[racket] clarification for beginners please

The traditional way of explaining eq? is that it works when one of the
arguments is a symbol or a mutable data structure.  In all other cases the
result is implementation dependent.  Is this explanation still valid for
racket?



On Fri, Apr 26, 2013 at 1:37 AM, Robby Findler
<robby at eecs.northwestern.edu>wrote:

> I don't think that either of these explanations are really the right way
> to think about eq?. The only way to really understand eq? on immutable
> values is to understand that it is exposing low-level details of the
> implementation to you (ostensibly for performance reasons). That is, if
> (eq? a b) returns #true (where a and b are immutable things like the
> structs in the previous messages), then that tells you that equal? will
> return also return #true. When it returns #false all you know is that
> equal? may or may not return #true (ie, you know nothing).
>
> This may seem like a strange primitive (and it is), but it can be very
> useful for performance reasons, since it takes constant time but equal? may
> take up to linear time in the size of its inputs.
>
> When the arguments are mutable objects, then Macros and Danny's
> explanations are a good way to start understanding them.
>
> (After all, if you punch one crab named "Joe", the other one doesn't get a
> bruise.)
>
> Robby
>
>
> On Thu, Apr 25, 2013 at 3:45 PM, Marco Morazan <morazanm at gmail.com> wrote:
>
>> Joe,
>>
>> Perhaps a more pedestrian explanation might help. There is a difference
>> between two items having the same value and two items being the same item.
>>
>> equal? tests if its two items have the same value.
>>
>> eq? tests if two items are the same item.
>>
>> Using your example:
>>
>>
>> (define A (make-posn 4 5))
>> (define B (make-posn (+ 3 1) (- 6 1)))
>>
>> Both A and B are items (in this case posns) that have the same value.
>> Thus, (equals? A B) is true.
>>
>> A and B, however, are not the same item (they are two different posns
>> that happen to have the same value). Thus, (eq? A B) is false.
>>
>> Consider:
>>
>> (define C B)
>>
>> Now, C and B are the same item (the same posn). Thus, (eq? C B) is true.
>> As C and A are not the same posn, we have (eq? C A) is false. As you would
>> expect (equal? C A) is true (because the have the same value).
>>
>> eq? returning true suffices to know that equal? returns true (if two
>> items are the same item, then clearly they have the same value). equal?
>> returning true does not suffice to know what eq? returns (given that two
>> different items may or may not have the same value).
>>
>> This difference is not relevant for most of HtDP, because it is not until
>> your reach the material with assignment that you need to understand/know if
>> two items are the same item or not. Before the assignment material, you are
>> always testing for value equality.
>>
>> Finally, eqv? is not used in HtDP and my advice would be to ignore its
>> existence with beginners.
>>
>> I hope this helps.
>>
>> Marco
>>
>>
>>
>>
>> On Thu, Apr 25, 2013 at 2:31 PM, Ford, Joe <jford at hollandhall.org> wrote:
>>
>>> I have a group of high school students with a question... can someone
>>> please explain to beginner Racket users the differences between these three
>>> boolean functions:   eq?   equal?   eqv?
>>>
>>> We have read the help menu verbage visible from DrRacket, but simply
>>> don't understand what it is saying.  Maybe that's lack of context or
>>> vocabulary... but we're struggling a bit.  To test simple variations of
>>> application, we wrote some simple code (shown below) but don't understand
>>> why the results are what they are:
>>>
>>> (define FOUR "four")
>>> (define A (make-posn 4 5))
>>> (define B (make-posn (+ 3 1) (- 6 1)))
>>> "-------------"
>>> (equal?  FOUR  "four")
>>> (equal?  4  (+ 1 3))
>>> (equal?  4 (posn-x (make-posn 4 3)))
>>> (equal? A B)
>>> "-------------"
>>> (eq?  FOUR  "four")
>>> (eq?  4  (+ 1 3))
>>> (eq?  4 (posn-x (make-posn 4 3)))
>>> (eq? A B)
>>> "---------------"
>>> (eqv?  FOUR  "four")
>>> (eqv?  4  (+ 1 3))
>>> (eqv?  4 (posn-x (make-posn 4 3)))
>>> (eqv? A B)
>>>
>>>
>>> Why in the world would the above-defined A and B be considered "equal?"
>>> but not "eq?" or "eqv?"?
>>>
>>> --
>>> *Joe Ford
>>> Technology Resources, Scheduling & Yearbook
>>> Holland Hall
>>> jford at hollandhall.org
>>>
>>> *
>>>
>>>
>>> ____________________
>>>   Racket Users list:
>>>   http://lists.racket-lang.org/users
>>>
>>>
>>
>>
>> --
>>
>> Cheers,
>>
>> Marco
>>
>> Have a´¨)
>> ¸.·´¸.·*´¨) ¸.·*¨)
>> (¸.·´ (¸.·´ * wonderful day! :)
>>
>> ____________________
>>   Racket Users list:
>>   http://lists.racket-lang.org/users
>>
>>
>
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