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
