Macros in LISP
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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