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Lisp - Call Functions using funcall
funcall is a very powerful function to call a function by its name or its object stored in a variable. funcall function is very useful in case of first class functions and it helps to determine a function to call at runtime. In this chapter, we'll explore ways to use funcall function.
Syntax - funcall function
(funcall function-object arg1 arg2 ...)
Where
-
function-object− a function to be called.
We can use operator symbols like '+, function name like 'compute or built-in construct like 'list etc.
We can use lambda expression as an anonymous function.
We can use function object returned by function.
arg1, arg2, ...− arguments to be passed to the function to be called.
Example - Calling a named function
main.lisp
; call + operator (print(funcall '+ 2 3)) ; create a new list (print(funcall 'list 1 2 3)) ; define a function (defun multiply-by-two (x) (* x 2)) (terpri) ; call named function (print(funcall 'multiply-by-two 5))
Output
When you execute the code, it returns the following result −
5 (1 2 3) 10
Example - Using variable to store function
main.lisp
; define a function (defun multiply-by-two (x) (* x 2)) ; set operation as + (setf operation '+) ; call the function; evaluates to 9 (print(funcall operation 4 5)) (terpri) ; set operation to function name (setf operation 'multiply-by-two) ; call function; evaluates to 12 (print(funcall operation 6))
Output
When you execute the code, it returns the following result −
9 12
Example - Calling lambda expression
main.lisp
; call a lambda expression; evaluates to 12 (print(funcall (lambda (x y) (* x y)) 3 4))
Output
When you execute the code, it returns the following result −
12
Example - Creating a High Order Function
main.lisp
; define a high order function (defun apply-operation (function arg1 arg2) (funcall function arg1 arg2)) ; call the + operator; evaluates to 15 (print(apply-operation '+ 10 5)) (terpri) ; call the * operator; evaluates to 50 (print(apply-operation '* 10 5)) (terpri) ; call a lambda expression; evaluates to 5 (print(apply-operation (lambda (a b) (/ a b)) 10 2))
Output
When you execute the code, it returns the following result −
15 50 5
Key Considerations
funcall function can be used to create high order functions which can take functions as arguments.
funcall function treats functions as first class objects.
funcall function is very useful in dynamic function dispatching.