[plt-scheme] Re: polymorphism of primitive types

On Oct 19, 2005, at 5:02 PM, David Van Horn wrote:

> Matthias Felleisen wrote:
>> On Oct 19, 2005, at 3:04 PM, David Van Horn wrote:
>>> Matthias Felleisen wrote:
>>>> But it would blow up on this:
>>>>
>>>>           (let/ec jump
>>>>             (mymap (lambda (x y) (if (= x y) (jump #f) (+ x y))) 
>>>> '(1 2 .
>>>> 3) '(1 . 2)))
>>>>
>>>> Should it? -- Matthias
>>>
>>> The jump is a red herring here.  The function breaks even without
>>> escaping continuations:
>>>
>>> (mymap (lambda (x y) (+ x y)) '(1 2 . 3) '(1 . 2)))
>>
>>
>> There is no proper list so it should throw up.
>
> Right.  I misunderstood your original question, but I see what you were
> getting at now.  I think the answer must be yes, it should.  The
> arguments don't meet the contract.  If we want to allow this sort of
> thing, the contract and verbal description of the function should be
> changed.

You are jumping to conclusions. Think "lazy contracts."
>
> (Moreover, the description of SRFI 1 map explicitly states the dynamic
> order in which the procedure is applied to the elements of the list is
> unspecified.  Your example relies on a left-to-right order to avoid the
> improper lists at the tail.)

I wasn't thinking in terms of srfi 1, but the general idea, and I am 
well aware that the contract is a sketch.

> There are other problems with this contract, [more stuff deleted]

Oh? Then how do you reject (map + '(1 2) "three").

> To enforce the condition that the procedure has the correct arity, you
> can do the following:
>
>   (provide/contract
>    [mymap  (->r ([proc
>                   (and/c procedure?
>                          (lambda (_)
>                            (procedure-arity-includes? proc
>                              (length arg-lists))))])
>
>               arg-lists (lambda (_) (ormap proper-list? arg-lists))
>
>               proper-list?)])
>
> Note that the arg-lists contract is applied to each element.  The only
> way I could think to state an aggregate property was to ignore each
> element and assert the property for the whole list, which means the
> contract repeats work needlessly.  Is there a better approach?

Good. I asked Robby to add ->r. I see it happened.

Contracts repeat work. See above to see how our thinking is evolving.

-- Matthias


>
> David
>
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