Problem understanding set-register syntax

Problem understanding set-register syntax

[Aurélien Bottazzini]

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Hello everyone,

in my init.el I have

(set-register ?i '(file . "~/.emacs.d/init.el"))

which allows me to quickly jump to my init file using C-x r j i
But I don't really understand the syntax.
Why do I have to use the exclamation mark when I set the register?

Why can't I just use:

(set-register i '(file . "~/.emacs.d/init.el")) ; will say Symbol's value as
variable is void:i

I tried to search in the documentation for set-register and variable
definition but I did not find
anything to really explain it.

Thanks for any help.

Aurélien

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Problem understanding set-register syntax

[Aurélien Bottazzini]

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(sorry if this email is a duplicate, but the first one was lost apparently)

Hello everyone,

in my init.el I have

(set-register ?i '(file . "~/.emacs.d/init.el"))

which allows me to quickly jump to my init file using C-x r j i
But I don't really understand the syntax.
Why do I have to use the exclamation mark when I set the register?

Why can't I just use:

(set-register i '(file . "~/.emacs.d/init.el")) ; will say Symbol's value as
variable is void:i

I tried to search in the documentation for set-register and variable
definition but I did not find
anything to really explain it.

Thanks for any help.

Aurélien

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RE: Problem understanding set-register syntax

[Drew Adams]

> in my init.el I have
> (set-register ?i '(file . "~/.emacs.d/init.el"))
> which allows me to quickly jump to my init file using C-x r j i
> But I don't really understand the syntax.
> Why do I have to use the exclamation mark when I set the register?

You mean question mark, I think.
	
> Why can't I just use:
> (set-register i '(file . "~/.emacs.d/init.el"))
> ; will say Symbol's value as variable is void:i

Because the first argument to `set-register' is a register name, which is a
character, and `?i' is the Lisp syntax for the character `i'.
	
> I tried to search in the documentation for set-register and
> variable definition but I did not find anything to really explain it.

Please file a doc bug for `C-h f set-register'.  The doc should say clearly what
each parameter is.  In this case it says nothing about the first parameter
except that it is a register.  It expects readers to somehow know that a
register (its name, that is) is of type character.

Relevant doc that will help you:

1. Elisp manual, node `Registers':

 "A register is a sort of variable used in Emacs editing that
  can hold a variety of different kinds of values.  Each
  register is named by a single character."

IOW, a register is like a variable in that it has a name and a value.  The name
is a character, however, not a symbol.

2. Emacs manual, node `Registers':

 "Emacs "registers" are compartments where you can save text, rectangles,
  positions, and other things for later use....
  Each register has a name that consists of a single character, which
  we will denote by R; R can be a letter (such as `a') or a number (such
  as `1'); case matters, so register `a' is not the same as register `A'."

Re: Problem understanding set-register syntax

[PJ Weisberg]

On Mon, Dec 6, 2010 at 5:47 AM, Aurélien Bottazzini
<aurelien.bottazzini@gmail.com> wrote:
> (sorry if this email is a duplicate, but the first one was lost apparently)
> Hello everyone,
> in my init.el I have
> (set-register ?i '(file . "~/.emacs.d/init.el"))
> which allows me to quickly jump to my init file using C-x r j i
> But I don't really understand the syntax.
> Why do I have to use the exclamation mark when I set the register?
> Why can't I just use:
> (set-register i '(file . "~/.emacs.d/init.el")) ; will say Symbol's value as
> variable is void:i
> I tried to search in the documentation for set-register and variable
> definition but I did not find
> anything to really explain it.
> Thanks for any help.
> Aurélien

English:
    !: exclamation mark (more commonly "exclamation point")
    ?: question mark

Emacs:
    ?i:  The character 'i' (which, technically, is equal to the number 105).
    i:  A variable represented by the symbol i.

Consider the difference between:

int i = 0;

and

int 'i' = 0;

-PJ

RE: Problem understanding set-register syntax

[Drew Adams]

> > Why can't I just use:
> > (set-register i '(file . "~/.emacs.d/init.el"))
> > ; will say Symbol's value as variable is void:i
> 
> Because the first argument to `set-register' is a register 
> name, which is a character, and `?i' is the Lisp syntax for
> the character `i'.

I should also have mentioned that using just i, without quoting it (i.e., 'i)
causes the symbol i to be evaluated.  And since the symbol i has no value here
(no global value) you get the error message saying that its value is void.

Some symbols, such as t and nil (and all keywords, such as :foobar) are
self-evaluating, meaning that they act just like variables whose values are the
symbols themselves.  You do not need to quote self-evaluating symbols in an
evaluation context.  Other symbols (e.g. i, foobar) you do need to quote, if you
want not the value of the symbol (e.g. it might not have a value) but the symbol
itself.

But all characters are self-evaluating, so ?i evaluates to ?i.

See the Emacs manual, node `Init Syntax'.
See the Elisp manual, node `Basic Char Syntax'.
(Use `g' in Info to go to a particular node.)