Nim Enhancement Proposal #1 - Standard Library Style Guide

Author: Clay Sweetser, Dominik Picheta
Version: 0.17.0


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:
      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.
      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.
      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.
      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.
      PathComponent = enum
  • Non-pure enum values should use camelCase whereas pure enum values should use PascalCase.
      PathComponent {.pure.} = enum
  • 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.

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:
  • 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.
      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.
      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)