Style Guide

Introduction

This guide is intended to provide coding conventions for writing solidity code. This guide should be thought of as an evolving document that will change over time as useful conventions are found and old conventions are rendered obsolete.

Many projects will implement their own style guides. In the event of conflicts, project specific style guides take precedence.

The structure and many of the recommendations within this style guide were taken from python’s pep8 style guide.

The goal of this guide is not to be the right way or the best way to write solidity code. The goal of this guide is consistency. A quote from python’s pep8 captures this concept well.

A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one module or function is most important. But most importantly: know when to be inconsistent – sometimes the style guide just doesn’t apply. When in doubt, use your best judgement. Look at other examples and decide what looks best. And don’t hesitate to ask!

Code Layout

Indentation

Use 4 spaces per indentation level.

Tabs or Spaces

Spaces are the preferred indentation method.

Mixing tabs and spaces should be avoided.

Blank Lines

Surround top level declarations in solidity source with two blank lines.

Yes:

contract A {
    ...
}


contract B {
    ...
}


contract C {
    ...
}

No:

contract A {
    ...
}
contract B {
    ...
}

contract C {
    ...
}

Within a contract surround function declarations with a single blank line.

Blank lines may be omitted between groups of related one-liners (such as stub functions for an abstract contract)

Yes:

contract A {
    function spam() public;
    function ham() public;
}


contract B is A {
    function spam() public {
        ...
    }

    function ham() public {
        ...
    }
}

No:

contract A {
    function spam() public {
        ...
    }
    function ham() public {
        ...
    }
}

Source File Encoding

UTF-8 or ASCII encoding is preferred.

Imports

Import statements should always be placed at the top of the file.

Yes:

import "owned";


contract A {
    ...
}


contract B is owned {
    ...
}

No:

contract A {
    ...
}


import "owned";


contract B is owned {
    ...
}

Order of Functions

Ordering helps readers identify which functions they can call and to find the constructor and fallback definitions easier.

Functions should be grouped according to their visibility and ordered:

  • constructor
  • fallback function (if exists)
  • external
  • public
  • internal
  • private

Within a grouping, place the constant functions last.

Yes:

contract A {
    function A() public {
        ...
    }

    function() public {
        ...
    }

    // External functions
    // ...

    // External functions that are constant
    // ...

    // Public functions
    // ...

    // Internal functions
    // ...

    // Private functions
    // ...
}

No:

contract A {

    // External functions
    // ...

    // Private functions
    // ...

    // Public functions
    // ...

    function A() public {
        ...
    }

    function() public {
        ...
    }

    // Internal functions
    // ...
}

Whitespace in Expressions

Avoid extraneous whitespace in the following situations:

Immediately inside parenthesis, brackets or braces, with the exception of single line function declarations.

Yes:

spam(ham[1], Coin({name: "ham"}));

No:

spam( ham[ 1 ], Coin( { name: "ham" } ) );

Exception:

function singleLine() public { spam(); }

Immediately before a comma, semicolon:

Yes:

function spam(uint i, Coin coin) public;

No:

function spam(uint i , Coin coin) public ;
More than one space around an assignment or other operator to align with
another:

Yes:

x = 1;
y = 2;
long_variable = 3;

No:

x             = 1;
y             = 2;
long_variable = 3;

Don’t include a whitespace in the fallback function:

Yes:

function() public {
    ...
}

No:

function () public {
    ...
}

Control Structures

The braces denoting the body of a contract, library, functions and structs should:

  • open on the same line as the declaration
  • close on their own line at the same indentation level as the beginning of the declaration.
  • The opening brace should be proceeded by a single space.

Yes:

contract Coin {
    struct Bank {
        address owner;
        uint balance;
    }
}

No:

contract Coin
{
    struct Bank {
        address owner;
        uint balance;
    }
}

The same recommendations apply to the control structures if, else, while, and for.

Additionally there should be a single space between the control structures if, while, and for and the parenthetic block representing the conditional, as well as a single space between the conditional parenthetic block and the opening brace.

Yes:

if (...) {
    ...
}

for (...) {
    ...
}

No:

if (...)
{
    ...
}

while(...){
}

for (...) {
    ...;}

For control structures whose body contains a single statement, omitting the braces is ok if the statement is contained on a single line.

Yes:

if (x < 10)
    x += 1;

No:

if (x < 10)
    someArray.push(Coin({
        name: 'spam',
        value: 42
    }));

For if blocks which have an else or else if clause, the else should be placed on the same line as the if’s closing brace. This is an exception compared to the rules of other block-like structures.

Yes:

if (x < 3) {
    x += 1;
} else if (x > 7) {
    x -= 1;
} else {
    x = 5;
}


if (x < 3)
    x += 1;
else
    x -= 1;

No:

if (x < 3) {
    x += 1;
}
else {
    x -= 1;
}

Function Declaration

For short function declarations, it is recommended for the opening brace of the function body to be kept on the same line as the function declaration.

The closing brace should be at the same indentation level as the function declaration.

The opening brace should be preceeded by a single space.

Yes:

function increment(uint x) public pure returns (uint) {
    return x + 1;
}

function increment(uint x) public pure onlyowner returns (uint) {
    return x + 1;
}

No:

function increment(uint x) public pure returns (uint)
{
    return x + 1;
}

function increment(uint x) public pure returns (uint){
    return x + 1;
}

function increment(uint x) public pure returns (uint) {
    return x + 1;
    }

function increment(uint x) public pure returns (uint) {
    return x + 1;}

The visibility modifiers for a function should come before any custom modifiers.

Yes:

function kill() public onlyowner {
    selfdestruct(owner);
}

No:

function kill() onlyowner public {
    selfdestruct(owner);
}

For long function declarations, it is recommended to drop each argument onto it’s own line at the same indentation level as the function body. The closing parenthesis and opening bracket should be placed on their own line as well at the same indentation level as the function declaration.

Yes:

function thisFunctionHasLotsOfArguments(
    address a,
    address b,
    address c,
    address d,
    address e,
    address f
)
    public
{
    doSomething();
}

No:

function thisFunctionHasLotsOfArguments(address a, address b, address c,
    address d, address e, address f) public {
    doSomething();
}

function thisFunctionHasLotsOfArguments(address a,
                                        address b,
                                        address c,
                                        address d,
                                        address e,
                                        address f) public {
    doSomething();
}

function thisFunctionHasLotsOfArguments(
    address a,
    address b,
    address c,
    address d,
    address e,
    address f) public {
    doSomething();
}

If a long function declaration has modifiers, then each modifier should be dropped to its own line.

Yes:

function thisFunctionNameIsReallyLong(address x, address y, address z)
    public
    onlyowner
    priced
    returns (address)
{
    doSomething();
}

function thisFunctionNameIsReallyLong(
    address x,
    address y,
    address z,
)
    public
    onlyowner
    priced
    returns (address)
{
    doSomething();
}

No:

function thisFunctionNameIsReallyLong(address x, address y, address z)
                                      public
                                      onlyowner
                                      priced
                                      returns (address) {
    doSomething();
}

function thisFunctionNameIsReallyLong(address x, address y, address z)
    public onlyowner priced returns (address)
{
    doSomething();
}

function thisFunctionNameIsReallyLong(address x, address y, address z)
    public
    onlyowner
    priced
    returns (address) {
    doSomething();
}

For constructor functions on inherited contracts whose bases require arguments, it is recommended to drop the base constructors onto new lines in the same manner as modifiers if the function declaration is long or hard to read.

Yes:

contract A is B, C, D {
    function A(uint param1, uint param2, uint param3, uint param4, uint param5)
        B(param1)
        C(param2, param3)
        D(param4)
        public
    {
        // do something with param5
    }
}

No:

contract A is B, C, D {
    function A(uint param1, uint param2, uint param3, uint param4, uint param5)
    B(param1)
    C(param2, param3)
    D(param4)
    public
    {
        // do something with param5
    }
}

contract A is B, C, D {
    function A(uint param1, uint param2, uint param3, uint param4, uint param5)
        B(param1)
        C(param2, param3)
        D(param4)
        public {
        // do something with param5
    }
}

When declaring short functions with a single statement, it is permissible to do it on a single line.

Permissible:

function shortFunction() public { doSomething(); }

These guidelines for function declarations are intended to improve readability. Authors should use their best judgement as this guide does not try to cover all possible permutations for function declarations.

Mappings

TODO

Variable Declarations

Declarations of array variables should not have a space between the type and the brackets.

Yes:

uint[] x;

No:

uint [] x;

Other Recommendations

  • Strings should be quoted with double-quotes instead of single-quotes.

Yes:

str = "foo";
str = "Hamlet says, 'To be or not to be...'";

No:

str = 'bar';
str = '"Be yourself; everyone else is already taken." -Oscar Wilde';
  • Surround operators with a single space on either side.

Yes:

x = 3;
x = 100 / 10;
x += 3 + 4;
x |= y && z;

No:

x=3;
x = 100/10;
x += 3+4;
x |= y&&z;
  • Operators with a higher priority than others can exclude surrounding whitespace in order to denote precedence. This is meant to allow for improved readability for complex statement. You should always use the same amount of whitespace on either side of an operator:

Yes:

x = 2**3 + 5;
x = 2*y + 3*z;
x = (a+b) * (a-b);

No:

x = 2** 3 + 5;
x = y+z;
x +=1;

Naming Conventions

Naming conventions are powerful when adopted and used broadly. The use of different conventions can convey significant meta information that would otherwise not be immediately available.

The naming recommendations given here are intended to improve the readability, and thus they are not rules, but rather guidelines to try and help convey the most information through the names of things.

Lastly, consistency within a codebase should always supercede any conventions outlined in this document.

Naming Styles

To avoid confusion, the following names will be used to refer to different naming styles.

  • b (single lowercase letter)
  • B (single uppercase letter)
  • lowercase
  • lower_case_with_underscores
  • UPPERCASE
  • UPPER_CASE_WITH_UNDERSCORES
  • CapitalizedWords (or CapWords)
  • mixedCase (differs from CapitalizedWords by initial lowercase character!)
  • Capitalized_Words_With_Underscores

Note

When using abbreviations in CapWords, capitalize all the letters of the abbreviation. Thus HTTPServerError is better than HttpServerError.

Names to Avoid

  • l - Lowercase letter el
  • O - Uppercase letter oh
  • I - Uppercase letter eye

Never use any of these for single letter variable names. They are often indistinguishable from the numerals one and zero.

Contract and Library Names

Contracts and libraries should be named using the CapWords style. Examples: SimpleToken, SmartBank, CertificateHashRepository, Player.

Event Names

Events should be named using the CapWords style. Examples: Deposit, Transfer, Approval, BeforeTransfer, AfterTransfer.

Function Names

Functions should use mixedCase. Examples: getBalance, transfer, verifyOwner, addMember, changeOwner.

Function Argument Names

Function arguments should use mixedCase. Examples: initialSupply, account, recipientAddress, senderAddress, newOwner.

When writing library functions that operate on a custom struct, the struct should be the first argument and should always be named self.

Local and State Variable Names

Use mixedCase. Examples: totalSupply, remainingSupply, balancesOf, creatorAddress, isPreSale, tokenExchangeRate.

Constants

Constants should be named with all capital letters with underscores separating words. Examples: MAX_BLOCKS, TOKEN_NAME, TOKEN_TICKER, CONTRACT_VERSION.

Modifier Names

Use mixedCase. Examples: onlyBy, onlyAfter, onlyDuringThePreSale.

Enums

Enums, in the style of simple type declarations, should be named using the CapWords style. Examples: TokenGroup, Frame, HashStyle, CharacterLocation.

Avoiding Naming Collisions

  • single_trailing_underscore_

This convention is suggested when the desired name collides with that of a built-in or otherwise reserved name.

General Recommendations

TODO