LISP - Symbols

Lisp - Code Generation Using Macros

Quiz

In LISP, macros is a powerful tool to extend language capability by writing a code which can generate code. Macros allows metaprogramming in LISP. In this chapter, we'll explore multiple examples of macros used in code generation.

Example - Create a Square Function using Macro

main.lisp

; define a macro accepting x and returns square of it
(defmacro my-square (x)
   `(* ,x ,x))
   
; (my-square 5) expands to (* 5 5)
(print (my-square 5))
(terpri)
; (my-square (+ 2 3)) expands to (* (+ 2 3)(+ 2 3))
(print (my-square (+ 2 3)))

Output

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

25
25

Explanation

defmacro my-square (x) − macro my-square is defined using defmacro with an argument x
`(* ,x ,x) − a template is created using backquote and using comma, value of x will be inserted in the template
(my-square 5) − expression is expanded (* 5 5) and is evaluated as 25.

Example - Create a Loop using Macro

main.lisp

; define a macro to create a loop
(defmacro my-loop (var start end body)
  `(do ((,var ,start (+ ,var 1)))
       ((> ,var ,end))
     ,body))

; call loop to print 1 to 5
(my-loop i 1 5 (print i))

Output

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

Explanation

defmacro my-loop − macro my-loop is defined using defmacro with an arguments to define a loop.
`(...) − a template is created using backquote and using do construct, a loop is run.
,... − comma is used to insert values in the template.

Example - Create a Loop using Macro

main.lisp

; define a macro to create a loop
(defmacro my-loop (var start end body)
  `(do ((,var ,start (+ ,var 1)))
       ((> ,var ,end))
     ,body))

; call loop to print 1 to 5
(my-loop i 1 5 (print i))

Output

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

Explanation

defmacro my-loop − macro my-loop is defined using defmacro with an arguments to define a loop.
(var start end body)− four parameters are passed, a loop variable, start value, end value and body of the loop.
`(...) − a template is created using backquote and using do construct, a loop is run.
,... − comma is used to insert values in the template.

Example - Create a Function using Macro

main.lisp

; define a macro to define a function
(defmacro def-my-function (name params &body body)
  `(defun ,name ,params
     ,@body))

; define the function add-two-numbers using macro
(def-my-function add-two-numbers (x y)
  (+ x y))

; prints 7
(print (add-two-numbers 3 4))

Output

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

Explanation

defmacro def-my-function − macro def-my-function is defined using defmacro with an arguments to define a function.
(name params &body body)− three parameters are passed, a function name, parameters to be pased and body of the function.
`(...) − a template is created using backquote and using do construct, a loop is run.
,... − comma is used to insert values in the template.
,@body − comma-at is used to insert body of the function.
def-my-function − macro is used to define a function by passing name of the function, parameters to be passed and a body.

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