如何在Ocaml中表示一个简单的有限状态机？ [英] How to represent a simple finite state machine in Ocaml?

问题描述

我已经用C ++和Java编写了一些状态机，但从来没有像Ocaml这样的函数式语言。

问题是我不知道我是否可以修改代码对象语言版本，因为在Ocaml记录和变体比类更强大;

因此，我需要一个事件驱动的有限状态机（类似于UML的层次结构），可以很容易地配置

有人在该领域有经验的人可以发表一个简单的例子吗？只是为了避免最常见的陷阱

感谢：）

编辑 16 / 03：有没有可能做到没有可变状态？
我想正确地将它封装在名称FSM下，我应该选择一个模块还是一个类？

解决方案

通常，您创建一个对应于自动机状态的记录，并且您有另一个类型的事件触发过渡到另一个状态。在状态记录中，您可以使用映射为每个事件查找新状态。

假设您的转换由字符串触发：

  type event = string 
 
 module EventMap = Map.Make（struct 
 type t = event 
 let compare = compare 
 end）
 
 type state = {
 state_info：...; （*该状态的内容，id，注释等*）
 mutable state_transitions：state EventMap.t; 
} 
 
让next_state current_state event = 
尝试
 EventMap.find事件current_state.state_transitions 
 with Not_found  - > current_state 
  

在这里，我假设未知的事件保持在相同的状态，但是你可能有一个错误状态在记录...

I have written some state machine in C++ and Java but never in a functional language like Ocaml

Problem is I don't know if I can just adapt code from the object languages versions, since in Ocaml records and variants are more powerful than class;

So, I need an event-driven finite state machine (hierarchical like in UML), easily configurable

Could someone experienced in the field post a simple sample of that ? Just to avoid the most common traps

thanks :)

EDIT 16/03 : Is it possible to do it without mutable state ? And I'd like to encapsulate it properly under the name "FSM", should I choose a module or a class ?

解决方案

Usually, you create a record corresponding to a state of the automata, and you have another type for the event triggering the transition to another state. In the state record, you have a map to find, for each event, the new state.

Let's suppose your transitions are triggered by strings:

type event = string

module EventMap = Map.Make(struct
    type t = event
    let compare = compare
  end)

type state = {
  state_info : ...; (* the content of that state, id, comment, etc. *)
  mutable state_transitions : state EventMap.t;
}

let next_state current_state event =
  try
    EventMap.find event current_state.state_transitions
  with Not_found -> current_state

Here, I supposed that unknown events stay on the same state, but you could have an error state in the record...

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