The (define (empty? l) (null? l)) is needed due to object-name. > (object-name null?) 'null? > (object-name empty?) 'empty? > (define my-empty? empty?) > (object-name my-empty?) 'empty?
/Jens Axel 2014-03-07 12:19 GMT+01:00 Ryan Davis <zenspi...@gmail.com>: > > On Mar 7, 2014, at 2:45, Daniel Carrera <dcarr...@gmail.com> wrote: > >> 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) > > For many/most things in racket, you can bring up the definition for something > inside of DrRacket: > > (define (first x) > (if (and (pair? x) (list? x)) > (car x) > (raise-argument-error 'first "(and/c list? (not/c empty?))" x))) > > I couldn't for car, so I'm assuming it is considered a primitive. > > last and cdr aren't synonymous: > > (define (last l) > (if (and (pair? l) (list? l)) > (let loop ([l l] [x (cdr l)]) > (if (pair? x) > (loop x (cdr x)) > (car l))) > (raise-argument-error 'last "(and/c list? (not/c empty?))" l))) > > > (define (empty? l) (null? l)) > > null? seems to be a primitive as well. Not sure why they're not direct > synonyms in this case. > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users -- -- Jens Axel Søgaard ____________________ Racket Users list: http://lists.racket-lang.org/users
