Convert Elixir to F#

Convert Elixir code to idiomatic F#. This skill extends meta-convert-dev with Elixir-to-F# specific type mappings, idiom translations, and tooling for transforming dynamic, actor-based Elixir code into functional-first, statically-typed F# on the .NET platform.

This Skill Extends

• meta-convert-dev - Foundational conversion patterns (APTV workflow, testing strategies)

For general concepts like the Analyze → Plan → Transform → Validate workflow, testing strategies, and common pitfalls, see the meta-skill first.

This Skill Adds

• Type mappings: Elixir dynamic types → F# static types with inference
• Idiom translations: Elixir pipelines/pattern matching → F# functional patterns
• Error handling: Tagged tuples {:ok, value} → Result<'T,'E>/Option<'T>
• Concurrency patterns: GenServer/processes → MailboxProcessor/async workflows
• Runtime translation: BEAM VM → .NET CLR with async/await
• Collection patterns: Elixir lists/maps → F# immutable collections

This Skill Does NOT Cover

• General conversion methodology - see meta-convert-dev
• Elixir language fundamentals - see lang-elixir-dev
• F# language fundamentals - see lang-fsharp-dev
• Reverse conversion (F# → Elixir) - see convert-fsharp-elixir
• OTP distributed systems - requires architectural redesign for .NET

---

Quick Reference

Elixir	F#	Notes
atom()	Discriminated union or string	:ok → Ok case, :error → Error case
integer()	int, int64, bigint	F# int is 32-bit
float()	float	64-bit double precision
binary()	string or byte[]	UTF-8 binary → UTF-16 string
list()	List<'T>	Both immutable, F# is singly-linked
tuple()	'T * 'U * ...	Direct mapping
map()	Map<'K, 'V>	Immutable maps
keyword list	List<'K * 'V>	List of tuples
pid()	MailboxProcessor<'T>	Actor-style processing
GenServer	MailboxProcessor + state loop	Actor pattern with message handling
Task	Async<'T> or Task<'T>	Async computations
Stream	Seq<'T>	Lazy sequences
{:ok, value}	Ok value (Result type)	Success case
{:error, reason}	Error reason (Result type)	Error case
nil	None (Option type)	Explicit nullable

---

When Converting Code

1. Analyze source thoroughly before writing target
2. Map types first - create type equivalence table, especially for atoms
3. Preserve functional semantics - both languages are functional-first
4. Translate actor patterns carefully - GenServer → MailboxProcessor requires state management
5. Use F# type inference - let compiler infer types when possible
6. Railway-oriented programming - translate {:ok, _}/{:error, _} to Result<'T,'E>
7. Leverage pipe operator - Elixir |> maps directly to F# |>
8. Test equivalence - same inputs → same outputs

---

Type System Mapping

Primitive Types

Elixir	F#	Notes
integer() (small)	int	32-bit signed integer
integer() (large)	int64	64-bit signed integer
integer() (arbitrary)	bigint	Arbitrary precision
float()	float	64-bit IEEE 754
boolean()	bool	true/false
atom()	Discriminated union	:ok, :error, etc. → DU cases
binary() (text)	string	UTF-8 → UTF-16
binary() (bytes)	byte[]	Raw bytes
bitstring()	Custom type	No direct equivalent
nil	None in Option<'T>	Must be wrapped
reference()	-	No direct equivalent
port()	-	BEAM-specific

Critical Note on Atoms: Elixir atoms are compile-time constants used extensively for tagging (:ok, :error, :atom_name). In F#, translate to:

• Discriminated unions for fixed sets (:ok/:error → Result type)
• Strings for dynamic/user-defined atoms
• Enums for simple value sets

Collection Types

Elixir	F#	Notes
list()	List<'T>	Both immutable, singly-linked
[h | t] pattern	head :: tail pattern	List destructuring
tuple()	'T * 'U * ...	Fixed-size product types
map()	Map<'K, 'V>	Immutable tree-based map
%{atom_key: value}	Record type { AtomKey: 'T }	Atom-keyed maps → records
%{string_key => value}	Map<string, 'T>	String-keyed maps
keyword list	List<string * 'T>	[key: value] → list of tuples
MapSet	Set<'T>	Immutable set
Range	seq { start..end }	Lazy range
Stream	Seq<'T>	Lazy sequences

Composite Types

Elixir	F#	Notes
struct	type Record = { }	Elixir structs → F# records
@type	type alias	Type aliases
Tagged tuple	Discriminated union	{:ok, value} → Ok value
@callback	Interface	Module behaviors → interfaces
defprotocol	Interface or type class	Protocol → interface
defimpl	Interface implementation	Implementation

---

Idiom Translation

Pattern 1: Pipe Operator (Direct Translation)

Elixir:

# Pipeline processing
"hello world"
|> String.upcase()
|> String.reverse()
|> String.split("")
|> Enum.join("-")

F#:

// Direct translation - pipe works the same!
"hello world"
|> String.toUpper
|> String.rev
|> Seq.toArray
|> String.concat "-"

// Or with built-in functions
"hello world"
|> (fun s -> s.ToUpper())
|> Seq.rev
|> String.concat "-"

Why this translation:

• Both languages use |> for left-to-right function chaining
• F# and Elixir share functional pipeline philosophy
• Minor differences in standard library function names

Pattern 2: Pattern Matching on Tagged Tuples

Elixir:

# Pattern matching on result tuples
case File.read("config.json") do
  {:ok, contents} ->
    Jason.decode(contents)
  {:error, :enoent} ->
    {:error, "File not found"}
  {:error, reason} ->
    {:error, "Read error: #{inspect(reason)}"}
end

F#:

// Using Result type with pattern matching
match System.IO.File.ReadAllText("config.json") |> tryRead with
| Ok contents ->
    Json.decode contents
| Error FileNotFound ->
    Error "File not found"
| Error (ReadError reason) ->
    Error $"Read error: {reason}"

// Helper to convert exceptions to Result
let tryRead path =
    try
        Ok (System.IO.File.ReadAllText(path))
    with
    | :? System.IO.FileNotFoundException -> Error FileNotFound
    | ex -> Error (ReadError ex.Message)

Why this translation:

• Elixir's {:ok, value} / {:error, reason} maps directly to F#'s Result<'T, 'E>
• Pattern matching syntax is similar in both languages
• F# requires explicit error type definition

Pattern 3: with Statement → Result Computation Expression

Elixir:

# Chain operations that can fail
def create_user(params) do
  with {:ok, validated} <- validate_params(params),
       {:ok, user} <- insert_user(validated),
       {:ok, email} <- send_welcome_email(user) do
    {:ok, user}
  else
    {:error, reason} -> {:error, reason}
  end
end

F#:

// Result computation expression
type ResultBuilder() =
    member _.Bind(x, f) = Result.bind f x
    member _.Return(x) = Ok x
    member _.ReturnFrom(x) = x

let result = ResultBuilder()

let createUser params =
    result {
        let! validated = validateParams params
        let! user = insertUser validated
        let! email = sendWelcomeEmail user
        return user
    }

// Or using built-in Result.bind (more verbose)
let createUser params =
    validateParams params
    |> Result.bind insertUser
    |> Result.bind (fun user ->
        sendWelcomeEmail user
        |> Result.map (fun _ -> user))

Why this translation:

• Elixir's with statement is railway-oriented programming
• F# computation expressions provide the same short-circuiting behavior
• let! in F# unwraps Result like <- unwraps tuples in Elixir

Pattern 4: GenServer → MailboxProcessor

Elixir:

defmodule Counter do
  use GenServer

  # Client API
  def start_link(initial_value) do
    GenServer.start_link(__MODULE__, initial_value, name: __MODULE__)
  end

  def increment do
    GenServer.call(__MODULE__, :increment)
  end

  def get do
    GenServer.call(__MODULE__, :get)
  end

  # Server Callbacks
  @impl true
  def init(initial_value) do
    {:ok, initial_value}
  end

  @impl true
  def handle_call(:increment, _from, state) do
    {:reply, state + 1, state + 1}
  end

  def handle_call(:get, _from, state) do
    {:reply, state, state}
  end
end

F#:

module Counter =
    // Message types (discriminated union)
    type Message =
        | Increment of AsyncReplyChannel<int>
        | Get of AsyncReplyChannel<int>

    // Agent wrapping MailboxProcessor
    type CounterAgent(initialValue: int) =
        let agent = MailboxProcessor.Start(fun inbox ->
            let rec loop state = async {
                let! msg = inbox.Receive()
                match msg with
                | Increment reply ->
                    let newState = state + 1
                    reply.Reply(newState)
                    return! loop newState
                | Get reply ->
                    reply.Reply(state)
                    return! loop state
            }
            loop initialValue
        )

        member _.Increment() = agent.PostAndReply(Increment)
        member _.Get() = agent.PostAndReply(Get)

    // Client API
    let start initialValue = CounterAgent(initialValue)

Why this translation:

• Both GenServer and MailboxProcessor implement the actor pattern
• F# uses discriminated unions for message types (typed vs Elixir's atoms)
• AsyncReplyChannel in F# replaces Elixir's from parameter for synchronous calls
• Recursive loop function maintains state like Elixir's callback chain

Pattern 5: Enum Functions → List/Seq Functions

Elixir:

# Enum operations
numbers = [1, 2, 3, 4, 5]

doubled = Enum.map(numbers, fn x -> x * 2 end)
evens = Enum.filter(numbers, fn x -> rem(x, 2) == 0 end)
sum = Enum.reduce(numbers, 0, fn x, acc -> x + acc end)

# With pipe operator
result =
  [1, 2, 3, 4, 5]
  |> Enum.filter(&(rem(&1, 2) == 0))
  |> Enum.map(&(&1 * 2))
  |> Enum.sum()

F#:

// List operations
let numbers = [1; 2; 3; 4; 5]

let doubled = numbers |> List.map (fun x -> x * 2)
let evens = numbers |> List.filter (fun x -> x % 2 = 0)
let sum = numbers |> List.fold (+) 0

// With pipe operator (idiomatic F#)
let result =
    [1; 2; 3; 4; 5]
    |> List.filter (fun x -> x % 2 = 0)
    |> List.map (fun x -> x * 2)
    |> List.sum

// Using Seq for lazy evaluation (like Elixir Stream)
let lazyResult =
    seq { 1..5 }
    |> Seq.filter (fun x -> x % 2 = 0)
    |> Seq.map (fun x -> x * 2)
    |> Seq.sum

Why this translation:

• Elixir Enum → F# List for eager evaluation
• Elixir Stream → F# Seq for lazy evaluation
• Function names are similar: map, filter, reduce/fold
• Both languages use pipe operator idiomatically

Pattern 6: Structs → Records

Elixir:

defmodule User do
  defstruct [:name, :email, age: 0, active: true]

  def new(name, email) do
    %User{name: name, email: email}
  end

  def activate(%User{} = user) do
    %{user | active: true}
  end
end

# Usage
user = %User{name: "Alice", email: "alice@example.com"}
user = User.activate(user)

F#:

module User =
    // Record type (immutable by default)
    type User = {
        Name: string
        Email: string
        Age: int
        Active: bool
    }

    // Constructor with defaults
    let create name email =
        { Name = name
          Email = email
          Age = 0
          Active = true }

    // Update function (returns new record)
    let activate user =
        { user with Active = true }

// Usage
let user = User.create "Alice" "alice@example.com"
let activeUser = User.activate user

Why this translation:

• Elixir structs and F# records are both immutable by default
• Copy-and-update syntax is similar: %{user | ...} vs { user with ... }
• F# records have structural equality automatically (like Elixir structs)

---

Error Handling

Elixir Tagged Tuples → F# Result Type

Elixir's error model:

# Return tagged tuples
def divide(a, b) when b != 0, do: {:ok, a / b}
def divide(_, 0), do: {:error, :division_by_zero}

# Pattern match on result
case divide(10, 2) do
  {:ok, result} -> "Result: #{result}"
  {:error, :division_by_zero} -> "Cannot divide by zero"
end

# With operator for chaining
with {:ok, a} <- divide(10, 2),
     {:ok, b} <- divide(20, 4),
     {:ok, c} <- divide(a, b) do
  {:ok, c}
else
  {:error, reason} -> {:error, reason}
end

F# Result type (idiomatic):

// Define error type
type MathError =
    | DivisionByZero

// Function returns Result<'T, 'E>
let divide a b =
    if b = 0.0 then
        Error DivisionByZero
    else
        Ok (a / b)

// Pattern match on result
let message =
    match divide 10.0 2.0 with
    | Ok result -> $"Result: {result}"
    | Error DivisionByZero -> "Cannot divide by zero"

// Result computation expression for chaining
let result =
    result {
        let! a = divide 10.0 2.0
        let! b = divide 20.0 4.0
        let! c = divide a b
        return c
    }

Translation guidelines:

• {:ok, value} → Ok value
• {:error, reason} → Error reason
• Atom errors (:not_found, :timeout) → discriminated union cases
• String errors remain strings or become union cases with data
• with statement → result { } computation expression

Option Type for Nullable Values

Elixir:

# nil represents absence
def find_user(id) do
  case Repo.get(User, id) do
    nil -> nil
    user -> user
  end
end

# Pattern matching on nil
case find_user(123) do
  nil -> "User not found"
  user -> "Found: #{user.name}"
end

F#:

// Option type for nullable
let findUser id =
    match Repo.get<User> id with
    | null -> None
    | user -> Some user

// Pattern matching on Option
match findUser 123 with
| None -> "User not found"
| Some user -> $"Found: {user.Name}"

// Or with Option.map
findUser 123
|> Option.map (fun user -> $"Found: {user.Name}")
|> Option.defaultValue "User not found"

Translation guidelines:

• nil → None (but wrapped in Option<'T>)
• Present value → Some value
• Pattern match or use Option module functions

---

Concurrency Patterns

Elixir Processes → F# MailboxProcessor

Elixir process communication:

# Spawn process
pid = spawn(fn ->
  receive do
    {:hello, sender} -> send(sender, {:ok, "Hello back!"})
  end
end)

# Send message
send(pid, {:hello, self()})

# Receive response
receive do
  {:ok, message} -> IO.puts(message)
after
  1000 -> IO.puts("Timeout!")
end

F# MailboxProcessor:

// Message type
type Message =
    | Hello of AsyncReplyChannel<string>

// Create mailbox processor
let processor = MailboxProcessor.Start(fun inbox ->
    async {
        let! msg = inbox.Receive()
        match msg with
        | Hello reply ->
            reply.Reply("Hello back!")
    }
)

// Send and receive (synchronous)
let message = processor.PostAndReply(Hello)
printfn "%s" message

// Or asynchronous with timeout
let! result = processor.PostAndAsyncReply(Hello, timeout = 1000)
match result with
| Some message -> printfn "%s" message
| None -> printfn "Timeout!"

Translation guidelines:

• spawn(fn -> ... end) → MailboxProcessor.Start(fun inbox -> async { ... })
• receive do ... end → let! msg = inbox.Receive()
• send(pid, msg) → processor.Post(msg) (async) or PostAndReply (sync)
• Elixir atoms for message tags → F# discriminated union cases

Task/async-await Pattern

Elixir:

# Async task
task1 = Task.async(fn -> fetch_data("url1") end)
task2 = Task.async(fn -> fetch_data("url2") end)

# Await results
result1 = Task.await(task1)
result2 = Task.await(task2)

# Parallel map
results =
  ["url1", "url2", "url3"]
  |> Task.async_stream(&fetch_data/1)
  |> Enum.to_list()

F#:

// Async computation
let task1 = async { return fetchData "url1" }
let task2 = async { return fetchData "url2" }

// Await results
let! result1 = task1
let! result2 = task2

// Parallel execution
let results =
    ["url1"; "url2"; "url3"]
    |> List.map (fun url -> async { return fetchData url })
    |> Async.Parallel
    |> Async.RunSynchronously

// Or with Task<'T> (closer to .NET idioms)
let task1 = Task.Run(fun () -> fetchData "url1")
let task2 = Task.Run(fun () -> fetchData "url2")

let results = Task.WaitAll([| task1; task2 |])

Translation guidelines:

• Task.async(fn -> ... end) → async { ... } or Task.Run
• Task.await(task) → let! result = task (in async block)
• Task.async_stream → Async.Parallel or parallel LINQ
• Elixir Task is eager; F# Async<'T> is lazy (runs when awaited)

Supervision Pattern

Elixir:

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      {Stack, []},
      {MyWorker, name: MyWorker, restart: :transient}
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

F# (using Akka.NET or custom supervision):

// Custom supervision with restart logic
module Supervisor =
    type SupervisedAgent<'State, 'Msg>(
        initialState: 'State,
        handler: MailboxProcessor<'Msg> -> 'State -> 'Msg -> Async<'State>) =

        let mutable agent = None

        let rec start() =
            let a = MailboxProcessor.Start(fun inbox ->
                let rec loop state = async {
                    try
                        let! msg = inbox.Receive()
                        let! newState = handler inbox state msg
                        return! loop newState
                    with
                    | ex ->
                        printfn "Agent crashed: %s" ex.Message
                        // Restart with initial state
                        return! loop initialState
                }
                loop initialState
            )
            agent <- Some a
            a

        member _.Agent = agent |> Option.defaultWith start

// Or use Akka.NET for production OTP-like supervision
open Akka.Actor
open Akka.FSharp

let system = System.create "my-system" (Configuration.load())

let supervisor = spawn system "supervisor" (fun mailbox ->
    // Akka.NET provides OTP-like supervision strategies
    actorOf2 (fun mailbox ->
        actor {
            // Actor logic
        })
)

Translation guidelines:

• Simple supervision: custom restart logic with MailboxProcessor
• Complex supervision: use Akka.NET (F#-friendly OTP alternative)
• Restart strategies map: :one_for_one, :one_for_all, :rest_for_one
• F# lacks built-in supervision; requires libraries or custom implementation

---

Common Pitfalls

1. Atom explosion: Elixir atoms are lightweight; F# strings are not. Use discriminated unions for fixed sets of atoms to get compile-time safety.

2. Dynamic typing assumptions: Elixir's dynamic typing allows %{id: 1, name: "Alice"} and %{id: 2, age: 30} in the same list. F# requires consistent types—use discriminated unions for heterogeneous data.

3. Process registry: Elixir's process registry (named processes) doesn't have a direct F# equivalent. Use dependency injection or singleton patterns for named services.

4. Hot code swapping: BEAM's hot code swapping is not available on .NET. Plan for deployment downtime or use blue-green deployment strategies.

5. String encoding: Elixir binaries are UTF-8; .NET strings are UTF-16. Be careful with string/binary conversion, especially for protocols.

6. Pattern matching on strings: Elixir can pattern match on binary structure (<<a, b, rest::binary>>). F# requires imperative slicing or active patterns.

7. Lazy evaluation differences: Elixir Stream is pull-based lazy; F# Seq is also lazy but behaves slightly differently with side effects.

8. Async cancellation: Elixir processes can be killed easily; F# async requires CancellationToken propagation.

---

Tooling

Tool	Purpose	Notes
dotnet CLI	Build, run, test F# projects	dotnet build, dotnet run
Ionide	VS Code F# extension	Provides IntelliSense, linting
Paket	Alternative package manager	Deterministic dependency resolution
FAKE	Build automation	F# Make (like Mix tasks)
Expecto	Testing framework	Similar to ExUnit
FsCheck	Property-based testing	Like Elixir's StreamData
Akka.NET	Actor framework	OTP-like supervision and distribution
Giraffe	Web framework	Functional HTTP (like Phoenix)

---

Examples

Example 1: Simple - Pattern Matching and Recursion

Before (Elixir):

defmodule Math do
  # Factorial using pattern matching
  def factorial(0), do: 1
  def factorial(n) when n > 0, do: n * factorial(n - 1)

  # FizzBuzz with pattern matching
  def fizzbuzz(n) do
    case {rem(n, 3), rem(n, 5)} do
      {0, 0} -> "FizzBuzz"
      {0, _} -> "Fizz"
      {_, 0} -> "Buzz"
      _ -> to_string(n)
    end
  end
end

After (F#):

module Math =
    // Factorial using pattern matching
    let rec factorial n =
        match n with
        | 0 -> 1
        | n when n > 0 -> n * factorial (n - 1)
        | _ -> failwith "Negative input not allowed"

    // FizzBuzz with pattern matching
    let fizzbuzz n =
        match (n % 3, n % 5) with
        | (0, 0) -> "FizzBuzz"
        | (0, _) -> "Fizz"
        | (_, 0) -> "Buzz"
        | _ -> string n

Example 2: Medium - Error Handling with Result

Before (Elixir):

defmodule UserService do
  def register_user(params) do
    with {:ok, email} <- validate_email(params["email"]),
         {:ok, password} <- validate_password(params["password"]),
         {:ok, user} <- create_user(email, password),
         {:ok, _} <- send_welcome_email(user) do
      {:ok, user}
    else
      {:error, :invalid_email} -> {:error, "Invalid email format"}
      {:error, :weak_password} -> {:error, "Password too weak"}
      {:error, :email_taken} -> {:error, "Email already registered"}
      {:error, reason} -> {:error, "Registration failed: #{reason}"}
    end
  end

  defp validate_email(email) when is_binary(email) do
    if String.contains?(email, "@") do
      {:ok, email}
    else
      {:error, :invalid_email}
    end
  end

  defp validate_password(password) when byte_size(password) >= 8 do
    {:ok, password}
  end
  defp validate_password(_), do: {:error, :weak_password}

  defp create_user(email, password) do
    # Simulate database insert
    if email == "taken@example.com" do
      {:error, :email_taken}
    else
      {:ok, %{id: 1, email: email, password_hash: hash_password(password)}}
    end
  end

  defp send_welcome_email(user) do
    # Simulate email sending
    {:ok, user}
  end

  defp hash_password(password), do: :crypto.hash(:sha256, password)
end

After (F#):

module UserService =
    // Error type
    type RegistrationError =
        | InvalidEmail
        | WeakPassword
        | EmailTaken
        | RegistrationFailed of string

    // User type
    type User = {
        Id: int
        Email: string
        PasswordHash: byte[]
    }

    // Result computation expression
    type ResultBuilder() =
        member _.Bind(x, f) = Result.bind f x
        member _.Return(x) = Ok x
        member _.ReturnFrom(x) = x

    let result = ResultBuilder()

    // Validation functions
    let validateEmail email =
        if String.contains email "@" then
            Ok email
        else
            Error InvalidEmail

    let validatePassword password =
        if String.length password >= 8 then
            Ok password
        else
            Error WeakPassword

    let hashPassword password =
        use sha256 = System.Security.Cryptography.SHA256.Create()
        Text.Encoding.UTF8.GetBytes(password)
        |> sha256.ComputeHash

    let createUser email password =
        // Simulate database insert
        if email = "taken@example.com" then
            Error EmailTaken
        else
            Ok { Id = 1; Email = email; PasswordHash = hashPassword password }

    let sendWelcomeEmail user =
        // Simulate email sending
        Ok user

    // Main registration function
    let registerUser params =
        result {
            let! email = Map.tryFind "email" params |> Option.toResult InvalidEmail
            let! validEmail = validateEmail email

            let! password = Map.tryFind "password" params |> Option.toResult WeakPassword
            let! validPassword = validatePassword password

            let! user = createUser validEmail validPassword
            let! _ = sendWelcomeEmail user
            return user
        }
        |> Result.mapError (fun error ->
            match error with
            | InvalidEmail -> "Invalid email format"
            | WeakPassword -> "Password too weak"
            | EmailTaken -> "Email already registered"
            | RegistrationFailed reason -> $"Registration failed: {reason}"
        )

    // Helper extension
    module Option =
        let toResult error option =
            match option with
            | Some value -> Ok value
            | None -> Error error

Example 3: Complex - GenServer to MailboxProcessor

Before (Elixir):

defmodule ChatRoom do
  use GenServer

  # Client API

  def start_link(room_name) do
    GenServer.start_link(__MODULE__, room_name, name: via_tuple(room_name))
  end

  def join(room_name, user_name) do
    GenServer.call(via_tuple(room_name), {:join, user_name})
  end

  def leave(room_name, user_name) do
    GenServer.cast(via_tuple(room_name), {:leave, user_name})
  end

  def send_message(room_name, user_name, message) do
    GenServer.cast(via_tuple(room_name), {:message, user_name, message})
  end

  def get_users(room_name) do
    GenServer.call(via_tuple(room_name), :get_users)
  end

  # Server Callbacks

  @impl true
  def init(room_name) do
    state = %{
      room_name: room_name,
      users: MapSet.new(),
      messages: []
    }
    {:ok, state}
  end

  @impl true
  def handle_call({:join, user_name}, _from, state) do
    if MapSet.member?(state.users, user_name) do
      {:reply, {:error, :already_joined}, state}
    else
      new_state = %{state | users: MapSet.put(state.users, user_name)}
      broadcast_message(new_state, "#{user_name} joined the room")
      {:reply, {:ok, :joined}, new_state}
    end
  end

  def handle_call(:get_users, _from, state) do
    {:reply, MapSet.to_list(state.users), state}
  end

  @impl true
  def handle_cast({:leave, user_name}, state) do
    new_state = %{state | users: MapSet.delete(state.users, user_name)}
    broadcast_message(new_state, "#{user_name} left the room")
    {:noreply, new_state}
  end

  def handle_cast({:message, user_name, message}, state) do
    formatted = "#{user_name}: #{message}"
    new_state = %{state | messages: [formatted | state.messages]}
    broadcast_message(new_state, formatted)
    {:noreply, new_state}
  end

  # Private Helpers

  defp via_tuple(room_name) do
    {:via, Registry, {ChatRegistry, room_name}}
  end

  defp broadcast_message(state, message) do
    # In real app, would use PubSub
    IO.puts("[#{state.room_name}] #{message}")
  end
end

After (F#):

module ChatRoom =
    // Types
    type User = string
    type Message = string

    type State = {
        RoomName: string
        Users: Set<User>
        Messages: Message list
    }

    // Message types
    type ChatMessage =
        | Join of user: User * AsyncReplyChannel<Result<unit, string>>
        | Leave of user: User
        | SendMessage of user: User * message: Message
        | GetUsers of AsyncReplyChannel<User list>

    // Chat room agent
    type ChatRoomAgent(roomName: string) =
        let agent = MailboxProcessor.Start(fun inbox ->
            let broadcastMessage state message =
                printfn "[%s] %s" state.RoomName message

            let rec loop state = async {
                let! msg = inbox.Receive()

                match msg with
                | Join (user, reply) ->
                    if Set.contains user state.Users then
                        reply.Reply(Error "Already joined")
                        return! loop state
                    else
                        let newState = { state with Users = Set.add user state.Users }
                        broadcastMessage newState $"{user} joined the room"
                        reply.Reply(Ok ())
                        return! loop newState

                | Leave user ->
                    let newState = { state with Users = Set.remove user state.Users }
                    broadcastMessage newState $"{user} left the room"
                    return! loop newState

                | SendMessage (user, message) ->
                    let formatted = $"{user}: {message}"
                    let newState = { state with Messages = formatted :: state.Messages }
                    broadcastMessage newState formatted
                    return! loop newState

                | GetUsers reply ->
                    reply.Reply(Set.toList state.Users)
                    return! loop state
            }

            let initialState = {
                RoomName = roomName
                Users = Set.empty
                Messages = []
            }

            loop initialState
        )

        // Client API
        member _.Join(user) =
            agent.PostAndReply(fun ch -> Join(user, ch))

        member _.Leave(user) =
            agent.Post(Leave user)

        member _.SendMessage(user, message) =
            agent.Post(SendMessage(user, message))

        member _.GetUsers() =
            agent.PostAndReply(GetUsers)

    // Room registry (simple dictionary-based)
    module Registry =
        open System.Collections.Concurrent

        let private rooms = ConcurrentDictionary<string, ChatRoomAgent>()

        let getOrCreate roomName =
            rooms.GetOrAdd(roomName, fun name -> ChatRoomAgent(name))

        let tryGet roomName =
            match rooms.TryGetValue(roomName) with
            | true, room -> Some room
            | false, _ -> None

// Usage example
let room = ChatRoom.Registry.getOrCreate "lobby"

match room.Join("Alice") with
| Ok () -> printfn "Alice joined successfully"
| Error msg -> printfn "Join failed: %s" msg

room.SendMessage("Alice", "Hello everyone!")

let users = room.GetUsers()
printfn "Current users: %A" users

room.Leave("Alice")

---

See Also

For more examples and patterns, see:

• meta-convert-dev - Foundational patterns with cross-language examples
• convert-erlang-fsharp - Similar BEAM → .NET conversion (Elixir's foundation)
• lang-elixir-dev - Elixir development patterns
• lang-fsharp-dev - F# development patterns

Cross-cutting pattern skills:

• patterns-concurrency-dev - Actor patterns, async workflows across languages
• patterns-serialization-dev - JSON handling, validation across languages
• patterns-metaprogramming-dev - Compile-time code generation patterns