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).

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 annontated 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.

Documentation Example

Documentation is inserted above each class, interface and function using the doxygen notation format.

  • 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 Vyper documentation.

The following example shows a contract and a function using all available tags. Note: NatSpec currently does NOT apply to public state variables (see solidity#3418), even if they are declared public and therefore do affect the ABI. 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.

pragma solidity ^0.5.6;

/// @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
contract Tree {
    /// @author Mary A. Botanist
    /// @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
    function age(uint256 rings) external pure returns (uint256) {
        return rings + 1;
    }
}

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, interface
@author The name of the author contract, interface, function
@notice Explain to an end user what this does contract, interface, function
@dev Explain to a developer any extra details contract, interface, function
@param Documents a parameter just like in doxygen (must be followed by parameter name) function
@return Documents the return type of a contract’s function function

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

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:

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

to the end-user as:

This function will multiply 10 by 7

if a function is being called and the input a is assigned a value of 7.

Specifying these dynamic expressions is outside the scope of the Solidity documentation and you may read more at the radspec project.

Inheritance Notes

Currently it is undefined whether a contract with a function having no NatSpec will inherit the NatSpec of a parent contract/interface for that same function.

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:

solc --userdoc --devdoc ex1.sol

And the output is below.

User Documentation

The above documentation will produce the following user documentation JSON file as output:

{
  "methods" :
  {
    "age(uint256)" :
    {
      "notice" : "Calculate tree age in years, rounded up, for live trees"
    }
  },
  "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 Contract ABI and not simply the function’s name.

Developer Documentation

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

{
  "author" : "Larry A. Gardner",
  "details" : "All function calls are currently implemented without side effects",
  "methods" :
  {
    "age(uint256)" :
    {
      "author" : "Mary A. Botanist",
      "details" : "The Alexandr N. Tetearing algorithm could increase precision",
      "params" :
      {
        "rings" : "The number of rings from dendrochronological sample"
      },
      "return" : "age in years, rounded up for partial years"
    }
  },
  "title" : "A simulator for trees"
}