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

Does this mean we shouldn't cdr functional lists but only use rest?
On Mar 7, 2014 12:02 PM, "Matthias Felleisen" <matthias at ccs.neu.edu> wrote:

>
> In a world of immutable cons-es you can cache the result so that testing
> list? becomes an O(1) operation.
>
>
>
>
> On Mar 7, 2014, at 11:52 AM, Eric Dong <yd2dong at uwaterloo.ca> wrote:
>
> > Forgive me if I am super terribly wrong. Isn't it the case that an
> improper list is only known to be improper if we walk to the end and find
> something other than an empty? So wouldn't that mean "first" and "rest"
> take linear time since they must make sure the argument is a list? Clearly
> that doesn't happen. What am I missing?
> >
> >
> > On Fri, Mar 7, 2014 at 10:16 AM, Daniel Carrera <dcarrera at gmail.com>
> wrote:
> > Ok. That makes sense. A list is either '() or something plus a list.
> >
> > Thanks.
> >
> > Cheers,
> > Daniel.
> >
> >
> > On 7 March 2014 14:46, Jon Zeppieri <zeppieri at gmail.com> wrote:
> > Oops, sorry about that empty message. I was going to say that your
> > definition of a list is close, but it's missing something, A list is
> > either:
> >
> > - the empty list; or
> > - a pair, the second element of which is a list
> >
> > (cons 3 2) is a pair, and sometimes non-list pairs are called
> > "improper lists," but they don't satisfy list?.
> >
> >
> > On Fri, Mar 7, 2014 at 8:43 AM, Jon Zeppieri <zeppieri at gmail.com> wrote:
> > > On Fri, Mar 7, 2014 at 8:39 AM, Daniel Carrera <dcarrera at gmail.com>
> wrote:
> > >> What is (cons 3 2) ? What is the definition of a list? I thought that
> a list
> > >> was defined as either '() or a pair.
> > >>
> > >> Cheers,
> > >> Daniel.
> > >>
> > >>
> > >> On 7 March 2014 13:49, Jens Axel Søgaard <jensaxel at soegaard.net>
> wrote:
> > >>>
> > >>> The value (cons 3 42) is not a list. The function car will extract 3,
> > >>> but first will fail.
> > >>>
> > >>> /Jens Axel
> > >>>
> > >>>
> > >>> 2014-03-07 13:40 GMT+01:00 Daniel Carrera <dcarrera at gmail.com>:
> > >>> > Thanks. That's a very useful tip (being able to get at the source
> code).
> > >>> > I
> > >>> > am a bit confused by the condition "(and (pair? x) (list? x))". It
> seems
> > >>> > to
> > >>> > me that this could just be replaced with "(pair? x)". The "list?"
> > >>> > doesn't
> > >>> > add anything. Am I wrong?
> > >>> >
> > >>> > Also, I don't see exactly how "first" and "car" behave different
> on a
> > >>> > non-list. They both raise an error. The errors are just worded
> > >>> > differently.
> > >>> >
> > >>> > On the same file, I found the definition of empty?
> > >>> >
> > >>> > (define empty? (lambda (l) (null? l)))
> > >>> >
> > >>> > Wouldn't it be more economical to write "(define empty? null?)" and
> > >>> > allow
> > >>> > them to be synonyms?
> > >>> >
> > >>> > Cheers,
> > >>> > Daniel.
> > >>> >
> > >>> >
> > >>> > On 7 March 2014 12:16, Jens Axel Søgaard <jensaxel at soegaard.net>
> wrote:
> > >>> >>
> > >>> >> For lists first/rest works the same as car/cdr.
> > >>> >> For non-lists there is a difference: first and rest signals an
> error.
> > >>> >> The names first and rest makes it easier for a human reader of
> > >>> >> a piece of code to see that the program works on lists only.
> > >>> >>
> > >>> >> For the curious, the definition of first is:
> > >>> >>
> > >>> >> (define (first x)
> > >>> >>   (if (and (pair? x) (list? x))
> > >>> >>     (car x)
> > >>> >>     (raise-argument-error 'first "(and/c list? (not/c empty?))"
> x)))
> > >>> >>
> > >>> >> I found this definition like this:
> > >>> >> 1. Entered this program in DrRacket:
> > >>> >>       #lang racket
> > >>> >>       first
> > >>> >> 2. Clicked the "Check Syntax" button
> > >>> >> 3. Right clicked the identifier first and chose "Open defining
> file"
> > >>> >> 4. Chose "first" in the definition-drop-down in the upper left
> corner.
> > >>> >>
> > >>> >> /Jens Axel
> > >>> >>
> > >>> >>
> > >>> >>
> > >>> >>
> > >>> >>
> > >>> >>
> > >>> >>
> > >>> >> 2014-03-07 11:45 GMT+01:00 Daniel Carrera <dcarrera at gmail.com>:
> > >>> >> > Hello,
> > >>> >> >
> > >>> >> > Is there any difference between `first` and `car`, or between
> `last`
> > >>> >> > and
> > >>> >> > `cdr`, or between `empty? and null?` ?
> > >>> >> >
> > >>> >> > I had assumed that these were just synonyms, added by Racket
> because
> > >>> >> > they
> > >>> >> > might be more memorable to a student. But apparently Racket
> doesn't
> > >>> >> > think
> > >>> >> > they are equal:
> > >>> >> >
> > >>> >> > -> (equal? first car)
> > >>> >> > #f
> > >>> >> > -> (equal? last cdr)
> > >>> >> > #f
> > >>> >> > -> (equal? empty? null?)
> > >>> >> > #f
> > >>> >> >
> > >>> >> >
> > >>> >> > I suppose that they could be separate functions that happen to
> do the
> > >>> >> > same
> > >>> >> > thing, but if so, my next question would be why they aren't just
> > >>> >> > aliases. As
> > >>> >> > in:
> > >>> >> >
> > >>> >> > -> (define myfirst car)
> > >>> >> > -> (equal? myfirst car)
> > >>> >> > #t
> > >>> >> >
> > >>> >> > Cheers,
> > >>> >> > Daniel.
> > >>> >> > --
> > >>> >> > When an engineer says that something can't be done, it's a code
> > >>> >> > phrase
> > >>> >> > that
> > >>> >> > means it's not fun to do.
> > >>> >> >
> > >>> >> > ____________________
> > >>> >> >   Racket Users list:
> > >>> >> >   http://lists.racket-lang.org/users
> > >>> >> >
> > >>> >>
> > >>> >>
> > >>> >>
> > >>> >> --
> > >>> >> --
> > >>> >> Jens Axel Søgaard
> > >>> >
> > >>> >
> > >>> >
> > >>> >
> > >>> > --
> > >>> > When an engineer says that something can't be done, it's a code
> phrase
> > >>> > that
> > >>> > means it's not fun to do.
> > >>>
> > >>>
> > >>>
> > >>> --
> > >>> --
> > >>> Jens Axel Søgaard
> > >>
> > >>
> > >>
> > >>
> > >> --
> > >> When an engineer says that something can't be done, it's a code
> phrase that
> > >> means it's not fun to do.
> > >>
> > >> ____________________
> > >>   Racket Users list:
> > >>   http://lists.racket-lang.org/users
> > >>
> >
> >
> >
> > --
> > When an engineer says that something can't be done, it's a code phrase
> that means it's not fun to do.
> >
> > ____________________
> >   Racket Users list:
> >   http://lists.racket-lang.org/users
> >
> >
> > ____________________
> >  Racket Users list:
> >  http://lists.racket-lang.org/users
>
>
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
>
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