LISP - Symbols

Lisp - + Method Combination

Quiz

Method combination is a useful technique in CLOS, Common LISP Object System. This technique allows us to combine methods when multiple methods are associated with a generic method. In this chapter, we're exploring how to use plus, + symbol to add numbers with length of strings.

Defining add function with method combination +

An add function is defined using following syntax

; define a generic function add
(defgeneric add (x y)
   (:method-combination +))

; add two numbers
(defmethod add + ((x number) (y number))
   (+ x y))

defgeneric add (x y) is used to define a generic function.
(:method-combination +) signifies that + method combination is to be used.
(defmethod add + ((x number) (y number)) − add method is specialized to add two numbers.
+ is essential for method combination. This method adds two numbers and returns the same.
Similarly, we're adding add() methods to add following parameters −
- add number, string (length)
- add string (length), number
- add string (length), string (length)

Example - Use of + Function

Following is the complete example of a + function.

main.lisp

; define a generic function add
(defgeneric add (x y)
   (:method-combination +))

; add two numbers
(defmethod add + ((x number) (y number))
   (+ x y))

; add a number with string length
(defmethod add + ((x number) (y string))
   (+ x (length y)))

; add string length with a number
(defmethod add + ((x string) (y number))
   (+ (length x) y))

; add strings length
(defmethod add + ((x string) (y string))
   (+ (length x) (length y)))

; add 10 with 5 to print 15
(print (add 10 5))
(terpri)
; add 10 with 4 to print 14
(print (add 10 "test"))
(terpri)
; add 4 with 20 to print 24
(print (add "test" 20))
(terpri)
; add 4 with 4 to print 8
(print (add "test" "code"))

Output

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

Explanation

Generic add function is called with different argument types. CLOS selects appropriate implementation methods based on argument types and combines the result using + method combination. + method combination treats result of each applicable method as numbers and sums them up.

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