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  ANSI Common Lisp   24 System Construction   24.2 Dictionary of System Construction

24.2.11 provide, require Function

provide module-name    implementation-dependent

require module-name &optional pathname-list    implementation-dependent

Arguments and Values:
module-name - a string designator.

pathname-list - nil, or a designator for a non-empty list of pathname designators. The default is nil.

provide adds the module-name to the list held by *modules*, if such a name is not already present.

require tests for the presence of the module-name in the list held by *modules*. If it is present, require immediately returns. Otherwise, an attempt is made to load an appropriate set of files as follows: The pathname-list argument, if non-nil, specifies a list of pathnames to be loaded in order, from left to right. If the pathname-list is nil, an implementation-dependent mechanism will be invoked in an attempt to load the module named module-name; if no such module can be loaded, an error of type error is signaled.

Both functions use string= to test for the presence of a module-name.


;;; This illustrates a nonportable use of REQUIRE, because it
;;; depends on the implementation-dependent file-loading mechanism.

(require "CALCULUS")

;;; This use of REQUIRE is nonportable because of the literal 
;;; physical pathname.  

(require "CALCULUS" "/usr/lib/lisp/calculus")

;;; One form of portable usage involves supplying a logical pathname,
;;; with appropriate translations defined elsewhere.

(require "CALCULUS" "lib:calculus")

;;; Another form of portable usage involves using a variable or
;;; table lookup function to determine the pathname, which again
;;; must be initialized elsewhere.

(require "CALCULUS" *calculus-module-pathname*)

Side Effects:
provide modifies *modules*.

Affected By:
The specific action taken by require is affected by calls to provide (or, in general, any changes to the value of *modules*).

Exceptional Situations:
Should signal an error of type type-error if module-name is not a string designator.

If require fails to perform the requested operation due to a problem while interacting with the file system, an error of type file-error is signaled.

An error of type file-error might be signaled if any pathname in pathname-list is a designator for a wild pathname.

See Also:
*modules*, Section 19.1.2 Pathnames as Filenames

The functions provide and require are deprecated.

If a module consists of a single package, it is customary for the package and module names to be the same.

Allegro CL Implementation Details:
See cl:provide and cl:require in implementation.htm for information on the implementation of provide and require. Note the links are to the documentation for the current Allegro CL version. Replace /current/ in the URL with the Allegro CL version number to see similar documentation in earlier releases.

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