[plt-scheme] Contract changes a parameter-procedure to a procedure. Why?

(The real issue is that real contracts can't be implemented as a  
library; they have to be 'baked into' the core of the language. But  
this is not for here or now.)


On Apr 16, 2010, at 1:45 PM, Carl Eastlund wrote:

> On Fri, Apr 16, 2010 at 1:30 PM, Greg Hendershott
> <greghendershott at gmail.com> wrote:
>> I had the bright (?) idea of specifying a contract for a parameter.  
>> But:
>>
>> #lang scheme
>>
>> (define p1
>>  (make-parameter #f))
>>
>> (parameter? p1) ; --> #t
>> p1 ; --> #<procedure:parameter-procedure>
>>
>> (define/contract p2
>>  (() (boolean?) . ->* . (or/c boolean? void?))
>>  (make-parameter #f))
>>
>> (parameter? p2) ; --> #f   Why?
>> p2 ; --> #<procedure>      Why?
>>
>> I discovered this when I made a contract for a procedure that  
>> requires
>> its argument to be a parameter. I can't use parameter? I'd have to
>> settle for procedure?.
>>
>> Is this because the contract is implemented as a wrapper procedure,
>> and the parameter? predicate only sees the contract wrapper procedure
>> not the parameter-procedure inside?
>
> That's exactly it.  Use parameter/c instead; it constructs a
> parameter-friendly wrapper.
>
> --Carl
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