Data commit

Task/Happy-numbers/Oz/happy-numbers.oz

@@ -0,0 +1,44 @@
+functor
+import
+  System
+define
+  fun {IsHappy N}
+     {IsHappy2 N nil}
+  end
+
+  fun {IsHappy2 N Seen}
+     if     N == 1          then true
+     elseif {Member N Seen} then false
+     else
+	Next = {Sum {Map {Digits N} Square}}
+     in
+	{IsHappy2 Next N|Seen}
+     end
+  end
+
+  fun {Sum Xs}
+     {FoldL Xs Number.'+' 0}
+  end
+
+  fun {Digits N}
+     {Map {Int.toString N} fun {$ D} D - &0 end}
+  end
+
+  fun {Square N} N*N end
+
+  fun lazy {Nat I}
+     I|{Nat I+1}
+  end
+
+  %% List.filter is eager. But we need a lazy Filter:
+  fun lazy {LFilter Xs P}
+     case Xs of X|Xr andthen {P X} then X|{LFilter Xr P}
+     [] _|Xr then {LFilter Xr P}
+     [] nil then nil
+     end
+  end
+
+  HappyNumbers = {LFilter {Nat 1} IsHappy}
+in
+  {System.show {List.take HappyNumbers 8}}
+end