common-lisp Functions as first class values Defining functions that take functions and return functions
Example
A simple example:
CL-USER> (defun make-apply-twice (fun) "return a new function that applies twice the function`fun' to its argument" (lambda (x) (funcall fun (funcall fun x)))) MAKE-APPLY-TWICE CL-USER> (funcall (make-apply-twice #'1+) 3) 5 CL-USER> (let ((pow4 (make-apply-twice (lambda (x) (* x x))))) (funcall pow4 3)) 81 The classical example of function composition: (f ∘ g ∘ h)(x) = f (g (h (x)):
CL-USER> (defun compose (&rest funs) "return a new function obtained by the functional compositions of the parameters" (if (null funs) #'identity (let ((rest-funs (apply #'compose (rest funs)))) (lambda (x) (funcall (first funs) (funcall rest-funs x)))))) COMPOSE CL-USER> (defun square (x) (* x x)) SQUARE CL-USER> (funcall (compose #'square #'1+ #'square) 3) 100 ;; => equivalent to (square (1+ (square 3))) 
