[racket] Is this a bug in contracts?

Thanks, you guys are great!

On Mon, Jan 14, 2013 at 9:27 AM, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
>
> And Harry, do define certain contracts separately, e.g.,
>
> (define shish/c (is-a?/c shish-D%))
>
>
>
> On Jan 14, 2013, at 12:24 AM, Asumu Takikawa wrote:
>
>> On 2013-01-13 22:02:44 -0500, Harry Spier wrote:
>>> Is this a bug in contracts?  When I ran the following code the
>>> contract in onion% didn't give me an error, yet the contract is on an
>>> "init sh" but the class doesn't have an "init sh" it has an
>>> "init-field sh".  See the line with the comment ;;;;;THIS LINE SHOULD
>>> USE init-field not init
>>
>> No, this should be correct. An `init` specifies a subset of `init-field`
>> (it's not mutually exclusive with it).
>>
>>> (define/contract onion%
>>>  (class/c [init (sh (is-a?/c shish-D%))]) ;;;;;THIS LINE SHOULD USE init-field not init
>>>  (class shish-D% (super-new)
>>>    (init-field sh)
>>>    (define/override (only-onions?)
>>>      (and (send sh only-onions?) #t))))
>>>
>>> (send (new onion% [sh (new onion% [sh (new skewer%)])]) only-onions?)
>>
>> Notice here that if we change this to:
>>
>>  (send (new onion% [sh 5]) only-onions?)
>>
>> A contract error is raised appropriately because the init argument was
>> wrong, so the contract is clearly protecting the init argument. This is
>> the correct behavior, because an `init-field` clause just defines both
>> an init argument and a field. The class system doesn't track a separate
>> `init-field` thing.  If you only specify the init contract, it only
>> protects the init part.
>>
>> If you set the field like this:
>>
>>  (set-field! sh (new onion% [sh (new onion% [sh (new skewer%)])]) 5)
>>
>> It does not raise a contract error. If you want to protect both, you
>> either need to revise the contract to
>>
>>  (class/c [init-field (sh (is-a?/c shish-D%))])
>>
>> or
>>
>>  (class/c [init (sh (is-a?/c shish-D%))]
>>           [field (sh (is-a?/c shish-D%))])
>>
>> Cheers,
>> Asumu
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>