free monad

FreeMonad.fs

// Define the types for the inputs and outputs
type AccountID = AccountID of int
type Balance = Balance of decimal
type Deposit = Deposit of AccountID * decimal
type LogMessage = string

// Define an error type for your operations
type BankError =
    | AccountNotFound of int
    | InsufficientFunds of decimal
    | DepositFailed of string
    | LogError of string
    // Add other error cases as needed


// Define the Free monad constructs including Log
type FaceInstruction<'a> =
    | CheckBalance of AccountID * (Balance -> 'a)
    | PerformDeposit of Deposit * (Balance -> 'a)
    | Log of LogMessage * (unit -> 'a)

let private mapI f = function
    | CheckBalance (x, next) -> CheckBalance (x, next >> f)
    | PerformDeposit (x, next) -> PerformDeposit (x, next >> f)
    | Log (msg, next) -> Log (msg, next >> f)

type FaceProgram<'a> =
    | Free of FaceInstruction<FaceProgram<'a>>
    | Pure of 'a

let rec bind f = function
    | Free x -> Free (mapI (bind f) x)
    | Pure x -> f x

type FaceBuilder () =
    member _.Bind (x, f) = bind f x
    member _.Return x = Pure x
    member _.ReturnFrom x = x
    member _.Zero () = Pure ()

let face = FaceBuilder ()

// Define operations using the FaceProgram type
let checkAccountBalance accountID = Free (CheckBalance (accountID, Pure))
let makeDeposit deposit = Free (PerformDeposit (deposit, Pure))
let log message = Free (Log (message, Pure)) // Log operation
open System

let bankOperations = face {
    do! log "Starting bank operations"
    let! initialBalance = checkAccountBalance (AccountID 1)
    do! log (sprintf "Initial balance: %A" initialBalance)

    let! postDepositBalance = makeDeposit (Deposit (AccountID 1, 50.0M))
    do! log (sprintf "Balance after deposit: %A" postDepositBalance)

    return postDepositBalance
}
//---- cut off

// Define mock implementations for the operations as async functions returning Result
let getAccountBalance (AccountID id) : Async<Result<Balance, BankError>> = async {
    // Simulate asynchronous operation
    do! Async.Sleep 100
    match id with
    | 1 -> return Ok (Balance 100.0M)
    | _ -> return Error (AccountNotFound id)
}

let processDeposit (Deposit (id, amount)) : Async<Result<Balance, BankError>> = async {
    // Simulate asynchronous operation
    do! Async.Sleep 100
    match id with
    |  AccountID 1 -> return Ok (Balance (100.0M + amount))
    | _ -> return Error (DepositFailed $"Account {id} not found.")
}

let logMessage (msg : LogMessage) : Async<Result<unit, BankError>> = async {
    // Simulate asynchronous logging
    do! Async.Sleep 50
    try
        printfn "%s" msg
        return Ok ()
    with ex ->
        return Error (LogError ex.Message)
}

// Define the interpreter for the operations including handling for Log
let rec interpret (program : FaceProgram<'a>) : Async<Result<'a, BankError>> =
    async {
        match program with
        | Pure x -> return Ok x
        | Free (CheckBalance (accountID, next)) ->
            let! result = getAccountBalance accountID
            match result with
            | Ok balance ->
                return! interpret (next balance)
            | Error e -> return Error e
        | Free (PerformDeposit (deposit, next)) ->
            let! result = processDeposit deposit
            match result with
            | Ok newBalance ->
                return! interpret (next newBalance)
            | Error e -> return Error e
        | Free (Log (message, next)) ->
            let! result = logMessage message
            match result with
            | Ok () ->
                return! interpret (next ())
            | Error e -> return Error e
    }

// Use the computation expression to define a bank operation workflow

// Run the bank operations and print the result

async {
    let! result = interpret bankOperations
    match result with
    | Ok finalBalance ->
        printfn "Final result: %A" finalBalance
    | Error error ->
        printfn "An error occurred: %A" error
    return ()
} |> Async.StartImmediate