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 >
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