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

From: John Cowan
Subject: Re: list-set! Wrong type argument in position 1 (expecting mutable pair)
Date: Wed, 26 Aug 2020 16:42:36 -0400

First of all, Racket's pairs are immutable, so it provides list-set
(non-destructive) rather than list-set!.

Second, it is an error to mutate a literal, since that amounts to
self-modifying code.  Some Schemes don't enforce that, but Guile does.  So
you need (list-set! (list "one" "twoooo" three") 2 "two"), which constructs
the list at run time using the list function.

On Wed, Aug 26, 2020 at 4:13 PM <> 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 List
> Modification. If I google I find
> 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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