[plt-scheme] Problem with Contracts - HtDP 21.1.3

and shared it privately with me.

It is a bad habit.  In the future I'll make my replies public to avoid
redundancies.
Felix, thank you for your comments.

In Matthias's original proposed solution, X was unconstrained...

Is there a simple way to define the difference between a constrained and
unconstrained variable?  Is it the same as the difference between bound and
unbound?  Judging by your comment, it seems that making the distinction can
be useful, but I don't know how to make it; further, I wouldn't appreciate
the significance of the distinction even if I could....

Thanks,
Dave

On Mon, Nov 10, 2008 at 11:52 AM, Matthias Felleisen
<matthias at ccs.neu.edu>wrote:

>
> David found my mistake in the pedagogic exercise and share it privately
> with me.
>
>
>
>
>
> On Nov 10, 2008, at 2:43 PM, Felix Klock's PLT scheme proxy wrote:
>
>  David (cc'ing Matthias and the plt-scheme list)-
>>
>> On Nov 7, 2008, at 11:57 AM, Matthias Felleisen wrote:
>>
>>   (X U -> U) W U Nat -> U
>>>
>>> 4. The second argument is used as the first input to f so U = W
>>>
>>>  (X U -> U) U U Nat -> U
>>>
>>> 5. No other constraints so we're fine
>>>
>>> And I may have made a mistake so check the calculation -- Matthias
>>>
>>
>> Step 4 makes an incorrect deduction.
>>
>> I believe it should be:
>>
>> 4. The second argument is used as the first input to f, so X = W
>>
>>  (X U -> U) X U Nat -> U
>>
>> The way I caught this: usually all of the parameters have some sort of
>> relationship with other parts of the contract (on either side of the ->
>> arrow).  In Matthias's original proposed solution, X was unconstrained,
>> which raised a red flag in my mind.
>>
>> ----
>>
>> Also, there are ways to revise the interface to natural-f that completely
>> eliminate the X from the contract, which may make for an interesting design
>> exercise.
>>
>> -Felix Klock
>>
>>
>>
>
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