Re: Optional Arguments

[pietru@caramail.com]

> Sent: Monday, December 07, 2020 at 9:16 AM
> From: "Alexis Roda" <alexis.roda.villalonga@gmail.com>
> To: pietru@caramail.com
> Cc: "Help Gnu Emacs" <help-gnu-emacs@gnu.org>
> Subject: Re: Optional Arguments
>
> Hi,
> 
> Not sure what your question is.

It is bit confusing.

I want to use typh-word-markers to compute ma and mb so I can use the
two values in another function.  Had put (interactive) in my effort to
test it, but created problems as you say.  The function would be called
when cursor is on a word, so that I get the word "beg" and "end".

(defun typh-word-markers (ma mb)
   (skip-chars-backward "[:alpha:]")
   (setq ma (point))
   (skip-chars-forward "[:alpha:]")
   (setq mb (point))
   (message "[ma,mb]: %s,%s" ma mb) )


 
> Regarding the error, I'm assuming that you execute the function with 'M-x
> typh-word-markers'. That won't work interactively because the function
> expects two arguments and 'interactive' don't tell emacs how to get them.
> It works if you execute it with 'M-: (typh-word-markers nil nil)', from
> IELM or from the scratch buffer.
> 
> That said, in your code the arguments are not required, they are used only
> as local variables, not to provide information to the function, so the
> function can be written with a 'let' [1][2] form defining local variables:
> 
> (defun typh-word-markers ()
>   (interactive)
>   (let ((ma)
>         (mb))
>     (skip-chars-backward "[:alpha:]")
>     (setq ma (point))
>     (skip-chars-forward "[:alpha:]")
>     (setq mb (point))
>     (message "[ma,mb]: %s,%s" ma mb)) )
> 
> In the first function '&optional' is not strictly required. Since you
> specify in 'interactive' the 'n' option [3] (A number, read with the
> minibuffer. If the input is not a number, the user has to try again. ‘n’
> never uses the prefix argument. Prompt.) the user will be prompted for a
> numeric value and the function will always get an argument. That's the
> opposite case to typh-word-markers.
> 
> Probably you want the 'p' code [3]: The numeric prefix argument. (Note that
> this ‘p’ is lower case.) No I/O.
> 
> (defun typh-skip-chars (n)
>    (interactive "p")
>    (if (= n 1)
>       (skip-chars-forward "[:alpha:]")
>      (skip-chars-backward "[:alpha:]")) )
> 
> That way executing 'M-x typh-skip-chars' will skip chars forward while 'C-u
> M-x typh-skip-chars' will skip chars backward. No interactive input will be
> requested to the user, the input to the function is provided with the
> prefix argument [4]. That's more idiomatic.
> 
> 
> Hope this helps
> 
> [1]
> https://www.gnu.org/software/emacs/manual/html_node/eintr/Parts-of-let-Expression.html
> [2]
> https://www.gnu.org/software/emacs/manual/html_node/eintr/Sample-let-Expression.html#Sample-let-Expression
> [3]
> https://www.gnu.org/software/emacs/manual/html_node/elisp/Interactive-Codes.html
> [4] https://www.gnu.org/software/emacs/manual/html_node/emacs/Arguments.html
> 
> 
> Missatge de l'adreça <pietru@caramail.com> del dia dl., 7 de des. 2020 a
> les 4:17:
> 
> > Would like to have two functions
> >
> > 1. skip-chars - skip to beg of word (if no argument) or end of word (if
> > argument is 1)
> >
> > 2. word-markers - get beg and end of a word position of word
> >
> > Have written the following. For the first one, I have not used optional
> > arguments before.
> > And have read documentation about functions.
> >
> > About the second, there errors when I try to execute.
> >
> > (defun typh-skip-chars (&optional n)
> >    (interactive "n Skip direction: ")
> >    (if (= n 1)
> >       (skip-chars-forward "[:alpha:]")
> >       (skip-chars-backward "[:alpha:]")) )
> >
> > (defun typh-word-markers (ma mb)
> >    (interactive)
> >    (skip-chars-backward "[:alpha:]")
> >    (setq ma (point))
> >    (skip-chars-forward "[:alpha:]")
> >    (setq mb (point))
> >    (message "[ma,mb]: %s,%s" ma mb) )
> >
> >
> >
> >
>