Lisp - Packages

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In general term of programming languages, a package is designed for providing a way to keep one set of names separate from another. The symbols declared in one package will not conflict with the same symbols declared in another. This way packages reduce the naming conflicts between independent code modules.

The LISP reader maintains a table of all the symbols it has found. When it finds a new character sequence, it creates a new symbol and stores in the symbol table. This table is called a package.

The current package is referred by the special variable *package*.

There are two predefined packages in LISP −

• common-lisp − it contains symbols for all the functions and variables defined.

• common-lisp-user − it uses the common-lisp package and all other packages with editing and debugging tools; it is called cl-user in short

Package Functions in LISP

The following table provides most commonly used functions used for creating, using and manipulating packages −

Creating a LISP Package

The defpackage function is used for creating an user defined package. It has the following syntax −

(defpackage :package-name
   (:use :common-lisp ...)
   (:export :symbol1 :symbol2 ...)
)

Where,

• package-name is the name of the package.

• The :use keyword specifies the packages that this package needs, i.e., packages that define functions used by code in this package.

• The :export keyword specifies the symbols that are external in this package.

The make-package function is also used for creating a package. The syntax for this function is −

make-package package-name &key :nicknames :use

The arguments and keywords has same meaning as before.

Using a Package

Once you have created a package, you can use the code in this package, by making it the current package. The in-package macro makes a package current in the environment.

Example

Create a new source code file named main.lisp and type the following code in it.

main.lisp

; create a package tom
(make-package :tom)
; create a package dick
(make-package :dick)
; create a package harry
(make-package :harry)
; go to package tom
(in-package tom)
; define a function hello in package tom
(defun hello () 
   (write-line "Hello! This is Tom's Tutorials Point")
)

; go to package dick
(in-package dick)
; define a function hello in package dick
(defun hello () 
   (write-line "Hello! This is Dick's Tutorials Point")
)

; go to package harry
(in-package harry)
; define a function hello in package harry
(defun hello () 
   (write-line "Hello! This is Harry's Tutorials Point")
)

; go to package tom
(in-package tom)
; call hello function
(hello)
; go to package dick
(in-package dick)
; call hello function
(hello)
; go to package harry
(in-package harry)
; call hello function
(hello)

Output

When you execute the code, it returns the following result −

Hello! This is Tom's Tutorials Point
Hello! This is Dick's Tutorials Point
Hello! This is Harry's Tutorials Point

Deleting a Package

The delete-package macro allows you to delete a package. The following example demonstrates this −

Example

Create a new source code file named main.lisp and type the following code in it.

main.lisp

; create a package tom
(make-package :tom)
; create a package dick
(make-package :dick)
; create a package harry
(make-package :harry)
; go to package tom
(in-package tom)
; create a function hello
(defun hello () 
   (write-line "Hello! This is Tom's Tutorials Point")
)
; go to package dick
(in-package dick)
; define a function hello
(defun hello () 
   (write-line "Hello! This is Dick's Tutorials Point")
)
; go to package harry
(in-package harry)
; define a function hello
(defun hello () 
   (write-line "Hello! This is Harry's Tutorials Point")
)

; go to package tom
(in-package tom)
; call hello function
(hello)
; go to package dick
(in-package dick)
; call hello function
(hello)
; go to package harry
(in-package harry)
; call hello function
(hello)
; delete package tom
(delete-package tom)
; go to package tom
(in-package tom)
; call hello function
(hello)

Output

When you execute the code, it returns the following result −

Hello! This is Tom's Tutorials Point
Hello! This is Dick's Tutorials Point
Hello! This is Harry's Tutorials Point
*** - EVAL: variable TOM has no value