Elixir Writing Docs
Quick Reference
First-Line Summary Rule
ExDoc and tools like mix docs extract the first paragraph of @moduledoc and @doc as a summary. Keep the opening line concise and self-contained.
# GOOD - first line works as a standalone summary
@moduledoc """
Handles payment processing through Stripe and local ledger reconciliation.
Wraps the Stripe API client and ensures each charge is recorded in the
local ledger before returning a confirmation to the caller.
"""
# BAD - first line is vague, forces reader to continue
@moduledoc """
This module contains various functions related to payments.
It uses Stripe and also updates the ledger.
"""
The same rule applies to @doc:
# GOOD
@doc """
Charges a customer's default payment method for the given amount in cents.
Returns `{:ok, charge}` on success or `{:error, reason}` when the payment
gateway rejects the request.
"""
# BAD
@doc """
This function is used to charge a customer.
"""
@moduledoc Structure
A well-structured @moduledoc follows this pattern:
defmodule MyApp.Inventory do
@moduledoc """
Tracks warehouse stock levels and triggers replenishment orders.
This module maintains an ETS-backed cache of current quantities and
exposes functions for atomic stock adjustments. It is designed to be
started under a supervisor and will restore state from the database
on init.
## Examples
iex> {:ok, pid} = MyApp.Inventory.start_link(warehouse: :east)
iex> MyApp.Inventory.current_stock(pid, "SKU-1042")
{:ok, 350}
## Configuration
Expects the following in `config/runtime.exs`:
config :my_app, MyApp.Inventory,
repo: MyApp.Repo,
low_stock_threshold: 50
"""
end
Key points:
- First paragraph is the summary (one to two sentences).
## Examples shows realistic usage. Use doctests when the example is runnable.## Configuration documents required config keys. Omit this section if the module takes no config.- Use second-level headings (
##) only. First-level (#) is reserved for the module name in ExDoc output.
Documenting Behaviour Modules
When defining a behaviour, document the expected callbacks:
defmodule MyApp.PaymentGateway do
@moduledoc """
Behaviour for payment gateway integrations.
Implementations must handle charging, refunding, and status checks.
See `MyApp.PaymentGateway.Stripe` for a reference implementation.
## Callbacks
* `charge/2` - Initiate a charge for a given amount
* `refund/2` - Refund a previously completed charge
* `status/1` - Check the status of a transaction
"""
@callback charge(amount :: pos_integer(), currency :: atom()) ::
{:ok, transaction_id :: String.t()} | {:error, term()}
@callback refund(transaction_id :: String.t(), amount :: pos_integer()) ::
:ok | {:error, term()}
@callback status(transaction_id :: String.t()) ::
{:pending | :completed | :failed, map()}
end
@doc Structure
@doc """
Reserves the given quantity of an item, decrementing available stock.
Returns `{:ok, reservation_id}` when stock is available, or
`{:error, :insufficient_stock}` when the requested quantity exceeds
what is on hand.
## Examples
iex> MyApp.Inventory.reserve("SKU-1042", 5)
{:ok, "res_abc123"}
iex> MyApp.Inventory.reserve("SKU-9999", 1)
{:error, :not_found}
## Options
* `:warehouse` - Target warehouse atom. Defaults to `:primary`.
* `:timeout` - Timeout in milliseconds. Defaults to `5_000`.
"""
@spec reserve(String.t(), pos_integer(), keyword()) ::
{:ok, String.t()} | {:error, :insufficient_stock | :not_found}
def reserve(sku, quantity, opts \\ []) do
# ...
end
Guidelines:
- State what the function does, then what it returns.
- Document each option in a bulleted
## Options section when the function accepts a keyword list.
- Place
@spec between @doc and def. This is the conventional ordering.
- Include doctests for pure functions. Skip them for side-effecting functions (see references/doctests.md).
@typedoc
Document custom types defined with @type or @opaque:
@typedoc """
A positive integer representing an amount in the smallest currency unit (e.g., cents).
"""
@type amount :: pos_integer()
@typedoc """
Reservation status returned by `status/1`.
* `:held` - Stock is reserved but not yet shipped
* `:released` - Reservation was cancelled and stock restored
* `:fulfilled` - Items have shipped
"""
@type reservation_status :: :held | :released | :fulfilled
@typedoc """
Opaque handle returned by `connect/1`. Do not pattern-match on this value.
"""
@opaque connection :: %__MODULE__{socket: port(), buffer: binary()}
For @opaque types, the @typedoc is especially important because callers cannot inspect the structure.
Metadata
@doc since and @doc deprecated
@doc since: "1.3.0"
@doc """
Transfers stock between two warehouses.
"""
def transfer(from, to, sku, quantity), do: # ...
@doc deprecated: "Use transfer/4 instead"
@doc """
Moves items between locations. Deprecated in favor of `transfer/4`
which supports cross-region transfers.
"""
def move_stock(from, to, sku, quantity), do: # ...
You can combine metadata and the docstring in one attribute:
@doc since: "2.0.0", deprecated: "Use bulk_reserve/2 instead"
@doc """
Reserves multiple items in a single call.
"""
def batch_reserve(items), do: # ...
@moduledoc since: works the same way for modules:
@moduledoc since: "1.2.0"
@moduledoc """
Handles webhook signature verification for Stripe events.
"""
When to Use @doc false / @moduledoc false
Suppress documentation when the module or function is not part of the public API:
# Private implementation module — internal to the application
defmodule MyApp.Inventory.StockCache do
@moduledoc false
# ...
end
# Protocol implementation — documented at the protocol level
defimpl String.Chars, for: MyApp.Money do
@moduledoc false
# ...
end
# Callback implementation — documented at the behaviour level
@doc false
def handle_info(:refresh, state) do
# ...
end
# Helper used only inside the module
@doc false
def do_format(value), do: # ...
Do NOT use @doc false on genuinely public functions. If a function is exported and callers depend on it, document it. If it should not be called externally, make it private with defp.
Documentation vs Code Comments
| Documentation (@moduledoc, @doc) | Code Comments (#) |
|---|
| Audience | Users of your API | Developers reading source |
| Purpose | Contract: what it does, what it returns | Why a particular implementation choice was made |
| Rendered | Yes, by ExDoc in HTML/epub | No, visible only in source |
| Required | All public modules and functions | Only where code intent is non-obvious |
@doc """
Validates that the given coupon code is active and has remaining uses.
"""
@spec validate_coupon(String.t()) :: {:ok, Coupon.t()} | {:error, :expired | :exhausted}
def validate_coupon(code) do
# We query the read replica here to avoid adding load to the
# primary during high-traffic discount events.
Repo.replica().get_by(Coupon, code: code)
|> check_expiry()
|> check_remaining_uses()
end
The @doc tells the caller what validate_coupon/1 does and returns. The inline comment explains an implementation decision that would otherwise be surprising.
Completing documentation (gates)
Finish with these sequenced checks. Skip a step when it does not apply.
- Doctests added or changed? Run the project’s doctest verification (usually
mix test for affected modules or the full suite). Pass: no doctest failures.
- Cross-references, backticks, or
m: links added or edited? Run mix docs. Pass: the command completes; resolve ExDoc warnings about missing modules, callbacks, or bad links.
- New or changed public API? Pass: every exported
def / defmacro has an intentional @doc or @doc false, and every public module has @moduledoc or @moduledoc false, consistent with your project’s policy.
When to Load References