Update cps to more closely match book text.

joyofclojure · Aug 16, 2021 · a2a0ba2 · a2a0ba2
1 parent f463afa
commit a2a0ba2
Showing 1 changed file with 36 additions and 10 deletions.
diff --git a/src/clj/joy/cps.clj b/src/clj/joy/cps.clj
@@ -2,21 +2,47 @@
   "Demostrations of continuation-passing style")
 
 (defn fac-cps [n k]
-  (letfn [(cont [v] (k (* v n)))] ;; #1_fac-cps: Next continuation
-    (if (zero? n)                 ;; #2_fac-cps: Accept continuation
-      (k 1)                       ;; #3_fac-cps: Return continuation
+  (letfn [(cont [v] (k (* v n)))] ;; #1_fac-cps: Next
+    (if (zero? n)                 ;; #2_fac-cps: Accept
+      (k 1)                       ;; #3_fac-cps: Return
       (recur (dec n) cont))))
 
+(defn fac [n]
+  (fac-cps n identity))
+
+(assert (= (fac 10) 3628800))
+
+;; defined use loop/recur
+(defn mk-cps [accept? kend kont]
+  (fn [n]
+    (loop [n n, k kend]
+       (if (accept? n) ;; accept
+         (k 1)         ;; return
+         (recur (dec n)
+                (fn [v]
+                  (k ((partial kont v) n)))))))) ;; next
+
+;; defined using fn instead of loop, as shown in book text
 (defn mk-cps [accept? kend kont]
   (fn [n]
     ((fn [n k]
-       (let [cont (fn [v] (k ((partial kont v) n)))] ;; #1_mk-cps: Next continuation
-         (if (accept? n) ;; #2_mk-cps: Accept continuation
-           (k 1)         ;; #3_mk-cps: Return continuation
+       (let [cont (fn [v]
+                    (k ((partial kont v) n)))] ;; #1_mk-cps: Next
+         (if (accept? n) ;; #2_mk-cps: Accept
+           (k 1)         ;; #3_mk-cps: Return
            (recur (dec n) cont))))
      n kend)))
-
-(def fac (mk-cps zero? identity #(* %1 %2))) ;; #4_mk-cps: Factorial fn
-
-(def tri (mk-cps zero? dec #(+ %1 %2)))
 
+(def fac               ; Factorial
+  (mk-cps zero?        ; ... ends when 0
+          identity     ; ... returns 1 at bottom of stack
+          #(* %1 %2))) ; ... multiplies up the stack
+
+(assert (= (fac 10) 3628800))
+
+(def tri               ; Triangles
+  (mk-cps #(== 1 %)    ; ... ends when 1
+          identity     ; ... returns 1 at bottom of stack
+          #(+ %1 %2))) ; ... sums up the stack
+
+(assert (= (tri 10) 55))