Closures in Common Lisp

August 19, 2006 at 6:01 pm Leave a comment

Martin Fowler published an online article about closures. He used the Ruby programming language for the examples. There are translations of the examples to Python and a language called Boo.

As a programming task for myself I did the examples in Common Lisp, which supported Closured already decades ago. A real lisper might be able to write the code in a more elegant way but I think the principle should become clear.

I didn’t use the Common Lisp Object System (CLOS) since for this simple example an ad-hoc modeling with hashes is simpler.

;; Ruby
; def managers(emps)
; return {|e| e.isManager}
; end

;; Common Lisp
(defun is-manager (emp) (getf emp :is-manager))

(defun managers (emps)
(remove-if-not (lambda (emp)
(when (is-manager emp) emp))

;; Ruby
; def highPaid(emps)
; threshold = 150
; return {|e| e.salary > threshold}
; end

;; Common Lisp
(defun high-paid (emps)
(let ((threshold 150))
(remove-if-not (lambda (emp)
(when (> (getf emp :salary) threshold) emp))

;; Ruby
; def paidMore(amount)
; return {|e| e.salary > amount}
; end

;; Common Lisp
(defun paid-more (amount)
(lambda (emp) (when (> (getf emp :salary) amount) emp)))

;; Ruby
; highPaid = paidMore(150)
; john =
; john.salary = 200
; print

;; Common Lisp
(let ((high-paid (paid-more 150))
(john '(:name John :is-manager nil :salary 200)))
(princ (funcall high-paid john)))

;; Tests
(defparameter manager-list
'((:name Stefan :is-manager nil :salary 150)
(:name Henning :is-manager t :salary 151)
(:name Martin :is-manager nil :salary 120)
(:name Christian :is-manager t :salary 200)))

(princ #\newline)
(princ "Test function managers")
(princ #\newline)
(princ (managers manager-list))
(princ #\newline)

(princ #\newline)
(princ "Test function high-paid")
(princ #\newline)
(princ (high-paid manager-list))
(princ #\newline)

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