Re: Is (provide 'foo) at the start good or bad?

[William Xu <william.xwl@gmail.com>]

"Stephen J. Turnbull" <stephen@xemacs.org> writes:

> Leo writes:
>
>  > > one less char).  So what is the benefit of providing it at the very
>  > > start? 
>  > 
>  > I have run into this problem before. I prefer putting provide at the end
>  > of the file.
>
> Putting the provide form at the beginning allows mutually recursive
> requires to succeed.  I also prefer it as a matter of style, sort of
> serving as a `declare-package'.

That makes sense.  But looks like doesn't work very well? I tried the
following: 

a.el
==== 

  (provide 'a)
  
  (require 'b)
  
  (defun a-hi ()
    )
  
  (b-hi)

b.el
==== 

  (provide 'b)
  
  (require 'a)
  
  (defun b-hi ()
    )
  
  (a-hi)

Put them under load-path, now if I try (require 'a), i got: 

,----
| Debugger entered--Lisp error: (void-function a-hi)
|   (a-hi)
|   eval-buffer(#<buffer  *load*<2>> nil "q:/.emacs.d/site-lisp/b.el" nil t)  ; Reading at buffer position 55
|   load-with-code-conversion("q:/.emacs.d/site-lisp/b.el" "q:/.emacs.d/site-lisp/b.el" nil t)
|   require(b)
|   eval-buffer(#<buffer  *load*> nil "q:/.emacs.d/site-lisp/a.el" nil t)  ; Reading at buffer position 27
|   load-with-code-conversion("q:/.emacs.d/site-lisp/a.el" "q:/.emacs.d/site-lisp/a.el" nil t)
|   require(a)
|   eval((require (quote a)))
|   eval-last-sexp-1(nil)
|   eval-last-sexp(nil)
|   call-interactively(eval-last-sexp nil nil)
`----

So this doesn't really resolve the recursive requires? 

> Regarding work-arounds, I would do the OP's task (customizing a
> variable) in a mode-hook using `add-to-list', rather than as an
> eval-after-load.

Unfortunately, ffap has no such hook available, and it is not a mode.
add-to-list itself recommends eval-after-load at some point somehow:

,----[ C-h f add-to-list RET ]
| add-to-list is a compiled Lisp function in `subr.el'.
| 
| (add-to-list LIST-VAR ELEMENT &optional APPEND COMPARE-FN)
| 
| Add ELEMENT to the value of LIST-VAR if it isn't there yet.
| The test for presence of ELEMENT is done with `equal',
| or with COMPARE-FN if that's non-nil.
| If ELEMENT is added, it is added at the beginning of the list,
| unless the optional argument APPEND is non-nil, in which case
| ELEMENT is added at the end.
| 
| The return value is the new value of LIST-VAR.
| 
| If you want to use `add-to-list' on a variable that is not defined
| until a certain package is loaded, you should put the call to `add-to-list'
| into a hook function that will be run only after loading the package.
| `eval-after-load' provides one way to do this.  In some cases
| other hooks, such as major mode hooks, can do the job.
`----

-- 
William

http://xwl.appspot.com