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 ____________________ Racket Users list: http://lists.racket-lang.org/users
