I just discovered that SRFI-1 defines first, second, third, ... eight. Except that first = car, second = cadr, and so on.
Cheers, Daniel. On 7 March 2014 19:57, Matthias Felleisen <matth...@ccs.neu.edu> wrote: > > I prefer first/rest for readability. > > When I write (first (second (first x))) though, my old Lisp heart comes > back and I switch to (caadar x) or something like that. > > > > > On Mar 7, 2014, at 1:40 PM, Daniel Carrera <dcarr...@gmail.com> wrote: > > > On the contrary, car and cdr are GUARANTEED to be O(1). > > > > car == return the first element in a pair. > > cdr == return the second element in a pair. > > > > The fact that the cdr of a list returns the rest of the list is simply > an incidental side-effect of the definition of list. > > > > Cheers, > > Daniel. > > > > > > On 7 March 2014 18:28, Deren Dohoda <deren.doh...@gmail.com> wrote: > > Does this mean we shouldn't cdr functional lists but only use rest? > > > > On Mar 7, 2014 12:02 PM, "Matthias Felleisen" <matth...@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 <yd2d...@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 <dcarr...@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 <zeppi...@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 <zeppi...@gmail.com> > wrote: > > > > On Fri, Mar 7, 2014 at 8:39 AM, Daniel Carrera <dcarr...@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 <jensa...@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 <dcarr...@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 <jensa...@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 <dcarr...@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 > > > > ____________________ > > 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. > > -- 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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