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

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.
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