如何在OCAML中使用List.Map跳过术语?

let map_partial f xs =
  let prepend_option x xs = match x with
  | None -> xs
  | Some x -> x :: xs in
  List.rev (List.fold_left (fun acc x -> prepend_option (f x) acc) [] xs)

用法:

map_partial (fun x -> if x <> 1 then Some (x+1) else None) [0;1;2;3]

会回来的 [1;3;4]

或者你可以用 filter_map from extlib

两者 Batteries 和 Extlib 提供相当于 mapPartial :它们的扩展 List 模块提供a filter_map ('a -> 'b option) -> 'a list -> 'b list

另一个解决方案是直接使用 foldl :

let f e l = if (e <> 1) 
            then (e + 1)::l 
            else l
in List.fold_left f [] list  

但我的偏好是 filter_map

或者,您可以过滤列表,然后在结果列表上应用映射,如下所示:

let map_bis predicate map_function lst =
    List.map map_function (List.filter predicate lst);;

# val map_bis : ('a -> bool) -> ('a -> 'b) -> 'a list -> 'b list = <fun>

# map_bis (fun e -> e<>1) (fun e -> e+1) [0;1;2;3];;
- : int list = [1; 3; 4]

如果希望保留值,也可以将值映射到单例列表;如果不想保留值,还可以将值映射到空列表,然后将结果合并。

List.concat (List.map (fun e -> if (e <> 1) then [e + 1] else []) my_list)

let rec process = function
  | 1 :: t -> process t
  | h :: t -> (h + 1) :: (process t)
  | []     -> []

或尾部递归

let process = 
  let rec f acc = function
    | 1 :: t -> f acc t
    | h :: t -> f ((h + 1) :: acc) t
    | []     -> List.rev acc in
  f []

或者由标准函数组成

let process l = 
  l |> List.filter ((<>)1)
    |> List.map ((+)1)