TypeSpec & Dialyzer

Types make data shapes and railway returns explicit — especially at module boundaries — so pure core and shell stay honest.

Canonical FP bar: [docs/fcis-engineering-rules.md](../../docs/fcis-engineering-rules.md).

Quick Reference

RULES — Follow these with no exceptions

1. @spec every public function — especially context/shell APIs and pure core entry points 2. @type t for structs/schemas — match real field types (NaiveDateTime.t() | nil for default timestamps) 3. Tagged-tuple success typing — {:ok, t()} | {:error, reason()} for fallible ops 4. Run Dialyzer in CI with PLT cache 5. Never ignore warnings silently — document in .dialyzerignore.exs via ignorewarnings config 6. Incremental adoption — core modules first 7. No any() as a habit — prefer unions and parameterized result types 8. Specs match reality — do not widen past success typing to silence Dialyzer

Specs at the FCIS boundary

❌ Bad: untyped public APIs

def get_user(id), do: Repo.get(User, id)
def create_user(attrs), do: %User{} |> User.changeset(attrs) |> Repo.insert()

✅ Good: explicit shapes and railway returns

defmodule MyApp.Orders.Pricing do
  @type line :: %{amount: non_neg_integer()}
  @type order :: %{lines: [line()]}

  @doc "Pure core — no side effects."
  @spec total(order()) :: non_neg_integer()
  def total(%{lines: lines}), do: Enum.reduce(lines, 0, &(&1.amount + &2))
end

defmodule MyApp.Orders do
  alias MyApp.Accounts.User
  alias MyApp.Repo

  @type id :: pos_integer()

  @spec get_user(id()) :: User.t() | nil
  def get_user(id), do: Repo.get(User, id)

  @spec create_user(map()) :: {:ok, User.t()} | {:error, Ecto.Changeset.t()}
  def create_user(attrs) do
    %User{}
    |> User.changeset(attrs)
    |> Repo.insert()
  end
end

Struct types that match the schema

❌ Bad: types that lie about the schema

@type t :: %__MODULE__{
        role: :admin | :editor,          # field is :string
        inserted_at: DateTime.t()        # timestamps() default is naive + nilable
      }

✅ Good: align @type t with fields (or use Ecto.Enum + matching atom type)

defmodule MyApp.Accounts.User do
  use Ecto.Schema

  @type t :: %__MODULE__{
          id: integer() | nil,
          email: String.t(),
          username: String.t(),
          role: String.t() | nil,
          inserted_at: NaiveDateTime.t() | nil,
          updated_at: NaiveDateTime.t() | nil
        }

  # When using Ecto.Enum, prefer: @type role :: :admin | :editor | :viewer
  @type role :: String.t()

  @type attrs :: %{
          optional(:email) => String.t(),
          optional(:username) => String.t(),
          optional(:role) => role(),
          optional(:password) => String.t()
        }

  schema "users" do
    field :email, :string
    field :username, :string
    field :role, :string
    field :password, :string, virtual: true
    timestamps()
  end

  @spec changeset(t(), map()) :: Ecto.Changeset.t()
  def changeset(user, attrs) do
    user
    |> cast(attrs, [:email, :username, :role, :password])
    |> validate_required([:email, :username])
  end
end

Result types (railway)

@type result(ok, err) :: {:ok, ok} | {:error, err}

@spec divide(number(), number()) :: result(float(), :division_by_zero)
def divide(_n, 0), do: {:error, :division_by_zero}
def divide(n, d), do: {:ok, n / d}

@type status :: :active | :inactive | :suspended
@spec update_status(User.t(), status()) :: {:ok, User.t()} | {:error, atom()}

Opaque types for encapsulation

defmodule MyApp.Token do
  @opaque t :: %__MODULE__{value: String.t(), expires_at: DateTime.t()}
  defstruct [:value, :expires_at]

  @spec new(String.t(), DateTime.t()) :: t()
  def new(value, expires_at), do: %__MODULE__{value: value, expires_at: expires_at}
end

Dialyxir setup

# mix.exs deps
{:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false}

# config/config.exs (or config/dev.exs)
config :dialyxir,
  ignore_warnings: ".dialyzer_ignore.exs"

# .dialyzer_ignore.exs
[
  {"lib/my_app/legacy_module.ex", :unknown_type},
  ~r/unknown_function/
]

❌ Bad: non-existent CLI flag

mix dialyzer --ignore-file .dialyzer_ignore.exs

✅ Good: configure ignore_warnings and use supported formats

mix dialyzer                 # builds PLT on first run
mix dialyzer --format short
mix dialyzer --format ignore_file   # generate ignore entries

Reading errors

mix dialyzer --format short
# fix → rerun until clean or documented ignore

CI + PLT cache

- name: Cache PLT
  uses: actions/cache@6f8efc29b200d32929f49075959781ed54ec270c # v3
  with:
    path: priv/plts
    key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
    restore-keys: ${{ runner.os }}-mix-

- name: Dialyzer
  run: mix dialyzer --format short

Common pitfalls

Integration

Companion: code-quality, credo-config.