The procedure har-set-cursor is (fortunately!) not allowed to modify
its argument.

This is from SRFI 14:

**
The procedures of this SRFI, by default, are "pure functional" -- they
do not alter their parameters. However, this SRFI defines a set of
"linear-update" procedures which have a hybrid
pure-functional/side-effecting semantics: they are allowed, but not
required, to side-effect one of their parameters in order to construct
their result. An implementation may legally implement these procedures
as pure, side-effect-free functions, or it may implement them using
side effects, depending upon the details of what is the most efficient
or simple to implement in terms of the underlying representation.

The linear-update routines all have names ending with "!".
**

In particular, a version that allowed the modification of an argument
would have been called char-set-cursor-next!.

NB The sentence "Note that these primitives are necessary to export an
iteration facility for char sets to loop macros." shows the deficiency
of RnRS for n <= 7 in that it has no delimited continuations. Such
primitives should not be necessary, and they wouldn't be with
delimited continuations.

Am Mo., 10. Juli 2023 um 04:20 Uhr schrieb Bradley Lucier <xxxxxx@purdue.edu>:
>
> The signature is
>
>   char-set-cursor-next cset cursor -> cursor
>
> The SRFI text says:
> ========================
> A cursor index is incremented with char-set-cursor-next; in this way,
> code can step through every character in a char set.
> ========================
> I don't have much experience with cursors.  I want to know whether
> char-set-cursor is allowed to modify the "cursor" argument it is passed.
>
> I ask because in my design the cursor is a vector of three elements, but
> with very fast increment (on average) if I can modify the cursor argument.
>
> Here's what some web pages say about char-set-cursor-next.
>
> http://marcomaggi.github.io/docs/vicare-libs.html/srfi-char_002dsets-spec-iter.html
> ========================
> char-set-cursor-next increments a cursor index and returns a new cursor
> indexing the next character in the set; in this way, code can step
> through every character in a char set.
> ========================
> I assume this means that the argument cursor is not modified.  The code
> in lib/vicare/containers/char-sets.vicare.sls seems not to modify the
> cursor, but to build an entirely new one.
>
> https://www.gnu.org/software/guile/manual/html_node/Iterating-Over-Character-Sets.html
> ========================
> Scheme Procedure: char-set-cursor-next cs cursor
> C Function: scm_char_set_cursor_next (cs, cursor)
>
>      Advance the character set cursor cursor to the next character in
> the character set cs. It is an error if the cursor given satisfies
> end-of-char-set?.
> ========================
> This sounds like the argument cursor is modified.  The code in
> libguile/srfi-14.c seems to modify the argument.
>
> I don't understand the code in gnu/kawa/slib/srfi14.scm to know what
> Kawa does (it appears that the cursor is an integer, though, so it
> wouldn't change the argument).
>
> Brad