Re: scheme set list function

[Gianmaria Lari]

On Sat, 6 Apr 2019 at 16:30, Thomas Morley <address@hidden> wrote:

Am Sa., 6. Apr. 2019 um 16:11 Uhr schrieb Gianmaria Lari
<address@hidden>:
>
> I wrote this function
>
> #(define (resetList l) (set! l '()))
>
>
> that I would like to set a list to an empty list. This is an example of use:
>
> \version "2.21.0"
> #(define mylist '(1 2 3))
> \markup #(object->string mylist) %this prints (1 2 3)
>
> #(define (resetList l) (set! l '()))
> #(resetList mylist)
> \markup #(object->string mylist) % this I would like it prints () but it prints (1 2 3)
>
> I think the problem is related to the fact resetList changes mylist locally (sort of) but I have no idea how I can fix it. Any help?
>
> Thank you, g.

One possibility is to use a macro:

#(define-macro (reset-list l) `(set! ,l '()))

Thank you, it works.

 I tried to make the same with the second of these functions:

#(define (allButLast l) (reverse (cdr (reverse l))))

#(define (numberInc l) (append (allButLast l) (list (1+ (last l)))))

 (it simply increments the last integer element of a list). Here it is a code that use it:

\version "2.21.0"

#(define (allButLast l) (reverse (cdr (reverse l))))

#(define (numberInc l) (append (allButLast l) (list (1+ (last l)))))

#(define mylist '(1 2 3))

\markup #(object->string mylist)

#(set! mylist (numberInc mylist))

\markup #(object->string mylist)

And here my try with macro:

#(define-macro (numberInc l) `(set! ,l (append (allButLast l) (list (1+ (last l))))))

and this is a complete code that does not work:

\version "2.21.0"

#(define (allButLast l) (reverse (cdr (reverse l))))

#(define-macro (numberInc l) `(set! ,l (append (allButLast l) (list (1+ (last l))))))

#(define mylist '(1 2 3))

\markup #(object->string mylist)

#(numberInc mylist)

\markup #(object->string mylist)

Does it make sense?

Thank you Harm