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Macros in LISP

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  • Last Updated : 18 Nov, 2021

Macro is similar to a function in popular languages like C++, java, etc. that takes arguments and returns a LISP form to be evaluated. It is useful when the same code has to be executed with a few variable changes.
For example, rather than writing the same code for squaring a number, we can define a macro that holds the code for squaring.

Defining a Macro:

To define a named macro in LISP, a default macro – defmacro is used.
Syntax:

(defmacro macroName (parameters)
  expressions
)

Where, 

  • macroName: It is the identifier of the macro, wherever it is used the macro expressions will be executed
  • parameters: Parameters that can be passed whenever calling a macro
  • expressions: Statements executed wherever the macro is used.

Example

Lisp




(defmacro sayHello ()
    ; This statement will be executed whenever
      ; we use the macro sayHello
    (write-line "Hello everyone")
)
; Using macro sayHello
(sayHello)
(write-line "GfG")
(sayHello)
(terpri)
  
  
(defmacro greet (name)
    ; This statement will be executed whenever
      ; we use the macro greet
    ; substituting name with the parameter value
    (format t "Welcome ~S ~%" name)
)
; Calling macro square with parameter strings
(greet "GfG audience")
(greet "Awesome people")
(terpri)
  
  
(defmacro sumup (a b)
    ; This line will be executed whenever
      ; we use the macro sum
    ; with two integer parameters
    (format t "~D + ~D : ~D" a b (+ a b))
)
; Calling macro sumup with parameters 7 and 18 
(sumup 7 18)
(terpri)
  
  
(defmacro square (num)
    ; This line will be executed
      ; whenever we use the macro square
    ; with the parameter name
    (format t "Square of ~D is ~D " num (* num num))
)
; Calling macro square with parameter 5
(square 5)
(terpri)

Output:

Hello everyone
GfG
Hello everyone

Welcome "GfG audience" 
Welcome "Awesome people" 

7 + 18 : 25
Square of 5 is 25 

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