Creating a Practical DSL in F#

Introduction

In this example, we will explore how to create a Domain-Specific Language (DSL) in F# that does more than just print out actions. Instead, it will perform meaningful work, including flow control and state management, making it suitable for practical applications like a game engine in a Multi-User Dungeon (MUD).

Defining the Domain

We start by defining the tasks our DSL will handle. Tasks can be simple steps, parallel tasks, sequences, conditionals, or loops.

type GameState = {
    Fuel: int
    Oil: int
    IsEngineRunning: bool
}

type Task =
    | Step of string * (GameState -> GameState) // Step with a name and an action that modifies the game state
    | Parallel of Task list
    | Sequence of Task list
    | Conditional of (GameState -> bool) * Task * Task option // If-Else structure
    | LoopUntil of (GameState -> bool) * Task // Loop while a condition is false

Implementing Actions and Conditions

Next, we define actions that modify the GameState and conditions that control the flow of our tasks.

let startEngine state =
    printfn "Starting the engine..."
    { state with IsEngineRunning = true }

let checkFuel state =
    if state.Fuel > 10 then
        printfn "Fuel level is sufficient."
        state
    else
        failwith "Not enough fuel!"

let checkOil state =
    if state.Oil > 5 then
        printfn "Oil level is sufficient."
        state
    else
        failwith "Not enough oil!"

let launch state =
    if state.IsEngineRunning then
        printfn "Launching the spaceship!"
        state
    else
        failwith "Cannot launch: Engine is not running!"

let shutdown state =
    printfn "Shutting down the spaceship."
    { state with IsEngineRunning = false }

let isEngineRunning state = state.IsEngineRunning
let isFuelLow state = state.Fuel < 5

Creating the Execution Engine

We now create an engine that will execute our tasks, handling the flow control defined in our DSL.

let rec executeTask state task =
    match task with
    | Step(name, action) ->
        printfn "Executing step: %s" name
        action state
    | Parallel tasks ->
        printfn "Executing tasks in parallel:"
        tasks |> List.fold (fun acc t -> executeTask acc t) state
    | Sequence tasks ->
        printfn "Executing tasks in sequence:"
        tasks |> List.fold (fun acc t -> executeTask acc t) state
    | Conditional(condition, thenTask, elseTask) ->
        if condition state then
            executeTask state thenTask
        else
            match elseTask with
            | Some task -> executeTask state task
            | None -> state
    | LoopUntil(condition, task) ->
        let rec loop s =
            if condition s then s
            else loop (executeTask s task)
        loop state

Defining a Workflow

Using the DSL, we can now define a workflow that includes meaningful actions and flow control.

let myWorkflow =
    sequence [
        step "Start Engine" startEngine
        conditional isFuelLow
            (step "Refuel" (fun state -> { state with Fuel = state.Fuel + 10 }))
            None
        parallel [
            step "Check Fuel" checkFuel
            step "Check Oil" checkOil
        ]
        sequence [
            step "Launch" launch
            loopUntil (fun state -> state.Fuel < 1)
                (step "Burn Fuel" (fun state -> { state with Fuel = state.Fuel - 1 }))
        ]
        step "Shutdown" shutdown
    ]

let initialState = { Fuel = 12; Oil = 7; IsEngineRunning = false }
let finalState = executeTask initialState myWorkflow

Explanation

• Meaningful Actions: Each step in the workflow performs an actual game action, altering the GameState.
• Flow Control: The workflow includes a conditional to check fuel levels and a loopUntil to simulate fuel consumption until the fuel runs out.
• Error Handling: The use of failwith in actions handles errors if the conditions are not met, though in a real-world application, you'd likely want more robust error handling.

Conclusion

This example demonstrates how to extend a simple DSL concept into a more practical and powerful tool in F#. By leveraging F#'s functional features, you can create a DSL that orchestrates complex logic, manages state, and controls the flow of execution dynamically. This makes the DSL a valuable tool for managing complex scenarios, such as in a game engine or other interactive systems.