From: Ramakrishnan Muthukrishnan Date: Sat, 10 Jul 2010 05:38:48 +0000 (+0530) Subject: some additional notes and code on addition with church numerals X-Git-Url: https://git.rkrishnan.org/%5B/frontends/...?a=commitdiff_plain;h=ae801c4f04234d70cd39cb1aa68963d626947b70;p=sicp.git some additional notes and code on addition with church numerals --- diff --git a/src/sicp/ex2_6.clj b/src/sicp/ex2_6.clj index 39fae1e..ce06f90 100644 --- a/src/sicp/ex2_6.clj +++ b/src/sicp/ex2_6.clj @@ -44,6 +44,17 @@ ;; similarly the other examples. ;; Definition of an add operator +;; To do m + n, we should have a function which is applied f, n+m times. +;; +;; i.e. +;; +;; (m f) = takes a function f and composes a new function which composes +;; f, m times and applies this new function to x. +;; +;; (lambda (f) (lambda (x) ((m f) ((n f) x)))) will create this new +;; function which compose a given function n+m times and hence is equiv +;; to addition of m+n. Some simple repl experiments seem to verify the +;; result. (defn add [m n] (fn [f] (fn [x] ((m f) ((n f) x))))) @@ -52,6 +63,12 @@ (((add one two) inc) 1) ;;=> 4 ) + +;; another definition of add is this: +;; m + n == (m + [1 + 1 + 1 + ... ] +(defn new-add [m n] + ((m add-1) n)) + ;; continueing the same logic, (add one two) => three ;; and inc applied three times to 1 is 4. This proves ;; that our definition of add is correct. @@ -67,4 +84,22 @@ (deftest test-church-to-numeral (are [x y] [= x y] (church-to-numeral one) 1 - (church-to-numeral two) 2)) \ No newline at end of file + (church-to-numeral two) 2)) + +;; church to roman: func to integer +(defn church-to-roman [f] + ((f (fn [x] (+ x 1))) 0)) + +;; integer to func +(defn roman-to-church [n] + (fn [f] (fn [x] (first (drop n (take (inc n) (iterate f x))))))) + +(defn church [n] + (loop [n n cf (fn [f] (fn [x] x))] + (if (zero? n) + cf + (recur (- n 1) (fn [f] (fn [x] + (f ((cf f) x)))))))) + +(defn unchurch [f] + ((f (fn [x] (+ x 1))) 0)) \ No newline at end of file