About features

About features

[Xue Fuqiao]

In (info "(elisp) Coding Conventions"):

    * Put a call to `provide' at the end of each separate Lisp file.
      *Note Named Features::.

    * If a file requires certain other Lisp programs to be loaded
      beforehand, then the comments at the beginning of the file should
      say so.  Also, use `require' to make sure they are loaded.  *Note
      Named Features::.

Why should we use `provide' and `require'?  Is 
`load'/`load-library'/`load-file' enough?  Thanks.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: About features

[Andreas Röhler]

Am 06.03.2013 11:08, schrieb Xue Fuqiao:
> In (info "(elisp) Coding Conventions"):
>
>     * Put a call to `provide' at the end of each separate Lisp file.
>       *Note Named Features::.
>
>     * If a file requires certain other Lisp programs to be loaded
>       beforehand, then the comments at the beginning of the file should
>       say so.  Also, use `require' to make sure they are loaded.  *Note
>       Named Features::.
>
> Why should we use `provide' and `require'?  Is `load'/`load-library'/`load-file' enough?  Thanks.
>

IMHO `provide' delivers a kind of city-map with street-names, which exists, before you will visit a street.
Also require, before you a looking for the location of a certain building, checks if the
street-in-my-example exists.

Re: About features

[Kevin Rodgers]

On 3/6/13 3:08 AM, Xue Fuqiao wrote:
> In (info "(elisp) Coding Conventions"):
>
> * Put a call to `provide' at the end of each separate Lisp file.
> *Note Named Features::.
>
> * If a file requires certain other Lisp programs to be loaded
> beforehand, then the comments at the beginning of the file should
> say so. Also, use `require' to make sure they are loaded. *Note
> Named Features::.
>
> Why should we use `provide' and `require'? Is `load'/`load-library'/`load-file'
> enough? Thanks.

`provide' allows you to `require' unconditionally, but the library/file will 
only be loaded if necessary.

-- 
Kevin Rodgers
Denver, Colorado, USA

Re: About features

[Barry Margolin]

In article <mailman.21537.1362564509.855.help-gnu-emacs@gnu.org>,
 Xue Fuqiao <xfq.free@gmail.com> wrote:

> In (info "(elisp) Coding Conventions"):
> 
>     * Put a call to `provide' at the end of each separate Lisp file.
>       *Note Named Features::.
> 
>     * If a file requires certain other Lisp programs to be loaded
>       beforehand, then the comments at the beginning of the file should
>       say so.  Also, use `require' to make sure they are loaded.  *Note
>       Named Features::.
> 
> Why should we use `provide' and `require'?  Is 
> `load'/`load-library'/`load-file' enough?  Thanks.

`require' doesn't reload the file if it has already been loaded.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***

Re: About features

[Xue Fuqiao]

>> Why should we use `provide' and `require'? Is `load'/`load-library'/`load-file'
>> enough? Thanks.
>
> `provide' allows you to `require' unconditionally, but the library/file will
> only be loaded if necessary.

Why?  Can you explain it more detailed?

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: About features

[Barry Margolin]

In article <mailman.21593.1362608491.855.help-gnu-emacs@gnu.org>,
 Xue Fuqiao <xfq.free@gmail.com> wrote:

> >> Why should we use `provide' and `require'? Is 
> >> `load'/`load-library'/`load-file'
> >> enough? Thanks.
> >
> > `provide' allows you to `require' unconditionally, but the library/file 
> > will
> > only be loaded if necessary.
> 
> Why?  Can you explain it more detailed?

Why don't you just look at the source code? Simplifying, it's something 
like:

(defun require (feature)
  (if (not (member feature features))
      (load feature)))

(defun provide (feature)
  (if (not (member feature features))
      (push feature features)))

As I said in my earlier response: require won't reload a library if it's 
already been loaded, and it uses the features variable to keep track of 
which libraries have been loaded this way.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***

Re: About features

[Xue Fuqiao]

>>>> Why should we use `provide' and `require'? Is
>>>> `load'/`load-library'/`load-file'
>>>> enough? Thanks.

>>> `provide' allows you to `require' unconditionally, but the library/file
>>> will
>>> only be loaded if necessary.

>> Can you explain it more detailed?

> Simplifying, it's something like:

> (defun require (feature)
>   (if (not (member feature features))
>       (load feature)))

> (defun provide (feature)
>   (if (not (member feature features))
>       (push feature features)))

> As I said in my earlier response: require won't reload a library if it's
> already been loaded, and it uses the features variable to keep track of
> which libraries have been loaded this way.

I see.  Thanks, I misunderstood your meaning before. (My English sucks.)

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao