Family Day update

Task/Power-set/D/power-set-3.d

@@ -0,0 +1,16 @@
+// Haskell definition:
+// foldr f z []     = z
+// foldr f z (x:xs) = x `f` foldr f z xs
+S foldr(T, S)(S function(T, S) f, S z, T[] rest) {
+    return (rest.length == 0) ? z : f(rest[0], foldr(f, z, rest[1..$]));
+}
+
+// Haskell definition:
+//powerSet = foldr (\x acc -> acc ++ map (x:) acc) [[]]
+T[][] powerset(T)(T[] set) {
+    import std.algorithm;
+    import std.array;
+    // Note: The types before x and acc aren't needed, so this could be made even more concise, but I think it helps
+    // to make the algorithm slightly clearer.
+    return foldr( (T x, T[][] acc) => acc ~ acc.map!(accx => x ~ accx).array , [[]], set );
+}