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

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