# Nim Enhancement Proposal #1 - Standard Library Style Guide

Dark Mode

- Manual
- Standard library
- Index

Search:

Author: | Clay Sweetser, Dominik Picheta
Version: | 1.2.8

# Introduction

Although Nim supports a variety of code and formatting styles, it is nevertheless beneficial that certain community efforts, such as the standard library, should follow a consistent set of style guidelines when suitable. This enhancement proposal aims to list a series of guidelines that the standard library should follow.

Note that there can be exceptions to these rules. Nim being as flexible as it is, there will be parts of this style guide that don't make sense in certain contexts. Furthermore, just as Python's style guide changes over time, this style guide will too.

These rules will only be enforced for contributions to the Nim codebase and official projects, such as the Nim compiler, the standard library, and the various official tools such as C2Nim.

## Spacing and Whitespace Conventions

- Lines should be no longer than 80 characters. Limiting the amount of information present on each line makes for more readable code - the reader has smaller chunks to process.
- Two spaces should be used for indentation of blocks; tabstops are not allowed (the compiler enforces this). Using spaces means that the appearance of code is more consistent across editors. Unlike spaces, tabstop width varies across editors, and not all editors provide means of changing this width.
- Although use of whitespace for stylistic reasons other than the ones endorsed by this guide are allowed, careful thought should be put into such practices. Not all editors support automatic alignment of code sections, and re-aligning long sections of code by hand can quickly become tedious.

      # This is bad, as the next time someone comes
      # to edit this code block, they
      # must re-align all the assignments again:
      type
        WordBool*    = int16
        CalType*     = int
        ... # 5 lines later
        CalId*       = int
        LongLong*    = int64
        LongLongPtr* = ptr LongLong

## Naming Conventions

Note: While the rules outlined below are the current naming conventions, these conventions have not always been in place. Previously, the naming conventions for identifiers followed the Pascal tradition of prefixes which indicated the base type of the identifier - PFoo for pointer and reference types, TFoo for value types, EFoo for exceptions, etc. Though this has since changed, there are many places in the standard library which still use this convention. Such style remains in place purely for legacy reasons, and will be changed in the future.

- Type identifiers should be in PascalCase. All other identifiers should be in camelCase with the exception of constants which may use PascalCase but are not required to.

      # Constants can start with either a lower case or upper case letter.
      const aConstant = 42
      const FooBar = 4.2

      var aVariable = "Meep" # Variables must start with a lowercase letter.

      # Types must start with an uppercase letter.
      type
        FooBar = object

  For constants coming from a C/C++ wrapper, ALL_UPPERCASE are allowed, but ugly. (Why shout CONSTANT? Constants do no harm, variables do!)

- When naming types that come in value, pointer, and reference varieties, use a regular name for the variety that is to be used the most, and add a "Obj", "Ref", or "Ptr" suffix for the other varieties. If there is no single variety that will be used the most, add the suffixes to the pointer variants only. The same applies to C/C++ wrappers.

      type
        Handle = object # Will be used most often
          fd: int64
        HandleRef = ref Handle # Will be used less often

- Exception and Error types should have the "Error" suffix.

      type
        UnluckyError = object of Exception

- Unless marked with the `{.pure.}` pragma, members of enums should have an identifying prefix, such as an abbreviation of the enum's name.

      type
        PathComponent = enum
          pcDir
          pcLinkToDir
          pcFile
          pcLinkToFile

- Non-pure enum values should use camelCase whereas pure enum values should use PascalCase.

      type
        PathComponent {.pure.} = enum
          Dir
          LinkToDir
          File
          LinkToFile

- In the age of HTTP, HTML, FTP, TCP, IP, UTF, WWW it is foolish to pretend these are somewhat special words requiring all uppercase. Instead treat them as what they are: Real words. So it's `parseUrl` rather than `parseURL`, `checkHttpHeader` instead of `checkHTTPHeader` etc.
- Operations like `mitems` or `mpairs` (or the now deprecated `mget`) that allow a mutating view into some data structure should start with an `m`.
- When both in-place mutation and 'returns transformed copy' are available the latter is a past participle of the former:
  - reverse and reversed in algorithm
  - sort and sorted
  - rotate and rotated
- When the 'returns transformed copy' version already exists like `strutils.replace` an in-place version should get an `-In` suffix (`replaceIn` for this example).

The stdlib API is designed to be easy to use and consistent. Ease of use is measured by the number of calls to achieve a concrete high level action. The ultimate goal is that the programmer can guess a name.

The library uses a simple naming scheme that makes use of common abbreviations to keep the names short but meaningful.

English word | To use | Notes
--- | --- | ---
initialize | initT | `init` is used to create a value type `T`
new | newP | `new` is used to create a reference type `P`
find | find | should return the position where something was found; for a bool result use `contains`
contains | contains | often short for `find() >= 0`
append | add | use `add` instead of `append`
compare | cmp | should return an int with the `< 0` `== 0` or `> 0` semantics; for a bool result use `sameXYZ`
put | put, `[]=` | consider overloading `[]=` for put
get | get, `[]` | consider overloading `[]` for get; consider to not use `get` as a prefix: `len` instead of `getLen`
length | len | also used for number of elements
size | size, len | size should refer to a byte size
capacity | cap |
memory | mem | implies a low-level operation
items | items | default iterator over a collection
pairs | pairs | iterator over (key, value) pairs
delete | delete, del | del is supposed to be faster than delete, because it does not keep the order; delete keeps the order
remove | delete, del | inconsistent right now
include | incl |
exclude | excl |
command | cmd |
execute | exec |
environment | env |
variable | var |
value | value, val | val is preferred, inconsistent right now
executable | exe |
directory | dir |
path | path | path is the string "/usr/bin" (for example), dir is the content of "/usr/bin"; inconsistent right now
extension | ext |
separator | sep |
column | col, column | col is preferred, inconsistent right now
application | app |
configuration | cfg |
message | msg |
argument | arg |
object | obj |
parameter | param |
operator | opr |
procedure | proc |
function | func |
coordinate | coord |
rectangle | rect |
point | point |
symbol | sym |
literal | lit |
string | str |
identifier | ident |
indentation | indent |

## Coding Conventions

- The 'return' statement should ideally be used when its control-flow properties are required. Use a procedure's implicit 'result' variable whenever possible. This improves readability.

      proc repeat(text: string, x: int): string =
        result = ""

        for i in 0 .. x:
          result.add($i)

- Use a proc when possible, only using the more powerful facilities of macros, templates, iterators, and converters when necessary.
- Use the `let` statement (not the `var` statement) when declaring variables that do not change within their scope. Using the `let` statement ensures that variables remain immutable, and gives those who read the code a better idea of the code's purpose.

## Conventions for multi-line statements and expressions

- Tuples which are longer than one line should indent their parameters to align with the parameters above it.

      type
        LongTupleA = tuple[wordyTupleMemberOne: int, wordyTupleMemberTwo: string,
                           wordyTupleMemberThree: float]

- Similarly, any procedure and procedure type declarations that are longer than one line should do the same thing.

      type
        EventCallback = proc (timeReceived: Time, errorCode: int, event: Event,
                              output: var string)

      proc lotsOfArguments(argOne: string, argTwo: int, argThree: float
                           argFour: proc(), argFive: bool): int
                          {.heyLookALongPragma.} =

- Multi-line procedure calls should continue on the same column as the opening parenthesis (like multi-line procedure declarations).

      startProcess(nimExecutable, currentDirectory, compilerArguments
                   environment, processOptions)