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Re: Emacs Lisp's Library System (tutorial)

 
 
Rivka Miller
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Posts: n/a
 
      07-14-2010
The first thing to understand elisp is to have the code of the elisp
interpreter in elisp. I dont think that probably is published. You
still have the choice of reading the C code which I referred to in an
earlier post in comp.lang.c.

http://xahlee.org/elisp/Writing-Emac...acs-Primitives

Maybe you can expand on this topic.

DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
doc: /* Eval args until one of them yields non-nil, then return
that
value. The remaining args are not evalled at all.
If all args return nil, return nil.
usage: (or CONDITIONS ...) */)
(args)
Lisp_Object args;
{
register Lisp_Object val = Qnil;
struct gcpro gcpro1;

GCPRO1 (args);

while (CONSP (args))
{
val = Feval (XCAR (args));
if (!NILP (val))
break;
args = XCDR (args);
}

UNGCPRO;
return val;
}


On Jul 13, 5:44*pm, Xah Lee <(E-Mail Removed)> wrote:
> • Emacs Lisp's Library System
> *http://xahlee.org/emacs/elisp_library_system.html
>
> plain text version follows
> --------------------------------------------------
> Emacs Lisp's Library System
>
> Xah Lee, 2010-07-13
>
> This page explains emacs library system. For example, what's the
> difference between library, package, features? And what's the
> difference between load-file, load, require, autoload?
>
> --------------------------------------------------
> What's Library, Package, Feature?
>
> ==============================
> No Namespace
>
> Emacs lisp the language does not have name spaces. Everything is
> global, with dynamic scope, with some shadowing mechanism. So, don't
> expect library or module to be language defined name space constructs
> that somewhat enforce name space and file name relation, as in Perl,
> Python, Java.
>
> ==============================
> What's the difference between a Package and Library?
>
> The terms “package” and “library”, are used losely in emacs/elisp
> manual to refer to any useful elisp file. They are not technical
> definitions in elisp.
>
> A “library” usually refers to elisp file containing library of lisp
> functions, to be called by other lisp source code. For example, the
> command backward-word is defined in “simple.el”, which is a library of
> functions.
>
> A “package” usually refers to something useful for emacs users, like a
> major mode or minor mode. The term “module” is not used by emacs.
>
> ==============================
> Emacs's Concept of “Feature”
>
> The term “feature” has some meaning in elisp, but is not mechanical. A
> “feature” is a elisp symbol, that is intended to represent the
> functionality provided by a emacs package. This elisp symbol, can be
> placed on the predefined global variable named “features”, which is a
> list of symbols. For example, here's part of the value of “features”
> when i do “Ctrl+x describe-variable Enter features”:
>
> ibuffer etags ring cc-mode cc-fonts cc-menus cc-cmds cc-styles
> cc-align cc-engine cc-vars cc-defs xlsl-mode encoded-kb speck
> sgml-mode dired info newcomment desktop recentf tree-widget wid-edit
> advice help-fns ...
>
> A elisp file can call “(provide ‹some symbol›)” near the end, which
> adds that symbol to the “features” list. The purpose of features and
> the “features” variable is to provide a way for emacs to know if
> something is already loaded.
>
> --------------------------------------------------
> Package/Library/Feature are not Managed
>
> There is no absolute relation between any concept of package/library/
> feature/autoload facilities and the file name.
>
> By convention, if a elisp file name is “xyz-mode.el”, it OFTEN
> provides a lisp symbol “xyz-mode” as its feature name (it may not),
> and the function to invoke the mode is OFTEN named “xyz-mode”.
> Sometimes the “mode” part is dropped in the file name, feature symbol,
> or function name.
>
> This is only a lose convention. There are a lot exceptions in many
> bundled emacs packages. For example, the file “lisp-mode.el” provides
> the symbol “lisp-mode” as feature, and is invoked by “emacs-lisp-
> mode”. The “cua-base.el” file provides symbols “cua-base” and “cua” as
> features, and is invoked by cua-mode. The “text-mode.el” file does not
> provide any symbol for feature, but is a quite useful mode invoked by
> text-mode. The file “desktop.el” provides “desktop” as feature, and is
> invoked by desktop-save-mode.
>
> --------------------------------------------------
> File/Package Loading Mechanisms
>
> [table]
> Emacs's library/module/package is a primitive system, centered on
> loading file (load-file), with some slightly high level things such as
> its “features”, “autoload”, “require”. However, nothing is strict or
> enforced by elisp.
> Function Name * Purpose Tech Detail * * Comment
> load-file * * * Load a file. * *Load one specific file. Use this if you have
> one very SPECIFIC file at one particular file path.
> load * *Load a file. * *Load a file by searching thru var “load-path”. Also,
> tries to load a compiled version (.elc) if exists. * * *Use this if there
> are several possible location for the file, and you are using a short
> file name such as “undo.el” or “undo”, and you want to load the
> compiled version if it exists (“undo.elc”).
> require Load a package if it has not already been loaded. * * * Checks the
> var “features”, if symbol is not there, then call “load” to load it.
> Best used in elisp source code, similar to other lang's “require” or
> “import”.
> autoload * * * *Load a file only when a function is called. * * Associate a
> function name with a file path. When the function is called, load the
> file, and execute the function. It is good to use this when you
> install a package. It save startup time and makes emacs use less
> system resource.
>
> All the above means, you could have a file named “xyz.el”, which
> provides a feature named “abc”, while it really just provide a mode to
> user with the command name “opq-mode” or sometimes just “opq”, and it
> might be displayed in mode line as “OPQ”, “OPQ helper”, or anything
> else. And, this file can be considered as a package as well as
> library.
>
> See also: How To Name Your Emacs Major Mode.
>
> --------------------------------------------------
> Isn't this all very bad?
>
> Yes it is.
>
> Though, it's just the state of software. Many most popular langs, such
> as C, C++, PHP, do worse. They don't have a module system neither, but
> worse, they load a file by “include”. Note that even Scheme didn't
> have module system, until R6RS, released in 2007, and the new module
> system defined in it widely criticized, and R6RS caused Scheme
> community to split.
>
> * Xah
> ∑http://xahlee.org/
>
> ☄


 
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