sicp -- Marginalia

e2-21-22-23 toc
	(ns e2-21-22-23 (:use clojure.test) (:use [clojure.contrib.generic.math-functions :only (sqr)]))
	(defn square-list [xs] (if (empty? xs) nil (cons (sqr (first xs)) ; Argh.. I can't recur from a non-TC optimizable position! ; (recur (rest xs))))) ; However, that's what expected in this exercise, so I'll leave this function unfinished. nil)))
	(defn square-list-map [xs] (map sqr xs))
	(deftest test-square-list-map (is (= (square-list-map [1 2 3 4]) [1 4 9 16])))
His first try produces the reverse list. Why? Well, he wrote an internal recurring function which takes the unprocessed (unsquared) list as its first argument and accumulates the result in the second argument. The accumulation process takes the square of the first argument of the unprocessed list and conses it with the already-processed result. Now lets examine a simple case of list-square'ing a [1 2] vector: 1. things = [1 2], answer = nil 2. things = [2], answer = [1] : nil 3. things = nil, answer = [4] : [1] : nil Second try isn't much better. Lets see what happens: 1. things = [1 2], answer = nil 2. things = [2], answer = nil : [1] This shouldn't even work, as (cons nil x) doesn't make any sense.	(defn square-list-2 [xs] (letfn [(iter [things answer] (if (empty? things) answer ; FIRST version ; (recur (rest things) ; (cons (sqr (first things)) answer))))] ; SECOND version (recur (rest things) (cons answer (sqr (first things))))))] (iter xs nil)))
	(deftest test-square-list-2 ; NOTE: ; you can't (ns :use clojure.test :only (thrown?)) as it's not public, but macro-generated. (is (thrown? java.lang.IllegalArgumentException (square-list-2 [1 2 3 4]))))
Performs an f for every x in xs and returns a nil	(defn do-for-each [f xs] (if (empty? xs) nil (do (f (first xs)) (recur f (rest xs)))))

e2-21-22-23