Typed-fsm example: ATM

#1

typed-fsm example: ATM

Use typed-fsm to solve the ATM problem.

The goal is to track the number of attempts based on the type and statically ensure that no more than three attempts are made during the password input phase.

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#2

typed-fsm example: todoList

The overall structure of todoList is similar to elm:

Elm programs always look something like this:

buttons

The Elm program produces HTML to show on screen, and then the computer sends back messages of what is going on. “They clicked a button!”

What happens within the Elm program though? It always breaks into three parts:

  • Model — the state of your application
  • View — a way to turn your state into HTML
  • Update — a way to update your state based on messages

Compared with elm, todoList introduces control states, which affect Model, View, Update, and Message at the same time.

Core idea

Let’s take a TodoList:

origin
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Each state here represents a page:

Main represents the main page showing the todolist content.

AreYouSure represents the page for selecting Yes or No.

Modify represents the page for modifying a todo.

Add represents the page for adding a todo.

Exit represents exit.

Each arrow represents a message.

You may notice that this looks a lot like fsm. Each state here represents a control flow or a page, and Message changes the control flow or page and may also change the data.
I will use typed-fsm to separate the control state from the all data.

Comparing elm and todoList

elm todoList
Mode Control State: ps , Data State: ds
View Sing t → ds → UI ps (t :: ps) a
Update Op ps ds IO () (to :: ps) (from ps)
Message Msg ps (from :: ps) (to :: ps)

The UI part uses the modified threepenny-gui, the main change is to change the original (UI a) to (UI ps (t::ps) a).

There are at least three advantages to doing this.

  1. The type of the View part has a clear control state, which can limit the type of Message and avoid sending error messages.

  2. The Update part can give full play to the advantages of typed-fsm, and typed-fsm takes over the entire control flow.

  3. Extract the common part and simplify the control state.

At the same time, typed-fsm supports singletons, and the definition of control state is also very easy:

$( singletons
    [d|
      data Todo
        = Main
        | Add
        | Delete
        | Modify
        | Exit
        | Action Todo Todo
        | AreYouSure Todo Todo
        deriving (Show, Eq, Ord)
      |]
 )
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