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emacselisp
Norman Ramsey
Norman Ramsey

Reputation: 202535

What Emacs Lisp function is to `require` as `autoload` is to `load`?

I am trying to program GNU Emacs 23 to issue require commands lazily, on demand, instead of up front in my .emacs file. If I wanted to delay the execution of a load command, I could use autoload. But require and load take different sorts of arguments.

Is there a predefined function that does for require the same job that autoload does for load? And if not, what tools would people recommend that I use to roll my own?

Upvotes: 8

Views: 3192

Answers (3)

phils
phils

Reputation: 73274

One more answer to help clarify (this was a little verbose for a comment):

  • autoload says "if this function isn't already defined, then load this file (if and when the function is called)."
  • require says "if this library isn't already loaded, then load this file (immediately)."

Note in particular that you don't need to use require to load a library; that's simply the way you ensure that you don't load it again (assuming you don't want to do that). The (provide 'FEATURE) expression in the library will be evaluated no matter how the library was loaded, which lets any future require know that it doesn't need to do anything.

It's a similar situation for autoload -- if the file has already been loaded (and therefore the function in question properly defined), then the autoload no longer has any effect.

Upvotes: 4

user355252
user355252

Reputation:

There is not difference between require and load with regards to autoload. require is just a frontend to load, which more or less comes down to:

(defun require (feature &optional filename noerror)
  (unless (featurep feature)
    (let ((filename (or filename (symbol-name feature))))
      (load filename noerror))))

As you can see, the symbol name given to require is equal to the filename given to load. As a matter of fact, the first (require 'foo) evaluated in an Emacs session is equivalent to (load "foo").

Thus, you can just use (auto-load 'foo-function "foo") for foo-function from the library foo, that you can load with (require 'foo).

Upvotes: 7

Drew
Drew

Reputation: 30699

What kind of "demand" do you have in mind for your "on demand"?

  1. If a given command or other function needs (or soft-needs) a given library, then that function itself can use (require 'foo) or (require 'foo nil t). The library will be loaded on demand from that function.

  2. Consider also whether you might need to load the file more than once, i.e., reload it in some situations, whether or not it has already been loaded.

For #2, for instance, my code that uses a library of Lisp macros, icicles-mac.el does not just use require, because I want to make sure that if a user gets a new version of that library (e.g., downloads a new source version and byte-compiles it) then that new version is used whenever s?he byte-compiles another library that needs it. (This is important -- when a library of macros changes, other libraries that use those macros generally need to be recompiled after loading the new macros file.) For that, instead of just (require 'icicles-mac) I use this:

 (eval-when-compile
  (or (condition-case nil
          (load-library "icicles-mac") ; Use load-library to ensure latest .elc.
        (error nil))
      (require 'icicles-mac)))         ; Require, so can load separately if not on `load-path'.

Upvotes: 2

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