3:41 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:

> On Mon, Jun 16, 2014 at 3:29 PM, Alexander D. Knauth
> <alexander at knauth.org> wrote:
>> 
>> On Jun 16, 2014, at 3:13 PM, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:
>> 
>>> On Mon, Jun 16, 2014 at 3:06 PM, Alexander D. Knauth
>>> <alexander at knauth.org> wrote:
>>>> 
>>>>> Typed Racket functions are
>>>>> just plain Racket functions. We could add some extra metadata to every
>>>>> value that held its type,
>>>> 
>>>> It wouldn’t have to be every value, it could just put that information in for :has-type expressions, right?
>>>> Would that still require fundamental changes to Racket?
>>> 
>>> The problem is that the information needs to get attached to the value
>>> where the value is created, not where you use `:has-type?`.  The
>>> `:has-type?` could even be in some other module, so you'd have to add
>>> this information to every value, everywhere.  That's why it would need
>>> to be part of the runtime.
>> 
>> Then how would ann do it?
>> 
>> I was thinking of something that would work sort of like ann, except that if it has the type, it would put true there (instead of the value), and
>> if it doesn’t, then it would put false there (instead of raising an error).
>> 
>> I understand that there’s no way to do this at ‘expansion’ time because it type checks it after it expands, but is there any way
>> to do this at ‘type-check’ time?  That’s what ann does, right?
>> 
>> That way the actual checking wouldn’t have to happen at runtime.
>> 
>> Or do I have this expansion-time, type-check-time, run-time thing wrong somewhere?
> 
> I think you have the various times right, but this proposal won't work
> either. In particular, now your original example won't work:
> 
> (cond [(:has-type? f (Number -> Any))
>       (f 1)]
>      [(:has-type? f (String -> Any))
>       (f "1")])
> 
> Imagine the `f` has type `Any at the start.  Then if you replace
> `:has-type?` with `ann`, you get a type error.
> 
> Unless you're suggesting that `f` would have to already have one of
> those types, in which case I'm not sure how it helps you.
> 
> Sam

Oh, I hadn’t realized that.  f would have to already have one of those types, so it wouldn’t really help.