.. _natspec:

##############
NatSpec Format
##############

Solidity contracts can use a special form of comments to provide rich
documentation for functions, return variables and more. This special form is
named the Ethereum Natural Language Specification Format (NatSpec).

.. note::

  NatSpec was inspired by `Doxygen <https://en.wikipedia.org/wiki/Doxygen>`_.
  While it uses Doxygen-style comments and tags, there is no intention to keep
  strict compatibility with Doxygen. Please carefully examine the supported tags
  listed below.

This documentation is segmented into developer-focused messages and end-user-facing
messages. These messages may be shown to the end user (the human) at the
time that they will interact with the contract (i.e. sign a transaction).

It is recommended that Solidity contracts are fully annotated using NatSpec for
all public interfaces (everything in the ABI).

NatSpec includes the formatting for comments that the smart contract author will
use, and which are understood by the Solidity compiler. Also detailed below is
output of the Solidity compiler, which extracts these comments into a machine-readable
format.

NatSpec may also include annotations used by third-party tools. These are most likely
accomplished via the ``@custom:<name>`` tag, and a good use case is analysis and verification
tools.

.. _header-doc-example:

Documentation Example
=====================

Documentation is inserted above each ``contract``, ``interface``, ``library``,
``function``, and ``event`` using the Doxygen notation format.
A ``public`` state variable is equivalent to a ``function``
for the purposes of NatSpec.

-  For Solidity you may choose ``///`` for single or multi-line
   comments, or ``/**`` and ending with ``*/``.

-  For Vyper, use ``"""`` indented to the inner contents with bare
   comments. See the `Vyper
   documentation <https://docs.vyperlang.org/en/latest/natspec.html>`__.

The following example shows a contract and a function using all available tags.

.. note::

  The Solidity compiler only interprets tags if they are external or
  public. You are welcome to use similar comments for your internal and
  private functions, but those will not be parsed.

  This may change in the future.

.. code-block:: solidity

    // SPDX-License-Identifier: GPL-3.0
    pragma solidity >=0.8.2 < 0.9.0;

    /// @title A simulator for trees
    /// @author Larry A. Gardner
    /// @notice You can use this contract for only the most basic simulation
    /// @dev All function calls are currently implemented without side effects
    /// @custom:experimental This is an experimental contract.
    contract Tree {
        /// @notice Calculate tree age in years, rounded up, for live trees
        /// @dev The Alexandr N. Tetearing algorithm could increase precision
        /// @param rings The number of rings from dendrochronological sample
        /// @return Age in years, rounded up for partial years
        /// @return Name of the tree
        function age(uint256 rings) external virtual pure returns (uint256, string memory) {
            return (rings + 1, "tree");
        }

        /// @notice Returns the amount of leaves the tree has.
        /// @dev Returns only a fixed number.
        function leaves() external virtual pure returns(uint256) {
            return 2;
        }
    }

    contract Plant {
        function leaves() external virtual pure returns(uint256) {
            return 3;
        }
    }

    contract KumquatTree is Tree, Plant {
        function age(uint256 rings) external override pure returns (uint256, string memory) {
            return (rings + 2, "Kumquat");
        }

        /// Return the amount of leaves that this specific kind of tree has
        /// @inheritdoc Tree
        function leaves() external override(Tree, Plant) pure returns(uint256) {
            return 3;
        }
    }

.. _header-tags:

Tags
====

All tags are optional. The following table explains the purpose of each
NatSpec tag and where it may be used. As a special case, if no tags are
used then the Solidity compiler will interpret a ``///`` or ``/**`` comment
in the same way as if it were tagged with ``@notice``.

=============== ====================================================================================== =============================
Tag                                                                                                    Context
=============== ====================================================================================== =============================
``@title``      A title that should describe the contract/interface                                    contract, library, interface, struct, enum
``@author``     The name of the author                                                                 contract, library, interface, struct, enum
``@notice``     Explain to an end user what this does                                                  contract, library, interface, function, public state variable, event, struct, enum, error
``@dev``        Explain to a developer any extra details                                               contract, library, interface, function, state variable, event, struct, enum, error
``@param``      Documents a parameter just like in Doxygen (must be followed by parameter name)        function, event, error
``@return``     Documents the return variables of a contract's function                                function, public state variable
``@inheritdoc`` Copies all missing tags from the base function (must be followed by the contract name) function, public state variable
``@custom:...`` Custom tag, semantics is application-defined                                           everywhere
=============== ====================================================================================== =============================

If your function returns multiple values, like ``(int quotient, int remainder)``
then use multiple ``@return`` statements in the same format as the ``@param`` statements.

Custom tags start with ``@custom:`` and must be followed by one or more lowercase letters or hyphens.
It cannot start with a hyphen however. They can be used everywhere and are part of the developer documentation.

.. _header-dynamic:

Dynamic expressions
-------------------

The Solidity compiler will pass through NatSpec documentation from your Solidity
source code to the JSON output as described in this guide. The consumer of this
JSON output, for example the end-user client software, may present this to the end-user directly or it may apply some pre-processing.

For example, some client software will render:

.. code:: Solidity

   /// @notice This function will multiply `a` by 7

to the end-user as:

.. code:: text

    This function will multiply 10 by 7

if a function is being called and the input ``a`` is assigned a value of 10.

.. _header-inheritance:

Inheritance Notes
-----------------

Functions without NatSpec will automatically inherit the documentation of their
base function. Exceptions to this are:

* When the parameter names are different.
* When there is more than one base function.
* When there is an explicit ``@inheritdoc`` tag which specifies which contract should be used to inherit.

.. _header-output:

Documentation Output
====================

When parsed by the compiler, documentation such as the one from the
above example will produce two different JSON files. One is meant to be
consumed by the end user as a notice when a function is executed and the
other to be used by the developer.

If the above contract is saved as ``ex1.sol`` then you can generate the
documentation using:

.. code-block:: shell

   solc --userdoc --devdoc ex1.sol

And the output is below.

.. note::
    Starting Solidity version 0.6.11 the NatSpec output also contains a ``version`` and a ``kind`` field.
    Currently the ``version`` is set to ``1`` and ``kind`` must be one of ``user`` or ``dev``.
    In the future it is possible that new versions will be introduced, deprecating older ones.

.. _header-user-doc:

User Documentation
------------------

The above documentation will produce the following user documentation
JSON file as output for the ``Tree`` contract:

.. code-block:: json

    {
      "version" : 1,
      "kind" : "user",
      "methods" :
      {
        "age(uint256)" :
        {
          "notice" : "Calculate tree age in years, rounded up, for live trees"
        }
        "leaves()" :
        {
            "notice" : "Returns the amount of leaves the tree has."
        }
      },
      "notice" : "You can use this contract for only the most basic simulation"
    }

Note that the key by which to find the methods is the function's
canonical signature as defined in the :ref:`Contract
ABI <abi_function_selector>` and not simply the function's
name.

.. _header-developer-doc:

Developer Documentation
-----------------------

Apart from the user documentation file, a developer documentation JSON
file should also be produced and should look like this:

.. code-block:: json

    {
      "version" : 1,
      "kind" : "dev",
      "author" : "Larry A. Gardner",
      "details" : "All function calls are currently implemented without side effects",
      "custom:experimental" : "This is an experimental contract.",
      "methods" :
      {
        "age(uint256)" :
        {
          "details" : "The Alexandr N. Tetearing algorithm could increase precision",
          "params" :
          {
            "rings" : "The number of rings from dendrochronological sample"
          },
          "returns" : {
            "_0" : "Age in years, rounded up for partial years",
            "_1" : "Name of the tree"
          }
        },
        "leaves()" :
        {
            "details" : "Returns only a fixed number."
        }
      },
      "title" : "A simulator for trees"
    }
