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SICP/exercise_1_46.janet

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+(defn iterative-improve [good-enough? next]
+  (fn [guess]
+    (defn iter [prev current]
+      (cond
+        (good-enough? prev current) current 
+        (iter current (next current))))
+    (iter guess (next guess))))
+
+(def tolerance 0.0001)
+(defn good-enough? [prev current]
+  (< (math/abs (- prev current))
+     tolerance))
+
+(defn improve-sqrt [x]
+  (fn [y] (/ x y)))
+
+(defn average-damp [f]
+  (fn [x] (* 0.5 (+ x (f x)))))
+
+(defn improve [x] (average-damp (improve-sqrt x)))
+
+(defn sqrt [x] 
+  (def run (iterative-improve good-enough? (improve x)))
+  (run 1.0))
+
+(defn fixed-point [f initial-guess]
+  (def run (iterative-improve good-enough? f))
+  (run initial-guess))
+
+(defn fix-sqrt [x]
+  (fixed-point (improve x) 1.0))
+
+(print (sqrt 5.0))
+(print (fix-sqrt 5.0))