[racket] first vs car ; last vs cdr ; empty? vs null?

The first/rest operations do not use a memoization table.  They test using
the list? primitive, which is built in and actually has a couple of tag
bits reserved on every cons object for memoizing its results.  So the
operation really is quite fast.

Carl Eastlund

On Fri, Mar 7, 2014 at 4:40 PM, Eric Dong <yd2dong at uwaterloo.ca> wrote:

> So if code is using first/rest instead of car/cdr when I already know the
> lists are proper, it could incur a significant performance penalty? After
> all, memoization still needs a table lookup each time I do a first/rest.
>
> (i.e. implementing datastructures in terms of lists should use car/cdr for
> performance right? any benchmarks?)
>
>
> On Friday, March 7, 2014, Sean Kanaley <skanaley at gmail.com> wrote:
>
>> Pardon me, but going back a bit where the blog about switching to
>> immutable lists was mentioned...I saw an interesting comment about
>> immutability and eq?.  Have any mathematicians come up with an answer for
>> the meaning of eq? on permanently distinct but equivalent and immutable
>> objects, e.g. (eq? (cons 3 2) (cons 3 2))?  What about on immutable objects
>> that have mutable internal state, e.g. memoizing functions?
>> Naive-but-memoized (fib 1000) is not necessarily the same as (fib 1000),
>> unless the answer doesn't depend on the number of universes that have
>> bigbanged meanwhile.
>>
>>
>>
>> On Fri, Mar 7, 2014 at 2:31 PM, Daniel Carrera <dcarrera at gmail.com>wrote:
>>
>>> Thanks.
>>>
>>>
>>> On 7 March 2014 20:22, Harry Spier <vasishtha.spier at gmail.com> wrote:
>>>
>>>> See also this previous thread on the Racket list.
>>>>  http://www.mail-archive.com/users%40racket-lang.org/msg12161.html
>>>>
>>>>
>>>>
>>>>
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>>>
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>>> that means it's not fun to do.
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