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

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.
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