Convert Elixir to Roc

Convert Elixir code to idiomatic Roc. This skill extends meta-convert-dev with Elixir-to-Roc specific type mappings, idiom translations, and architectural patterns for moving from dynamic, actor-based concurrency to static, pure functional programming with platform-provided effects.

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 → Roc static types
• Paradigm translation: GenServer/OTP patterns → Pure state machines + platform Tasks
• Idiom translations: Elixir functional patterns → Roc functional patterns
• Error handling: Elixir error tuples + exceptions → Result types
• Concurrency: Processes/GenServer → Roc platform Tasks
• Module system: Elixir modules → Roc platform/application architecture
• Metaprogramming: Elixir macros → Roc abilities and external codegen

This Skill Does NOT Cover

• General conversion methodology - see meta-convert-dev
• Elixir language fundamentals - see lang-elixir-dev
• Roc language fundamentals - see lang-roc-dev
• Reverse conversion (Roc → Elixir) - see convert-roc-elixir

---

Quick Reference

Elixir	Roc	Notes
:atom	[Tag]	Atoms become tags in tag unions
integer()	I64 / U64	Specify signedness explicitly
float()	F64	64-bit float
String.t()	Str	UTF-8 strings
binary()	List U8	Byte sequences
list()	List a	Homogeneous lists
tuple()	(a, b, c)	Fixed-size tuples
map()	Dict k v	Key-value maps (requires Hash+Eq keys)
keyword()	List (Str, a)	List of tuples (no duplicate keys guaranteed)
{:ok, value}	Ok(value)	Success result
{:error, reason}	Err(reason)	Error result
nil	None in tag union	Optional values
pid()	-	No direct equivalent (platform handles processes)
fn(x) -> x end	\x -> x	Lambda syntax
%{__struct__: Name}	Opaque type Name := ...	Structs become opaque types

When Converting Code

1. Analyze OTP patterns before writing Roc - GenServers become pure state machines
2. Identify process boundaries - these become platform Task interactions
3. Map dynamic patterns to static types - use tag unions for variants
4. Redesign supervision trees - Roc platforms handle failure differently
5. Extract pure logic - separate computation from effects (Task, IO)
6. Handle hot code loading - not a language feature in Roc
7. Test equivalence - verify behavior matches despite different architecture

---

Type System Mapping

Primitive Types

Elixir	Roc	Notes
:atom	[Tag]	Atoms as tags in tag unions
integer()	I64	Default signed 64-bit
integer()	U64	Unsigned variant
integer() (small)	I32 / U32	For smaller values
integer() (arbitrary)	-	Roc has fixed-size integers
float()	F64	64-bit floating point
boolean()	Bool	Direct mapping
String.t()	Str	UTF-8 strings
binary()	List U8	Byte sequences
bitstring()	List U8	Byte-aligned only in Roc
nil	None	In tag union [Some a, None]
pid()	-	Processes don't exist in Roc
port()	-	Platform handles I/O
reference()	-	No direct equivalent
function()	Function types	See function mappings below

Collection Types

Elixir	Roc	Notes
list()	List a	Homogeneous lists (all same type)
[T]	List T	Type-safe, uniform elements
tuple()	(A, B, C)	Fixed-size tuples
{A, B, C}	(A, B, C)	Direct structural mapping
map()	Dict k v	Key-value dictionary (requires Hash+Eq for keys)
%{K => V}	Dict K V	Keys must implement Hash and Eq abilities
MapSet.t()	Set a	Unique values (requires Hash+Eq)
keyword()	List (Str, a)	List of tuples, no uniqueness guarantee
[{:key, value}]	List (Str, a)	Keyword lists as tuple lists
Range.t()	-	Use List.range to generate lists
Stream.t()	-	Roc has lazy iterators but no Stream type

Composite Types

Elixir	Roc	Notes
defstruct	{ field : Type }	Structs become records
%Name{field: value}	{ field: value }	Record literals
Tagged tuple {:tag, value}	Tag(value)	Tags with payloads
@type name :: spec	Name : Type	Type alias
@opaque name :: spec	Opaque type Name := Type	Hidden implementation
@spec annotations	Type signatures	Enforced in Roc

Function Types

Elixir	Roc	Notes
(-> R)	({} -> R)	Zero-argument function
(A -> R)	(A -> R)	Single argument
(A, B -> R)	(A, B -> R)	Multiple arguments (curried)
Variable arity	-	Roc doesn't support varargs
Default arguments	-	Use separate function signatures

Error Types

Elixir	Roc	Notes
{:ok, value}	Ok(value)	Success case
{:error, reason}	Err(reason)	Error case
:ok atom	Ok({})	Success with no value
{:ok, V} | {:error, R}	Result V R	Result type
Exception raising	Err variant	No exceptions, use Result
rescue clause	Pattern match on Result	Explicit error handling

---

Idiom Translation

Pattern 1: Simple Module Conversion

Elixir:

defmodule MathUtils do
  @doc "Add two numbers"
  def add(a, b), do: a + b

  @doc "Square a number"
  def square(n), do: n * n

  # Private function
  defp internal_helper(x), do: x * 2
end

Roc:

interface MathUtils
    exposes [add, square]
    imports []

## Add two numbers
add : I64, I64 -> I64
add = \a, b -> a + b

## Square a number
square : I64 -> I64
square = \n -> n * n

# Private helper (not exposed)
internalHelper : I64 -> I64
internalHelper = \x -> x * 2

Why this translation:

• Elixir modules become Roc interfaces
• Public functions go in exposes
• Private functions are simply not exposed
• Doctests become inline expect tests in Roc
• Type signatures are explicit (not optional like typespecs)

Pattern 2: Pattern Matching on Tagged Tuples

Elixir:

def process_result({:ok, data}) do
  {:success, data}
end

def process_result({:error, reason}) do
  {:failure, reason}
end

def process_result(_unknown) do
  {:failure, :unknown_result}
end

Roc:

processResult : [Ok Data, Err Reason, Unknown] -> [Success Data, Failure Reason]
processResult = \result ->
    when result is
        Ok(data) -> Success(data)
        Err(reason) -> Failure(reason)
        Unknown -> Failure(UnknownResult)

Why this translation:

• Elixir tagged tuples map to Roc tags
• Pattern matching syntax is similar (when vs pattern match in function head)
• Tag unions make all cases explicit in type signature
• Type system ensures exhaustiveness (can't forget cases)

Pattern 3: Pipe Operator

Elixir:

def process_data(data) do
  data
  |> String.trim()
  |> String.downcase()
  |> String.split(",")
  |> Enum.map(&String.trim/1)
  |> Enum.reject(&(&1 == ""))
end

Roc:

processData : Str -> List Str
processData = \data ->
    data
    |> Str.trim
    |> Str.toLowercase
    |> Str.split(",")
    |> List.map(Str.trim)
    |> List.keepIf(\s -> s != "")

Why this translation:

• Both languages have pipe operators with similar semantics
• Roc uses keepIf instead of reject with negated predicate
• Capture operator & in Elixir becomes explicit lambdas in Roc
• Type inference works similarly in both

Pattern 4: Enum Operations

Elixir:

def sum(list), do: Enum.reduce(list, 0, &+/2)

def squares(list), do: Enum.map(list, fn x -> x * x end)

def evens(list), do: Enum.filter(list, fn x -> rem(x, 2) == 0 end)

def find_first_large(list) do
  Enum.find(list, fn x -> x > 100 end)
end

Roc:

sum : List I64 -> I64
sum = \list ->
    List.walk(list, 0, Num.add)

squares : List I64 -> List I64
squares = \list ->
    List.map(list, \x -> x * x)

evens : List I64 -> List I64
evens = \list ->
    List.keepIf(list, \x -> x % 2 == 0)

findFirstLarge : List I64 -> [Some I64, None]
findFirstLarge = \list ->
    list
    |> List.findFirst(\x -> x > 100)
    |> Result.map(Some)
    |> Result.withDefault(None)

Why this translation:

• Enum.reduce becomes List.walk (fold)
• Enum.filter becomes List.keepIf
• Enum.find returns Result, needs conversion to option type
• Function captures (&+/2) become explicit function references (Num.add)
• Roc requires explicit handling of "not found" cases

Pattern 5: Struct to Record

Elixir:

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

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

  def update_age(user, new_age) do
    %{user | age: new_age}
  end

  def greet(%User{name: name}), do: "Hello, #{name}!"
end

Roc:

interface User
    exposes [User, new, updateAge, greet]
    imports []

User : {
    name : Str,
    email : Str,
    age : U32,
}

new : Str, Str -> User
new = \name, email ->
    { name, email, age: 0 }

updateAge : User, U32 -> User
updateAge = \user, newAge ->
    { user & age: newAge }

greet : User -> Str
greet = \{ name } ->
    "Hello, \(name)!"

Why this translation:

• Elixir structs map directly to Roc records
• Record update syntax is similar (%{user | age:} vs { user & age:})
• Pattern matching on records works similarly
• Roc records are structural (no __struct__ metadata)
• Default values in struct definition become default in constructor

Pattern 6: GenServer → Pure State Machine

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.cast(__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(:get, _from, state) do
    {:reply, state, state}
  end

  @impl true
  def handle_cast(:increment, state) do
    {:noreply, state + 1}
  end
end

Roc:

# Pure state machine - no processes
interface Counter
    exposes [State, init, increment, get]
    imports []

State : I64

init : State
init = 0

increment : State -> State
increment = \count ->
    count + 1

get : State -> I64
get = \count ->
    count

# If you need effects, use platform Tasks
# The platform would provide state management and concurrency
# Application code remains pure

Why this translation:

• GenServer becomes pure state functions
• No process lifecycle - just data transformation
• State is explicit parameter and return value
• call operations become synchronous functions that return new state
• cast operations become functions that just transform state
• Effects and concurrency are platform responsibilities, not shown here
• Platform provides state management if needed

Pattern 7: With Statement Error Handling

Elixir:

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

Roc:

createUser : Params -> Result User [ValidationErr, InsertErr, EmailErr]
createUser = \params ->
    validated = validateParams!(params)
    user = insertUser!(validated)
    _email = sendWelcomeEmail!(user)
    Ok(user)

Why this translation:

• Elixir with becomes Roc ! operator (try/propagate)
• ! suffix propagates errors automatically
• Error types are unified in tag union
• Early return on error is implicit with !
• More concise than explicit pattern matching

Pattern 8: Optional Values

Elixir:

def find_user(id, users) do
  Enum.find(users, fn user -> user.id == id end)
end

def get_email(nil), do: "no email"
def get_email(user), do: user.email

# Using in pipeline
def display_email(id, users) do
  users
  |> find_user(id)
  |> case do
    nil -> "User not found"
    user -> user.email
  end
end

Roc:

findUser : U64, List User -> [Some User, None]
findUser = \id, users ->
    users
    |> List.findFirst(\user -> user.id == id)
    |> Result.map(Some)
    |> Result.withDefault(None)

getEmail : [Some User, None] -> Str
getEmail = \maybeUser ->
    when maybeUser is
        Some({ email }) -> email
        None -> "no email"

displayEmail : U64, List User -> Str
displayEmail = \id, users ->
    when findUser(id, users) is
        Some(user) -> user.email
        None -> "User not found"

Why this translation:

• Elixir nil maps to None in tag union
• Enum.find returns nil or value; Roc findFirst returns Result
• Explicit pattern matching on option types
• Type system prevents forgetting to handle None case
• Must convert Result to option type with Some/None

Pattern 9: List Comprehensions

Elixir:

def squares(list) do
  for x <- list, do: x * x
end

def evens(list) do
  for x <- list, rem(x, 2) == 0, do: x
end

def cartesian_product(list1, list2) do
  for x <- list1, y <- list2, do: {x, y}
end

def filtered_map(list) do
  for x <- list, x > 0, into: %{}, do: {x, x * x}
end

Roc:

squares : List I64 -> List I64
squares = \list ->
    List.map(list, \x -> x * x)

evens : List I64 -> List I64
evens = \list ->
    List.keepIf(list, \x -> x % 2 == 0)

cartesianProduct : List a, List b -> List (a, b)
cartesianProduct = \list1, list2 ->
    List.joinMap(list1, \x ->
        List.map(list2, \y -> (x, y))
    )

filteredMap : List I64 -> Dict I64 I64
filteredMap = \list ->
    list
    |> List.keepIf(\x -> x > 0)
    |> List.map(\x -> (x, x * x))
    |> Dict.fromList

Why this translation:

• Comprehensions become map/keepIf operations
• Nested comprehensions use joinMap (flatMap/concatMap)
• Filters become keepIf
• into: %{} becomes Dict.fromList
• More verbose but explicit
• Type signatures make intent clear

Pattern 10: Protocol Implementation

Elixir:

defprotocol Serializable do
  @doc "Serialize to JSON"
  def to_json(data)
end

defimpl Serializable, for: Map do
  def to_json(map) do
    Jason.encode!(map)
  end
end

defimpl Serializable, for: List do
  def to_json(list) do
    Jason.encode!(list)
  end
end

# Usage
Serializable.to_json(%{name: "Alice"})

Roc:

# Roc uses abilities (similar to typeclasses)
# Built-in Encode ability is auto-derived for most types

# For custom encoding:
toJson : a -> Str where a implements Encode
toJson = \value ->
    value
    |> Encode.toBytes(Json.utf8)
    |> Str.fromUtf8

# Automatic for records and tags
user = { name: "Alice", age: 30 }
json = toJson(user)  # Works automatically

# For custom types with specific behavior:
User := { name : Str, age : U32 }

toJsonUser : User -> Str
toJsonUser = \@User({ name, age }) ->
    """
    {\"name\": \"\(name)\", \"age\": \(Num.toStr(age))}
    """

Why this translation:

• Elixir protocols map to Roc abilities
• Many abilities are auto-derived (Encode, Decode, Eq, Hash, Inspect)
• For custom behavior, write explicit functions
• No dynamic dispatch in Roc - abilities are compile-time
• Roc's approach is more restrictive but type-safe

---

Concurrency Patterns

Elixir Process Model vs Roc Task Model

Elixir's concurrency is built on lightweight BEAM processes with message passing. Roc has no built-in concurrency - it's all platform-provided through Tasks.

Mental Model Shift:

Elixir Concept	Roc Approach	Key Insight
Process with state	Pure state machine + platform Task	Data and behavior separated
Message passing	Function parameters and results	Explicit data flow, no mailboxes
spawn	Platform Task creation	Effects are platform capability
GenServer	Pure functions + Task orchestration	Business logic pure, I/O delegated
Supervisor	Platform-level concern	Fault tolerance in host
Hot code reload	Not available	Platform restart required

Pattern: Spawned Task

Elixir:

defmodule Worker do
  def start do
    spawn(fn -> loop() end)
  end

  defp loop do
    receive do
      {:work, from, data} ->
        result = process(data)
        send(from, {:result, result})
        loop()
      :stop ->
        :ok
    end
  end

  defp process(data), do: String.upcase(data)
end

# Usage
pid = Worker.start()
send(pid, {:work, self(), "hello"})
receive do
  {:result, result} -> IO.puts(result)
end

Roc:

# Pure processing function
process : Str -> Str
process = \data ->
    Str.toUppercase(data)

# If concurrency is needed, platform provides it
import pf.Task exposing [Task]

processAsync : Str -> Task Str []
processAsync = \data ->
    # Platform handles concurrent execution
    Task.await(Task.fromThunk(\{} -> process(data)))

# Usage (in Task context)
main : Task {} []
main =
    result = processAsync!("hello")
    Stdout.line!(result)

Why this translation:

• No process spawning in Roc - platform handles concurrency
• Pure function for business logic (process)
• Platform Task for effects
• No message passing - direct function composition
• Platform decides execution strategy (sync/async/parallel)

Pattern: GenServer State

Elixir:

defmodule Cache do
  use GenServer

  def start_link do
    GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
  end

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

  def put(key, value) do
    GenServer.cast(__MODULE__, {:put, key, value})
  end

  @impl true
  def init(_), do: {:ok, %{}}

  @impl true
  def handle_call({:get, key}, _from, state) do
    {:reply, Map.get(state, key), state}
  end

  @impl true
  def handle_cast({:put, key, value}, state) do
    {:noreply, Map.put(state, key, value)}
  end
end

Roc:

# Pure state functions
State : Dict Str Str

init : State
init = Dict.empty({})

get : State, Str -> [Some Str, None]
get = \state, key ->
    Dict.get(state, key)
    |> Result.map(Some)
    |> Result.withDefault(None)

put : State, Str, Str -> State
put = \state, key, value ->
    Dict.insert(state, key, value)

# Platform would provide state management
# For example, a hypothetical Agent-like platform API:
#
# import pf.Agent exposing [Agent, start, getState, updateState]
#
# cacheAgent : Agent State
# cacheAgent = start(init)
#
# getValue : Str -> Task [Some Str, None] []
# getValue = \key ->
#     Agent.getState(cacheAgent, \state -> get(state, key))
#
# putValue : Str, Str -> Task {} []
# putValue = \key, value ->
#     Agent.updateState(cacheAgent, \state -> put(state, key, value))

Why this translation:

• GenServer becomes pure state transformations
• State management delegated to platform (Agent, Store, etc.)
• No process registered names - platform handles references
• No call vs cast distinction - just sync vs async Task
• Fault tolerance handled by platform, not supervision tree

Pattern: Task.async

Elixir:

def fetch_multiple(urls) do
  tasks = Enum.map(urls, fn url ->
    Task.async(fn -> fetch_url(url) end)
  end)

  Task.await_many(tasks, 5000)
end

defp fetch_url(url) do
  case HTTPoison.get(url) do
    {:ok, %{body: body}} -> {:ok, body}
    {:error, reason} -> {:error, reason}
  end
end

Roc:

import pf.Http
import pf.Task exposing [Task]

fetchMultiple : List Str -> Task (List Str) [HttpErr]
fetchMultiple = \urls ->
    urls
    |> List.map(\url -> Http.get(url))
    |> Task.sequence  # Platform may execute concurrently
    |> Task.map(\responses -> List.map(responses, \r -> r.body))

# Platform-specific concurrent execution
# Some platforms may provide parallel primitives:
# fetchMultipleParallel : List Str -> Task (List Str) [HttpErr]
# fetchMultipleParallel = \urls ->
#     Task.parallel(List.map(urls, Http.get))

Why this translation:

• Task.async maps to platform Task creation
• Platform decides concurrency strategy
• Task.sequence combines multiple Tasks
• Timeout is platform-specific (may be in Task API)
• No separate "await" - composition with ! or Task.map

Pattern: Agent State

Elixir:

defmodule Counter do
  use Agent

  def start_link(_opts) do
    Agent.start_link(fn -> 0 end, name: __MODULE__)
  end

  def increment do
    Agent.update(__MODULE__, &(&1 + 1))
  end

  def get do
    Agent.get(__MODULE__, & &1)
  end
end

Roc:

# Pure state functions
State : I64

init : State
init = 0

increment : State -> State
increment = \count -> count + 1

get : State -> I64
get = \count -> count

# Hypothetical platform API for stateful agents
# (actual API depends on platform)
#
# import pf.Agent exposing [Agent]
#
# counterAgent : Agent State
# counterAgent = Agent.start(init)
#
# incrementCounter : Task {} []
# incrementCounter =
#     Agent.update(counterAgent, increment)
#
# getCounter : Task I64 []
# getCounter =
#     Agent.get(counterAgent, get)

Why this translation:

• Agent pattern becomes pure state + platform state management
• No global registered names - agent references are explicit
• Updates are explicit functions, not anonymous lambdas
• Platform provides concurrency-safe state updates
• Business logic (increment, get) remains pure and testable

---

Error Handling

Elixir Error Model → Roc Result Type

Elixir Pattern	Roc Pattern	Translation Strategy
{:ok, value}	Ok(value)	Direct mapping
{:error, reason}	Err(reason)	Direct mapping
raise/1	Err variant	No exceptions in Roc
rescue clause	Pattern match on Result	Explicit error handling
try/rescue	Result combinators	Compose with Result.try, Result.map
Multiple error tuples	Tag union error type	[Err1, Err2, Err3]
! (bang) functions	Regular functions returning Result	All errors explicit

Pattern: Error Propagation

Elixir:

def calculate(a, b, c) do
  with {:ok, x} <- divide(a, b),
       {:ok, y} <- divide(x, c) do
    {:ok, y}
  else
    {:error, reason} -> {:error, reason}
  end
end

# Or using the ! convention:
def calculate!(a, b, c) do
  x = divide!(a, b)
  y = divide!(x, c)
  y
end

Roc:

calculate : I64, I64, I64 -> Result I64 [DivByZero]
calculate = \a, b, c ->
    x = divide!(a, b)  # Propagates error automatically
    y = divide!(x, c)
    Ok(y)

# Or explicitly:
calculateExplicit : I64, I64, I64 -> Result I64 [DivByZero]
calculateExplicit = \a, b, c ->
    divide(a, b)
    |> Result.try(\x ->
        divide(x, c)
    )

Why this translation:

• Elixir with becomes Roc ! operator
• ! in Roc is try/propagate (not unwrap/crash like Elixir!)
• Both patterns allow early return on error
• Roc's approach is more concise
• Type system ensures all errors are handled

Pattern: Multiple Error Types

Elixir:

defmodule Parser do
  def parse_and_save(input) do
    with {:ok, data} <- parse(input),
         {:ok, validated} <- validate(data),
         :ok <- save(validated) do
      {:ok, validated}
    else
      {:error, :invalid_format} -> {:error, "Invalid format"}
      {:error, :validation_failed, msg} -> {:error, "Validation: #{msg}"}
      {:error, :save_failed} -> {:error, "Could not save"}
    end
  end
end

Roc:

parseAndSave : Str -> Result Data [InvalidFormat, ValidationFailed Str, SaveFailed]
parseAndSave = \input ->
    data = parse!(input)  # Returns Result Data [InvalidFormat]
    validated = validate!(data)  # Returns Result Data [ValidationFailed Str]
    save!(validated)  # Returns Result {} [SaveFailed]
    Ok(validated)

# Error handling with descriptive messages
parseAndSaveWithMsg : Str -> Result Data Str
parseAndSaveWithMsg = \input ->
    parseAndSave(input)
    |> Result.mapErr(\err ->
        when err is
            InvalidFormat -> "Invalid format"
            ValidationFailed(msg) -> "Validation: \(msg)"
            SaveFailed -> "Could not save"
    )

Why this translation:

• Elixir error atoms map to Roc tag variants
• Error payloads become tag parameters
• ! operator unifies error types automatically
• Result.mapErr converts to user-friendly messages
• Type signature documents all possible errors

Pattern: Raise and Rescue

Elixir:

def process_file(path) do
  content = File.read!(path)  # Raises on error
  parse(content)
rescue
  e in File.Error -> {:error, "File error: #{e.message}"}
  e in ArgumentError -> {:error, "Invalid arguments"}
end

# Safer alternative
def process_file_safe(path) do
  case File.read(path) do
    {:ok, content} -> parse(content)
    {:error, reason} -> {:error, "File error: #{inspect(reason)}"}
  end
end

Roc:

# Roc has no exceptions - all errors are Result types
import pf.File

processFile : Str -> Result Data [FileErr Str, ParseErr]
processFile = \path ->
    content = File.readUtf8!(path)  # ! propagates error
    parse!(content)

# Explicit error handling
processFileExplicit : Str -> Result Data Str
processFileExplicit = \path ->
    when File.readUtf8(path) is
        Ok(content) ->
            when parse(content) is
                Ok(data) -> Ok(data)
                Err(ParseErr) -> Err("Parse error")
        Err(err) -> Err("File error: \(File.errorToStr(err))")

Why this translation:

• No raise/rescue in Roc - all errors are values
• File I/O returns Result, never throws
• Bang functions in Roc propagate errors (not panic like Elixir)
• Error handling is always explicit
• Can't forget to handle errors (type system enforces)

---

Metaprogramming

Elixir Macros → Roc Alternatives

Roc has no macro system. Elixir's metaprogramming capabilities must be replaced with alternative patterns.

Elixir Feature	Roc Alternative	Notes
defmacro	External codegen	Build-time code generation
use Module	Interface composition	Import and compose interfaces
quote / unquote	-	Not available
Protocols	Abilities	Auto-derivation, compile-time
@derive	Automatic	Most abilities auto-derived
Compile-time code gen	Build scripts	External tools generate Roc
AST manipulation	-	Not supported

Pattern: Use Directive

Elixir:

defmodule MyGenServer do
  use GenServer  # Injects callbacks and helpers

  @impl true
  def init(state), do: {:ok, state}

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

Roc:

# No macro injection - explicit structure
interface MyServer
    exposes [State, init, handleGet]
    imports []

State : I64

init : I64 -> State
init = \initialValue -> initialValue

handleGet : State -> (I64, State)
handleGet = \state ->
    (state, state)

# Platform would provide GenServer-like behavior
# but without macro magic - explicit function signatures

Why this translation:

• No compile-time code injection in Roc
• All functions are explicit
• Behavior is defined by interface contract, not macros
• Platform provides runtime, application provides logic
• More verbose but clearer

Pattern: Protocol Derivation

Elixir:

defmodule User do
  @derive {Jason.Encoder, only: [:name, :email]}
  @derive Jason.Decoder

  defstruct [:name, :email, :age]
end

# Automatic implementation
Jason.encode!(%User{name: "Alice", email: "alice@example.com", age: 30})

Roc:

# Abilities are auto-derived for most types
User : {
    name : Str,
    email : Str,
    age : U32,
}

# Encode/Decode automatically available
user = { name: "Alice", email: "alice@example.com", age: 30 }
encoded = Encode.toBytes(user, Json.utf8)

# For custom encoding (rare):
encodeUser : User -> List U8
encodeUser = \{ name, email, age } ->
    # Manual JSON construction if needed
    jsonStr = """
    {"name":"\(name)","email":"\(email)","age":\(Num.toStr(age))}
    """
    Str.toUtf8(jsonStr)

Why this translation:

• Most abilities (Eq, Hash, Inspect, Encode, Decode) auto-derived
• No need for @derive - happens automatically
• Custom encoding requires explicit functions
• More restrictive but simpler
• No runtime overhead (compile-time derivation)

Pattern: Compile-Time Configuration

Elixir:

defmodule MyApp do
  @api_url Application.compile_env(:my_app, :api_url, "https://default.com")

  def fetch_data do
    HTTPoison.get(@api_url <> "/data")
  end
end

Roc:

# No compile-time config in language
# Use build scripts or environment-specific modules

# Option 1: Separate config modules (build-time switch)
# config/prod.roc
apiUrl : Str
apiUrl = "https://prod.com"

# config/dev.roc
apiUrl : Str
apiUrl = "https://dev.com"

# Option 2: Platform environment variables
import pf.Env

fetchData : Task Response [HttpErr, EnvErr]
fetchData =
    apiUrl = Env.var!("API_URL")  # Runtime config
    Http.get("\(apiUrl)/data")

# Option 3: External codegen
# Build script generates config.roc from environment

Why this translation:

• No compile-time metaprogramming in Roc
• Config is either build-time (separate modules) or runtime (Env vars)
• External tools can generate Roc modules
• Simpler language, more predictable builds
• Clear separation of build vs runtime config

Pattern: Macros for DSLs

Elixir:

defmodule Router do
  import MyWeb.Router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
  end

  scope "/", MyWeb do
    pipe_through :browser

    get "/", PageController, :index
    get "/users/:id", UserController, :show
  end
end

Roc:

# No DSL macros - use plain data structures
Route : {
    method : [Get, Post, Put, Delete],
    path : Str,
    handler : Request -> Task Response [],
}

Pipeline : List (Request -> Task Request [])

browserPipeline : Pipeline
browserPipeline = [
    acceptsHtml,
    fetchSession,
]

routes : List Route
routes = [
    { method: Get, path: "/", handler: pageIndex },
    { method: Get, path: "/users/:id", handler: userShow },
]

# Platform provides routing engine that interprets data
# No macros needed - just data and functions

Why this translation:

• DSLs become data structures
• Macros become data transformation functions
• Platform interprets routing data
• More verbose but explicit
• Easier to reason about (no compile-time magic)

---

Module System Translation

Elixir Modules → Roc Interfaces

Elixir	Roc	Notes
defmodule	interface	Module declaration
@moduledoc	Doc comments ##	Module documentation
@doc	Doc comments ##	Function documentation
def (public)	In exposes list	Public functions
defp (private)	Not in exposes	Private functions
import	import	Import modules
alias	-	Roc uses full names
use	-	No macro injection

Pattern: Module Organization

Elixir:

# lib/my_app/accounts/user.ex
defmodule MyApp.Accounts.User do
  @moduledoc """
  User account management.
  """

  defstruct [:name, :email, :age]

  @doc "Create a new user"
  def new(name, email) do
    %__MODULE__{name: name, email: email, age: 0}
  end

  @doc "Update user age"
  def update_age(user, age) do
    %{user | age: age}
  end

  defp validate(user), do: # ...
end

# lib/my_app/accounts.ex
defmodule MyApp.Accounts do
  alias MyApp.Accounts.User

  def create_user(name, email) do
    User.new(name, email)
  end
end

Roc:

# MyApp/Accounts/User.roc
## User account management
interface User
    exposes [User, new, updateAge]
    imports []

User : {
    name : Str,
    email : Str,
    age : U32,
}

## Create a new user
new : Str, Str -> User
new = \name, email ->
    { name, email, age: 0 }

## Update user age
updateAge : User, U32 -> User
updateAge = \user, age ->
    { user & age }

# Private function (not exposed)
validate : User -> Bool
validate = \user -> # ...

# MyApp/Accounts.roc
interface Accounts
    exposes [createUser]
    imports [User]

createUser : Str, Str -> User.User
createUser = \name, email ->
    User.new(name, email)

Why this translation:

• File structure mirrors namespace
• Interfaces replace modules
• exposes replaces public def
• Documentation uses ## comments
• Imports are explicit (no alias shorthand)
• Nested namespaces use directory structure

Pattern: Behaviours and Callbacks

Elixir:

defmodule MyBehaviour do
  @callback handle(term()) :: {:ok, term()} | {:error, term()}
  @callback init(term()) :: {:ok, term()}
end

defmodule MyImplementation do
  @behaviour MyBehaviour

  @impl true
  def init(config), do: {:ok, config}

  @impl true
  def handle(data), do: {:ok, process(data)}

  defp process(data), do: data
end

Roc:

# Behaviours become ability constraints or explicit interfaces

# Option 1: Ability (for polymorphism)
Handler implements
    handle : a, Request -> Result Response Err where a implements Handler
    init : Config -> Result a Err where a implements Handler

# Option 2: Interface contract (simpler, common case)
interface MyHandler
    exposes [State, init, handle]
    imports []

State : { config : Config }

init : Config -> Result State Err
init = \config -> Ok({ config })

handle : State, Request -> Result Response Err
handle = \state, request ->
    Ok(process(request))

# Private
process : Request -> Response
process = \request -> # ...

Why this translation:

• Behaviours map to abilities (for polymorphism) or interfaces (simpler)
• No @impl annotations needed (type system checks)
• Callbacks become function signatures in interface
• Type system ensures implementation matches contract
• More restrictive but type-safe

---

Testing Strategy

Elixir ExUnit → Roc Expect

Elixir	Roc	Notes
ExUnit.Case	expect statements	Inline tests
test "..."	expect ...	No test names
assert	expect x == y	Boolean expressions
refute	expect !(x == y)	Negation
setup	-	No test lifecycle hooks
describe	Comments #	Organizational comments
Doctest	expect in docs	Inline in documentation
assert_receive	-	No message-based testing

Pattern: Basic Testing

Elixir:

defmodule MathUtilsTest do
  use ExUnit.Case

  describe "add/2" do
    test "adds two positive numbers" do
      assert MathUtils.add(2, 3) == 5
    end

    test "adds negative numbers" do
      assert MathUtils.add(-1, 1) == 0
    end
  end

  describe "divide/2" do
    test "divides successfully" do
      assert MathUtils.divide(10, 2) == {:ok, 5.0}
    end

    test "returns error on division by zero" do
      assert MathUtils.divide(10, 0) == {:error, :division_by_zero}
    end
  end
end

Roc:

interface MathUtils
    exposes [add, divide]
    imports []

add : I64, I64 -> I64
add = \a, b -> a + b

# Tests for add
expect add(2, 3) == 5
expect add(-1, 1) == 0
expect add(0, 0) == 0

divide : F64, F64 -> Result F64 [DivisionByZero]
divide = \a, b ->
    if b == 0 then
        Err(DivisionByZero)
    else
        Ok(a / b)

# Tests for divide
expect divide(10, 2) == Ok(5.0)
expect divide(10, 0) == Err(DivisionByZero)
expect
    when divide(20, 4) is
        Ok(result) -> result == 5.0
        Err(_) -> Bool.false

Why this translation:

• No test framework setup needed
• expect statements run with roc test
• No test names - expectations speak for themselves
• Inline with code (closer to doctest philosophy)
• Can use pattern matching in expects

Pattern: Doctests

Elixir:

defmodule Calculator do
  @doc """
  Adds two numbers.

  ## Examples

      iex> Calculator.add(2, 3)
      5

      iex> Calculator.add(-1, 1)
      0
  """
  def add(a, b), do: a + b
end

Roc:

interface Calculator
    exposes [add]
    imports []

## Adds two numbers.
##
## Examples:
##
## ```
## expect add(2, 3) == 5
## expect add(-1, 1) == 0
## ```
add : I64, I64 -> I64
add = \a, b -> a + b

# Actual test expectations (run with roc test)
expect add(2, 3) == 5
expect add(-1, 1) == 0

Why this translation:

• Documentation includes example expect statements
• Actual tests are separate expect statements
• No special doctest syntax
• Examples in docs can be copy-pasted as tests
• Simpler mental model

Pattern: Setup and Teardown

Elixir:

defmodule DatabaseTest do
  use ExUnit.Case

  setup do
    {:ok, conn} = Database.connect()
    on_exit(fn -> Database.disconnect(conn) end)
    {:ok, conn: conn}
  end

  test "inserts user", %{conn: conn} do
    assert {:ok, user} = Database.insert(conn, %User{name: "Alice"})
    assert user.name == "Alice"
  end
end

Roc:

# No setup/teardown in Roc tests
# Tests are pure - create test data inline

interface Database
    exposes [connect, disconnect, insert, User]
    imports []

User : { name : Str }

# Tests create their own state
expect
    conn = testConnection  # Helper for test connections
    user = insert(conn, { name: "Alice" })
    when user is
        Ok(u) -> u.name == "Alice"
        Err(_) -> Bool.false

# Helper for test data
testConnection : Connection
testConnection = # Create test connection

Why this translation:

• No setup/teardown hooks
• Tests are pure functions
• Create test data inline or with helper functions
• No shared mutable state between tests
• Simpler but requires more explicit test data creation

Pattern: Property-Based Testing

Elixir:

defmodule StringPropertiesTest do
  use ExUnit.Case
  use ExUnitProperties

  property "reversing twice returns original" do
    check all string <- string(:printable) do
      assert string |> String.reverse() |> String.reverse() == string
    end
  end

  property "length is preserved when reversing" do
    check all string <- string(:alphanumeric) do
      assert String.length(string) == String.length(String.reverse(string))
    end
  end
end

Roc:

# Roc doesn't have built-in property-based testing yet
# Write tests for specific cases or use external tools

# Specific test cases
expect
    str = "hello"
    str == (str |> Str.reverse |> Str.reverse)

expect
    str = "Roc Lang"
    str == (str |> Str.reverse |> Str.reverse)

expect
    str = ""
    str == (str |> Str.reverse |> Str.reverse)

# Length preservation
expect
    str = "testing"
    Str.countUtf8Bytes(str) == Str.countUtf8Bytes(Str.reverse(str))

Why this translation:

• No built-in property-based testing library yet
• Write representative test cases manually
• Consider external codegen for test generation
• Simpler but less comprehensive
• May evolve as ecosystem matures

---

Build System and Dependencies

Mix → Roc Platform System

Elixir (Mix)	Roc	Notes
mix.exs	app / package header	Project definition
deps	Platform URL	Dependencies via platforms
mix deps.get	Automatic	Platforms fetched automatically
mix compile	roc build	Build command
mix test	roc test	Test runner
mix format	roc format	Code formatter
mix run	roc run	Run application

Pattern: Project Configuration

Elixir:

# mix.exs
defmodule MyApp.MixProject do
  use Mix.Project

  def project do
    [
      app: :my_app,
      version: "0.1.0",
      elixir: "~> 1.14",
      start_permanent: Mix.env() == :prod,
      deps: deps()
    ]
  end

  def application do
    [
      extra_applications: [:logger],
      mod: {MyApp.Application, []}
    ]
  end

  defp deps do
    [
      {:phoenix, "~> 1.7"},
      {:jason, "~> 1.4"},
      {:httpoison, "~> 2.0"}
    ]
  end
end

Roc:

# MyApp.roc - Application header
app [main] {
    pf: platform "https://github.com/roc-lang/basic-cli/releases/download/0.10.0/vNe6s9hWzoTZtFmNkvEICPErI9ptji_ySjicO6CkucY.tar.br"
}

import pf.Stdout
import pf.Task exposing [Task]

main : Task {} []
main =
    Stdout.line!("Hello, World!")

# No separate config file
# Platform URL includes all dependencies
# Version is embedded in URL hash

Why this translation:

• No mix.exs equivalent - project is defined in app header
• Dependencies come through platform (not direct package deps)
• Version pinning via URL hash
• Simpler dependency model but less flexible
• Platform provides cohesive set of APIs

Pattern: Custom Mix Tasks

Elixir:

# lib/mix/tasks/generate_docs.ex
defmodule Mix.Tasks.GenerateDocs do
  use Mix.Task

  @shortdoc "Generate custom documentation"

  def run(_args) do
    Mix.shell().info("Generating documentation...")
    # Custom logic
  end
end

# Usage: mix generate_docs

Roc:

# No mix task equivalent
# Use external build scripts or just/make

# Justfile
generate-docs:
    #!/usr/bin/env bash
    echo "Generating documentation..."
    roc run generate_docs.roc

# Or shell script: scripts/generate_docs.sh
#!/usr/bin/env bash
roc build docs_generator.roc
./docs_generator

Why this translation:

• No task system in Roc
• Use external build tools (just, make, shell scripts)
• Write Roc programs for custom tooling
• Platform provides basic commands only
• Simpler language, external orchestration

---

Common Pitfalls

1. Process-Based Thinking in Pure Code

Pitfall:

# ❌ Trying to replicate GenServer with state
# This doesn't work - Roc has no processes
counter = 0  # Can't have mutable module-level state

increment = \{} ->
    counter = counter + 1  # Error: counter is not mutable

Solution:

# ✓ Use explicit state passing
State : I64

increment : State -> State
increment = \count -> count + 1

# State is passed explicitly, not hidden in process
# Platform provides state management if needed

2. Expecting Dynamic Types

Pitfall:

# ❌ Trying to use heterogeneous lists
users = [
    { name: "Alice", age: 30 },
    "Invalid user",  # Error: different type
    42,              # Error: different type
]

Solution:

# ✓ Use tag unions for variants
User : [Valid { name : Str, age : U32 }, Invalid Str, Unknown I64]

users : List User
users = [
    Valid({ name: "Alice", age: 30 }),
    Invalid("Invalid user"),
    Unknown(42),
]

3. Message Passing Patterns

Pitfall:

# ❌ Trying to use message passing
# No send/receive in Roc
send(pid, message)  # Error: no send function

Solution:

# ✓ Use function composition
# Messages become function parameters
handleMessage : Message -> State -> State
handleMessage = \msg, state ->
    when msg is
        Increment -> increment(state)
        Decrement -> decrement(state)
        Get -> state

4. Hot Code Loading Expectations

Pitfall: Expecting to reload modules without restart (Elixir/BEAM feature).

Solution:

• Roc requires full program restart
• Design for stateless or externalizable state
• Use platform-level state persistence if needed
• Accept different deployment model

5. Macro-Heavy Code

Pitfall:

# Elixir macro-heavy DSL
defmacro assert_valid(condition, message) do
  quote do
    unless unquote(condition) do
      raise ArgumentError, unquote(message)
    end
  end
end

Solution:

# ✓ Use plain functions
assertValid : Bool, Str -> Result {} [ValidationErr Str]
assertValid = \condition, message ->
    if condition then
        Ok({})
    else
        Err(ValidationErr(message))

6. nil vs None Confusion

Pitfall:

# ❌ Expecting nil to work like Elixir
user = nil  # Error: no nil value

Solution:

# ✓ Use tag unions for optional values
User : [Some { name : Str }, None]

user : User
user = None

# Or simpler:
MaybeUser : [Some { name : Str }, None]

7. String vs Atom Usage

Pitfall:

# ❌ Using strings where tags are better
status = "pending"  # Strings don't get exhaustiveness checking

Solution:

# ✓ Use tags for enumerations
Status : [Pending, Approved, Rejected]

status : Status
status = Pending

# Compiler ensures all cases handled
when status is
    Pending -> "waiting"
    Approved -> "done"
    Rejected -> "failed"
    # Forgot a case? Compiler error!

---

Tooling

Purpose	Elixir	Roc	Notes
Build	mix compile	roc build	Compile project
Run	mix run	roc run	Execute application
Test	mix test	roc test	Run tests
Format	mix format	roc format	Code formatting
REPL	iex	roc repl	Interactive shell
Docs	mix docs (ExDoc)	roc docs	Generate documentation
Dependency fetch	mix deps.get	Automatic	Platform URLs fetched automatically
Type checking	Dialyzer	Built-in	Type checking
Hot reload	iex -S mix	-	Not available in Roc
Code analysis	Credo	-	No equivalent yet

---

Examples

Example 1: Simple - Basic List Processing

Before (Elixir):

defmodule ListUtils do
  @doc "Sum all numbers in a list"
  def sum(list), do: Enum.reduce(list, 0, &+/2)

  @doc "Double all numbers in a list"
  def double(list), do: Enum.map(list, &(&1 * 2))

  @doc "Filter even numbers"
  def evens(list), do: Enum.filter(list, &(rem(&1, 2) == 0))
end

# Tests
ExUnit.start()

defmodule ListUtilsTest do
  use ExUnit.Case

  test "sum/1" do
    assert ListUtils.sum([1, 2, 3, 4, 5]) == 15
  end

  test "double/1" do
    assert ListUtils.double([1, 2, 3]) == [2, 4, 6]
  end

  test "evens/1" do
    assert ListUtils.evens([1, 2, 3, 4, 5]) == [2, 4]
  end
end

After (Roc):

interface ListUtils
    exposes [sum, double, evens]
    imports []

## Sum all numbers in a list
sum : List I64 -> I64
sum = \list ->
    List.walk(list, 0, Num.add)

## Double all numbers in a list
double : List I64 -> List I64
double = \list ->
    List.map(list, \x -> x * 2)

## Filter even numbers
evens : List I64 -> List I64
evens = \list ->
    List.keepIf(list, \x -> x % 2 == 0)

# Tests (inline)
expect sum([1, 2, 3, 4, 5]) == 15
expect double([1, 2, 3]) == [2, 4, 6]
expect evens([1, 2, 3, 4, 5]) == [2, 4]

Example 2: Medium - Result Type and Error Handling

Before (Elixir):

defmodule Calculator do
  @doc "Safely divide two numbers"
  def divide(a, b) when b != 0, do: {:ok, a / b}
  def divide(_, 0), do: {:error, :division_by_zero}

  @doc "Chain multiple divisions"
  def chain_divide(a, b, c) do
    with {:ok, x} <- divide(a, b),
         {:ok, y} <- divide(x, c) do
      {:ok, y}
    else
      {:error, reason} -> {:error, reason}
    end
  end

  @doc "Safely parse and divide"
  def parse_and_divide(a_str, b_str) do
    with {:ok, a} <- parse_int(a_str),
         {:ok, b} <- parse_int(b_str),
         {:ok, result} <- divide(a, b) do
      {:ok, result}
    else
      {:error, :invalid_integer} -> {:error, "Invalid number format"}
      {:error, :division_by_zero} -> {:error, "Cannot divide by zero"}
    end
  end

  defp parse_int(str) do
    case Integer.parse(str) do
      {int, ""} -> {:ok, int}
      _ -> {:error, :invalid_integer}
    end
  end
end

# Tests
defmodule CalculatorTest do
  use ExUnit.Case

  test "divide/2 success" do
    assert Calculator.divide(10, 2) == {:ok, 5.0}
  end

  test "divide/2 by zero" do
    assert Calculator.divide(10, 0) == {:error, :division_by_zero}
  end

  test "chain_divide/3" do
    assert Calculator.chain_divide(20, 2, 2) == {:ok, 5.0}
    assert Calculator.chain_divide(10, 0, 2) == {:error, :division_by_zero}
  end

  test "parse_and_divide/2" do
    assert Calculator.parse_and_divide("10", "2") == {:ok, 5.0}
    assert Calculator.parse_and_divide("abc", "2") == {:error, "Invalid number format"}
    assert Calculator.parse_and_divide("10", "0") == {:error, "Cannot divide by zero"}
  end
end

After (Roc):

interface Calculator
    exposes [divide, chainDivide, parseAndDivide]
    imports []

## Safely divide two numbers
divide : F64, F64 -> Result F64 [DivisionByZero]
divide = \a, b ->
    if b == 0 then
        Err(DivisionByZero)
    else
        Ok(a / b)

## Chain multiple divisions
chainDivide : F64, F64, F64 -> Result F64 [DivisionByZero]
chainDivide = \a, b, c ->
    x = divide!(a, b)  # ! propagates error
    y = divide!(x, c)
    Ok(y)

## Safely parse and divide
parseAndDivide : Str, Str -> Result F64 [InvalidInteger, DivisionByZero]
parseAndDivide = \aStr, bStr ->
    a = parseFloat!(aStr)  # Returns Result F64 [InvalidInteger]
    b = parseFloat!(bStr)
    divide!(a, b)

# Helper for parsing
parseFloat : Str -> Result F64 [InvalidInteger]
parseFloat = \str ->
    when Str.toF64(str) is
        Ok(num) -> Ok(num)
        Err(_) -> Err(InvalidInteger)

# Convert to user-friendly messages
parseAndDivideMsg : Str, Str -> Result F64 Str
parseAndDivideMsg = \aStr, bStr ->
    parseAndDivide(aStr, bStr)
    |> Result.mapErr(\err ->
        when err is
            InvalidInteger -> "Invalid number format"
            DivisionByZero -> "Cannot divide by zero"
    )

# Tests
expect divide(10, 2) == Ok(5.0)
expect divide(10, 0) == Err(DivisionByZero)
expect chainDivide(20, 2, 2) == Ok(5.0)
expect chainDivide(10, 0, 2) == Err(DivisionByZero)
expect
    when parseAndDivideMsg("10", "2") is
        Ok(result) -> result == 5.0
        Err(_) -> Bool.false
expect parseAndDivideMsg("abc", "2") == Err("Invalid number format")
expect parseAndDivideMsg("10", "0") == Err("Cannot divide by zero")

Example 3: Complex - GenServer to Pure State Machine with Platform Tasks

Before (Elixir):

defmodule UserCache do
  use GenServer

  # Client API

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
  end

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

  def put(user_id, user) do
    GenServer.cast(__MODULE__, {:put, user_id, user})
  end

  def delete(user_id) do
    GenServer.cast(__MODULE__, {:delete, user_id})
  end

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

  # Server Callbacks

  @impl true
  def init(_args) do
    {:ok, %{}}
  end

  @impl true
  def handle_call({:get, user_id}, _from, state) do
    {:reply, Map.get(state, user_id), state}
  end

  def handle_call(:all_users, _from, state) do
    {:reply, Map.values(state), state}
  end

  @impl true
  def handle_cast({:put, user_id, user}, state) do
    {:noreply, Map.put(state, user_id, user)}
  end

  def handle_cast({:delete, user_id}, state) do
    {:noreply, Map.delete(state, user_id)}
  end
end

# Supervision tree
defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      UserCache
    ]

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

# Tests
defmodule UserCacheTest do
  use ExUnit.Case

  setup do
    {:ok, _pid} = start_supervised(UserCache)
    :ok
  end

  test "get and put user" do
    user = %{name: "Alice", email: "alice@example.com"}

    :ok = UserCache.put(1, user)
    assert UserCache.get(1) == user
  end

  test "get non-existent user returns nil" do
    assert UserCache.get(999) == nil
  end

  test "delete user" do
    user = %{name: "Bob", email: "bob@example.com"}

    :ok = UserCache.put(2, user)
    assert UserCache.get(2) == user

    :ok = UserCache.delete(2)
    assert UserCache.get(2) == nil
  end

  test "all_users returns all cached users" do
    user1 = %{name: "Alice", email: "alice@example.com"}
    user2 = %{name: "Bob", email: "bob@example.com"}

    :ok = UserCache.put(1, user1)
    :ok = UserCache.put(2, user2)

    users = UserCache.all_users()
    assert length(users) == 2
    assert user1 in users
    assert user2 in users
  end
end

After (Roc):

# Pure state machine - no GenServer
interface UserCache
    exposes [
        State,
        User,
        init,
        get,
        put,
        delete,
        allUsers,
    ]
    imports []

User : {
    name : Str,
    email : Str,
}

State : Dict U64 User

## Initialize empty cache
init : State
init = Dict.empty({})

## Get user by ID
get : State, U64 -> [Some User, None]
get = \state, userId ->
    Dict.get(state, userId)
    |> Result.map(Some)
    |> Result.withDefault(None)

## Put user in cache
put : State, U64, User -> State
put = \state, userId, user ->
    Dict.insert(state, userId, user)

## Delete user from cache
delete : State, U64 -> State
delete = \state, userId ->
    Dict.remove(state, userId)

## Get all users
allUsers : State -> List User
allUsers = \state ->
    Dict.values(state)

# Tests
expect
    state = init
    user = { name: "Alice", email: "alice@example.com" }
    newState = put(state, 1, user)
    get(newState, 1) == Some(user)

expect
    state = init
    get(state, 999) == None

expect
    state = init
    user = { name: "Bob", email: "bob@example.com" }
    stateWithUser = put(state, 2, user)
    get(stateWithUser, 2) == Some(user)

    stateAfterDelete = delete(stateWithUser, 2)
    get(stateAfterDelete, 2) == None

expect
    state = init
    user1 = { name: "Alice", email: "alice@example.com" }
    user2 = { name: "Bob", email: "bob@example.com" }

    state
    |> put(1, user1)
    |> put(2, user2)
    |> allUsers
    |> List.len
    |> \len -> len == 2

# Hypothetical platform integration (if state management needed)
# This would be provided by the platform, not shown here:
#
# import pf.Agent exposing [Agent]
#
# cacheAgent : Agent State
# cacheAgent = Agent.start(init)
#
# getUser : U64 -> Task [Some User, None] []
# getUser = \userId ->
#     Agent.get(cacheAgent, \state -> get(state, userId))
#
# putUser : U64, User -> Task {} []
# putUser = \userId, user ->
#     Agent.update(cacheAgent, \state -> put(state, userId, user))
#
# deleteUser : U64 -> Task {} []
# deleteUser = \userId ->
#     Agent.update(cacheAgent, \state -> delete(state, userId))
#
# getAllUsers : Task (List User) []
# getAllUsers =
#     Agent.get(cacheAgent, allUsers)

---

See Also

For more examples and patterns, see:

• meta-convert-dev - Foundational patterns with cross-language examples
• convert-erlang-roc - Related BEAM → Roc conversion (similar paradigm shift)
• lang-elixir-dev - Elixir development patterns
• lang-roc-dev - Roc development patterns

Cross-cutting pattern skills (for areas not fully covered by lang-*-dev):

• patterns-concurrency-dev - Process model vs Task model comparison
• patterns-serialization-dev - JSON, validation across languages
• patterns-metaprogramming-dev - Macros vs abilities comparison