Re: let assoc-set! append to the right

[Urs Liska]

Am 29.01.2016 um 10:29 schrieb David Kastrup:
> Urs Liska <address@hidden> writes:
>
>> Hi all,
>>
>> assoc-set! either replaces a value in an alist *in-place* or prepends a
>> new pair if the key isn't present in the alist.
>> However, I need a function the *appends* the entry if it is created newly.
>>
>> I came up with swapping the first element to the end if assoc-set! has
>> modified the length of the list, but somehow this looks unnecessarily
>> complicated:
>>
>> \version "2.19.35"
>>
>> #(define alst '((one . 1) (two . 2)))
>>
>> #(define (append-alist alst key-name value)
>>    (let ((intermediate (assoc-set! alst key-name value)))
>>      (if (> (length intermediate) (length alst))
>>          (append (cdr intermediate) (list (car intermediate)))
>>          intermediate)))
>>
>> #(display (append-alist alst 'three 3))
>> #(newline)
>>
>> #(display (append-alist alst 'one 4))
>> #(newline)
>>
>> Is there a more straightforward way to have the behaviour of assoc-set!
>> but with appending instead of prepending newly created elements?
> (define (append-alist alst key-name value)
>   (let ((where (assoc key-name alst)))
>     (if where
>         (begin (set-cdr! where value) alst)
>         (append! alst (list (cons key-name value))))))
>
> If you really want to use assoc-set!, just check whether the result of
> assoc-set! is eq to the original alst.  If it is, you are done,
> otherwise you'll need to bump your new first element to the end.
>
> By the way: if you are using assoc-set! anyway, you are planning on
> modifying the list, so you might as well use the more efficient append!.
> Unless it's important to you that assoc-set! retains the _spine_ of the
> list even while modifying its members.
>

Thank you for the explanations - learning each day ...
No, I don't really want to use assoc-set! as the function should just
return a list.

Urs