kapranov-anton · September 6, 2019 23:03 · Sep 6, 2019
diff --git a/Example.elm b/Example.elm
@@ -0,0 +1,50 @@
+import Html exposing (text)
+
+type NodeF v a
+    = Empty v 
+    | Elem (List a)
+
+nodeMap : (a -> b) -> NodeF v a -> NodeF v b
+nodeMap f node =
+    case node of
+        Empty v -> Empty v
+        Elem items -> Elem (List.map f items)
+
+type FixNode v = FixNode (NodeF v (FixNode v))
+
+nodeCata : (NodeF v a -> a) -> FixNode v -> a
+nodeCata alg (FixNode node) =
+  alg (nodeMap (nodeCata alg) node)
+
+
+main = text <| Debug.toString <| {
+  sum = nodeCata sumNode ff,
+  mul = nodeCata mulNode ff,
+  list = nodeCata aggNode ff}
+
+ff = FixNode (Elem [
+  FixNode (Empty 3),
+  FixNode (Empty 2),
+  FixNode (Elem [
+    FixNode (Empty 5),
+    FixNode (Empty 6),
+    FixNode (Empty 1)
+  ])
+  ])
+sumNode : NodeF Int Int -> Int
+sumNode n =
+  case n of
+    Empty v -> v
+    Elem items -> List.sum items
+
+mulNode : NodeF Int Int -> Int
+mulNode n =
+  case n of
+    Empty v -> v
+    Elem items -> List.product items
+
+aggNode : NodeF v (List v) -> List v
+aggNode n =
+  case n of
+    Empty v -> [v]
+    Elem items -> List.concat items