6
这是我的树实现fold
(左)的企图(这是非常简化的版本,但仔细再现真实的树形结构):折叠树OCaml中
type 'a tree = Leaf of 'a | Node of 'a * 'a tree list
let rec fold t acc f =
match t with
| Leaf x -> f acc x None
| Node (x, lst) ->
let deferred acc =
List.fold_left (fun acc t' -> fold t' acc f) acc lst in
f acc x (Some deferred)
的想法是使用延迟通话为子树。它让我们:
- 跳过子树遍历如果需要
- 初始化子树遍历并撰写结果
,一种玩具,例如:
open Printf
let() =
let tree = Node (3, [Leaf 5; Leaf 3; Node (11, [Leaf 1; Leaf 2]); Leaf 18]) in
fold tree "" (fun str x nested ->
let plus str = if String.length str = 0 then "" else "+" in
let x = string_of_int x in
match nested with
| None -> str^(plus str)^x
| Some f -> str^(plus str)^x^"*("^(f "")^")"
)
|> printf "%s=";
fold tree 0 (fun acc x -> function
| None -> acc + x
| Some f -> x * (f 0) + acc
)
|> printf "%d\n";
我想这是发明了很多次已经。这种技术有什么名字吗?任何着名的canonic形式?任何想法如何使它变得更好?
相关 - http://stackoverflow.com/questions/4434292/catamorphism-and-tree-traversing-in-haskell – nlucaroni