[racket] clarification for beginners please

You wouldn't happen to have a citation for this traditional way, would you?

tia,
Robby


On Thu, Apr 25, 2013 at 9:11 PM, Pierpaolo Bernardi <olopierpa at gmail.com>wrote:

> Ah, yes, right.
>
>
>
> On Fri, Apr 26, 2013 at 4:09 AM, Robby Findler <
> robby at eecs.northwestern.edu> wrote:
>
>> Yes, except that we do know something about what happens when it returns
>> #t in the "in all other cases" case.
>>
>> Robby
>>
>>
>> On Thu, Apr 25, 2013 at 9:03 PM, Pierpaolo Bernardi <olopierpa at gmail.com>wrote:
>>
>>> 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
>>>>>
>>>>>
>>>>
>>>> ____________________
>>>>   Racket Users list:
>>>>   http://lists.racket-lang.org/users
>>>>
>>>>
>>>
>>
>
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