Re: list-set! Wrong type argument in position 1 (expecting mutable pair)

[Tim Van den Langenbergh <tmt_vdl@gmx.com>]

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Hey,

yeah, a quoted list is immutable (at least in Guile).

You could do something like...

(define my-list (list "zero" "one" "twooo")
(list-set! my-list 2 "two")
my-list

Don't forget that list-set! returns the new element, rather than the modified list.

Cheers,

-Tim

On Wednesday, 26 August 2020 22:12:32 CEST you wrote:
>
> Hi,
> One shortcoming I find working with Guile is the limited number of
> examples of how to use code. Consider list-set!. The only reference in
> the entire Guile manual is the definition in section 6.6.9.6 List
> Modification. If I google I find
> https://docs.racket-lang.org/reference/pairs.html which gives me the
> example (list-set '(zero one two) 2 "two") ==> '(zero one "two") which
> is what I deduced from the Guile manual.  If I try something similar
> in Guile:
>
> scheme@(guile-user)> (list-set! '("zero" "one" "twooooo") 2 "two")
> ice-9/boot-9.scm:1669:16: In procedure raise-exception:
> In procedure set-car!: Wrong type argument in position 1 (expecting
> mutable pair): ("twooooo")
>
> Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> scheme@(guile-user) [1]> ,bt
>             1 (list-set! ("zero" "one" "twooooo") 2 "two")
> In ice-9/boot-9.scm:
> 1669:16  0 (raise-exception _ #:continuable? _)
>
> Could someone provide me with an example of how to use list-set! ?
> Alternatively what I really need is how to modify a specified element
> in a list.
> Thanks
> Mortimer
>

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