FP in Erlang/Elixir -- Functional Software Crafter Skill

Cross-references: fp-principles | fp-domain-modeling | pbt-erlang-elixir

When to Choose Erlang/Elixir

• Best for: fault-tolerant distributed systems | massive concurrency (millions of lightweight processes) | soft real-time (telecom, messaging, payments) | hot code reload without downtime
• Not ideal for: CPU-bound numeric computation | teams needing static types at compile time | small scripts where the BEAM's operational surface (release, clustering) is pure overhead

Elixir is shown below (more common entry point); Erlang differs in syntax, not in the model -- both compile to BEAM bytecode and share OTP, so every pattern below applies to Erlang under a different concrete syntax.

[STARTER] Quick Setup

mix new order_service --sup
cd order_service
# Add stream_data to mix.exs deps for property-based testing
mix deps.get && mix test

Test runner: mix test. Add {:stream_data, "~> 1.0", only: :test} to deps.

[STARTER] Type System for Domain Modeling

Erlang/Elixir has no compile-time type checker in mainstream use (Dialyzer is opt-in, gradual, and advisory, not enforced) -- domain shape is a RUNTIME discipline: pattern matching on tagged tuples/structs plus guard clauses, never an ad-hoc map with an implicit shape.

Choice Types (Tagged Tuples and Structs)

defmodule PaymentMethod do
  @type t ::
          {:credit_card, card_number :: String.t(), expiry :: Date.t()}
          | {:bank_transfer, account_number :: String.t()}
          | :cash
end

Record Types and Domain Wrappers

defmodule Customer do
  @enforce_keys [:customer_id, :customer_name, :customer_email]
  defstruct [:customer_id, :customer_name, :customer_email]
end

defmodule EmailAddress do
  @opaque t :: %__MODULE__{value: String.t()}
  defstruct [:value]

  @spec new(String.t()) :: {:ok, t()} | {:error, :invalid_email}
  def new(raw) do
    if String.contains?(raw, "@"),
      do: {:ok, %__MODULE__{value: raw}},
      else: {:error, :invalid_email}
  end
end

@enforce_keys and @opaque push construction through new/1 -- no direct %EmailAddress{value: raw} literal outside the module can skip validation, because the field is opaque to callers.

[INTERMEDIATE] Validated Construction with with

def validate_order(raw) do
  with {:ok, customer} <- Customer.validate(raw.customer),
       {:ok, lines} <- OrderLines.validate(raw.lines) do
    {:ok, %ValidatedOrder{customer: customer, lines: lines}}
  end
end

with short-circuits on the first non-matching clause, returning that clause's own value -- no manual case-nesting.

[INTERMEDIATE] Composition Style

Pipe Operator for Left-to-Right Pipelines

def place_order(raw_order) do
  raw_order
  |> validate_order()
  |> price_order()
  |> confirm_order()
end

|> threads the LEFT expression as the first argument of the RIGHT call -- reads as a pipeline, executes as nested calls.

Pattern Matching as Control Flow

def process_order({:ok, order}), do: confirm(order)
def process_order({:error, reason}), do: {:error, reason}

Multiple function clauses dispatch on the SHAPE of the argument -- no if/case needed when the shapes are already distinct tuples.

Standard with short-circuits on the first error; accumulate explicitly via Enum.reduce/3 over a list of {:ok, _} | {:error, _} results when the caller needs every validation failure, not just the first.

[INTERMEDIATE] Effect Management -- OTP IS the Effect System

Erlang/Elixir has no IO type or effect monad: the process model itself is the effect boundary. A pure function computes and returns; a side effect happens by SENDING A MESSAGE to a process (the mailbox), never by calling a side-effecting function inline in domain logic.

# Pure domain logic -- ordinary functions, no process interaction
defmodule OrderDomain do
  def calculate_discount(order) when length(order.lines) > 10, do: 0.1
  def calculate_discount(_order), do: 0.0
end

# Effectful boundary -- a GenServer owns the mutable state and I/O
defmodule OrderServer do
  use GenServer

  def place_order(pid, raw_order), do: GenServer.call(pid, {:place_order, raw_order})

  @impl true
  def handle_call({:place_order, raw_order}, _from, state) do
    case OrderDomain.validate_order(raw_order) do
      {:ok, validated} ->
        priced = OrderDomain.calculate_discount(validated)
        {:reply, {:ok, priced}, Map.put(state, validated.id, priced)}

      {:error, _} = err ->
        {:reply, err, state}
    end
  end
end

[ADVANCED] Supervision Trees as the Failure-Recovery Layer

defmodule OrderService.Application do
  use Application

  def start(_type, _args) do
    children = [
      {OrderServer, name: OrderServer},
      {DBConnection.Pool, name: OrderDB}
    ]

    # :one_for_one -- a crashed child restarts alone, siblings keep running
    Supervisor.start_link(children, strategy: :one_for_one)
  end
end

"Let it crash": a process that hits an unexpected state terminates instead of defensively guarding every branch; its supervisor restarts it into a KNOWN-GOOD initial state. This replaces most of what a try/catch railway does in other languages -- the recovery unit is the PROCESS, not the call stack.

[INTERMEDIATE] Testing

Frameworks: ExUnit (built-in) | StreamData (property-based testing, use ExUnitProperties) | Mox (behaviour-based mocking for OTP boundaries). See pbt-erlang-elixir for detailed PBT patterns.

Property Test Example

use ExUnit.Case
use ExUnitProperties

property "round-trips through serialization" do
  check all(order <- order_generator()) do
    assert {:ok, order} == order |> serialize() |> deserialize()
  end
end

property "validated orders always have a positive total" do
  check all(raw <- raw_order_generator()) do
    case OrderDomain.validate_order(raw) do
      {:error, _} -> :ok
      {:ok, validated} -> assert validated.total > 0
    end
  end
end

Custom Generator

def email_generator do
  gen all(
        user <- StreamData.string(:alphanumeric, min_length: 1),
        domain <- StreamData.string(:alphanumeric, min_length: 1)
      ) do
    "#{user}@#{domain}.com"
  end
end

[ADVANCED] Idiomatic Patterns

Protocols for Data-Directed Polymorphism

defprotocol Priceable do
  @spec price(t()) :: Money.t()
end

defimpl Priceable, for: PhysicalOrder do
  def price(order), do: order.subtotal + order.shipping
end

defimpl Priceable, for: DigitalOrder do
  def price(order), do: order.subtotal
end

Protocols dispatch on the STRUCT's own type at the call site -- open for new types without touching existing defimpl blocks, the Elixir analogue of a type class.

Immutability Is Not Optional

Every value in Erlang/Elixir is immutable by construction of the runtime -- there is no mutable-cell escape hatch comparable to Haskell's IORef or Clojure's atom reached casually; process state changes ONLY by a GenServer returning a new state from its own callback.

Maturity and Adoption

• Dialyzer is advisory, not enforced: success typing catches real bugs but is unsound by design and opt-in per project. A clean Dialyzer run is not a type-safety guarantee.
• BEAM has real CPU-bound limits: scheduling favors I/O-bound concurrency; a hot numeric loop belongs in a NIF (Rust via rustler, C), not pure Elixir.
• Smaller ecosystem outside web/telecom: Phoenix/OTP are mature; general-purpose libraries (ML, scientific computing) are thinner than JVM or Python.

Common Pitfalls

1. Business logic inside GenServer.handle_call: keep the callback thin (pattern-match, delegate); put validation/pricing/decision logic in pure modules the callback CALLS, so it is testable without a running process.
2. Blocking calls inside a GenServer callback: a synchronous DB call inside handle_call blocks the whole process's mailbox; use Task for I/O that should not serialize behind other messages.
3. Catching what should crash: wrapping ordinary business errors in try/rescue defeats "let it crash" -- reserve rescue for genuinely exceptional, unrecoverable-locally conditions; return {:error, reason} for everything else.
4. Unbounded process mailboxes: a producer faster than its consumer grows the mailbox unbounded and OOMs the node; back-pressure explicitly (GenStage, Broadway) for high-throughput pipelines.