Nim Standard Library

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Author:Andreas Rumpf

Nim's library is divided into pure libraries, impure libraries, and wrappers.

Pure libraries do not depend on any external *.dll or lib*.so binary while impure libraries do. A wrapper is an impure library that is a very low-level interface to a C library.

Read this document for a quick overview of the API design.


Nim's standard library only covers the basics, check out for a list of 3rd party packages.

Pure libraries

Automatic imports

  • system Basic procs and operators that every program needs. It also provides IO facilities for reading and writing text and binary files. It is imported implicitly by the compiler. Do not import it directly. It relies on compiler magic to work.
  • threads Basic Nim thread support. Note: This is part of the system module. Do not import it explicitly. Enabled with --threads:on.
  • channels_builtin Nim message passing support for threads. Note: This is part of the system module. Do not import it explicitly. Enabled with --threads:on.


  • atomics Types and operations for atomic operations and lockless algorithms.
  • bitops Provides a series of low-level methods for bit manipulation.
  • cpuinfo This module implements procs to determine the number of CPUs / cores.
  • endians This module contains helpers that deal with different byte orders.
  • lenientops Provides binary operators for mixed integer/float expressions for convenience.
  • locks Locks and condition variables for Nim.
  • macrocache Provides an API for macros to collect compile-time information across modules.
  • macros Contains the AST API and documentation of Nim for writing macros.
  • rlocks Reentrant locks for Nim.
  • typeinfo Provides (unsafe) access to Nim's run-time type information.
  • typetraits This module defines compile-time reflection procs for working with types.
  • volatile This module contains code for generating volatile loads and stores, which are useful in embedded and systems programming.


  • algorithm This module implements some common generic algorithms like sort or binary search.
  • enumutils This module adds functionality for the built-in enum type.
  • sequtils This module implements operations for the built-in seq type which were inspired by functional programming languages.
  • setutils This module adds functionality for the built-in set type.


  • critbits This module implements a crit bit tree which is an efficient container for a sorted set of strings, or a sorted mapping of strings.
  • deques Implementation of a double-ended queue. The underlying implementation uses a seq.
  • heapqueue Implementation of a binary heap data structure that can be used as a priority queue.
  • intsets Efficient implementation of a set of ints as a sparse bit set.
  • lists Nim linked list support. Contains singly and doubly linked lists and circular lists ("rings").
  • options The option type encapsulates an optional value.
  • packedsets Efficient implementation of a set of ordinals as a sparse bit set.
  • sets Nim hash set support.
  • sharedlist Nim shared linked list support. Contains a shared singly-linked list.
  • sharedtables Nim shared hash table support. Contains shared tables.
  • tables Nim hash table support. Contains tables, ordered tables, and count tables.

String handling

  • cstrutils Utilities for cstring handling.
  • editdistance This module contains an algorithm to compute the edit distance between two Unicode strings.
  • encodings Converts between different character encodings. On UNIX, this uses the iconv library, on Windows the Windows API.
  • parseutils This module contains helpers for parsing tokens, numbers, identifiers, etc.
  • pegs This module contains procedures and operators for handling PEGs.
  • punycode Implements a representation of Unicode with the limited ASCII character subset.
  • ropes This module contains support for a rope data type. Ropes can represent very long strings efficiently; in particular, concatenation is done in O(1) instead of O(n).
  • strbasics This module provides some high performance string operations.
  • strformat Macro based standard string interpolation/formatting. Inspired by Python's f-strings.
  • strmisc This module contains uncommon string handling operations that do not fit with the commonly used operations in strutils.
  • strscans This module contains a scanf macro for convenient parsing of mini languages.
  • strtabs The strtabs module implements an efficient hash table that is a mapping from strings to strings. Supports a case-sensitive, case-insensitive and style-insensitive modes.
  • strutils This module contains common string handling operations like changing case of a string, splitting a string into substrings, searching for substrings, replacing substrings.
  • unicode This module provides support to handle the Unicode UTF-8 encoding.
  • unidecode It provides a single proc that does Unicode to ASCII transliterations. Based on Python's Unidecode module.
  • wordwrap This module contains an algorithm to wordwrap a Unicode string.

Time handling

  • monotimes The monotimes module implements monotonic timestamps.
  • times The times module contains support for working with time.

Generic Operating System Services

  • distros This module implements the basics for OS distribution ("distro") detection and the OS's native package manager. Its primary purpose is to produce output for Nimble packages, but it also contains the widely used Distribution enum that is useful for writing platform-specific code. See packaging for hints on distributing Nim using OS packages.
  • dynlib This module implements the ability to access symbols from shared libraries.
  • marshal Contains procs for serialization and deserialization of arbitrary Nim data structures.
  • memfiles This module provides support for memory-mapped files (Posix's mmap) on the different operating systems.
  • os Basic operating system facilities like retrieving environment variables, reading command line arguments, working with directories, running shell commands, etc.
  • osproc Module for process communication beyond os.execShellCmd.
  • streams This module provides a stream interface and two implementations thereof: the FileStream and the StringStream which implement the stream interface for Nim file objects (File) and strings. Other modules may provide other implementations for this standard stream interface.
  • terminal This module contains a few procedures to control the terminal (also called console). The implementation simply uses ANSI escape sequences and does not depend on any other module.

Math libraries

  • complex This module implements complex numbers and relevant mathematical operations.
  • fenv Floating-point environment. Handling of floating-point rounding and exceptions (overflow, zero-divide, etc.).
  • math Mathematical operations like cosine, square root.
  • random Fast and tiny random number generator.
  • rationals This module implements rational numbers and relevant mathematical operations.
  • stats Statistical analysis.
  • sums Accurate summation functions.
  • sysrand Cryptographically secure pseudorandom number generator.

Internet Protocols and Support

  • asyncdispatch This module implements an asynchronous dispatcher for IO operations.
  • asyncfile This module implements asynchronous file reading and writing using asyncdispatch.
  • asyncftpclient This module implements an asynchronous FTP client using the asyncnet module.
  • asynchttpserver This module implements an asynchronous HTTP server using the asyncnet module.
  • asyncnet This module implements asynchronous sockets based on the asyncdispatch module.
  • asyncstreams This module provides FutureStream - a future that acts as a queue.
  • cgi This module implements helpers for CGI applications.
  • cookies This module contains helper procs for parsing and generating cookies.
  • httpclient This module implements a simple HTTP client which supports both synchronous and asynchronous retrieval of web pages.
  • mimetypes This module implements a mimetypes database.
  • nativesockets This module implements a low-level sockets API.
  • net This module implements a high-level sockets API. It replaces the sockets module.
  • selectors This module implements a selector API with backends specific to each OS. Currently, epoll on Linux and select on other operating systems.
  • smtp This module implements a simple SMTP client.
  • uri This module provides functions for working with URIs.



  • htmlparser This module parses an HTML document and creates its XML tree representation.
  • json High-performance JSON parser.
  • jsonutils This module implements a hookable (de)serialization for arbitrary types.
  • lexbase This is a low-level module that implements an extremely efficient buffering scheme for lexers and parsers. This is used by the diverse parsing modules.
  • parsecfg The parsecfg module implements a high-performance configuration file parser. The configuration file's syntax is similar to the Windows .ini format, but much more powerful, as it is not a line based parser. String literals, raw string literals, and triple quote string literals are supported as in the Nim programming language.
  • parsecsv The parsecsv module implements a simple high-performance CSV parser.
  • parsejson This module implements a JSON parser. It is used and exported by the json module, but can also be used in its own right.
  • parseopt The parseopt module implements a command line option parser.
  • parsesql The parsesql module implements a simple high-performance SQL parser.
  • parsexml The parsexml module implements a simple high performance XML/HTML parser. The only encoding that is supported is UTF-8. The parser has been designed to be somewhat error-correcting, so that even some "wild HTML" found on the web can be parsed with it.


  • packages/docutils/highlite Source highlighter for programming or markup languages. Currently, only a few languages are supported, other languages may be added. The interface supports one language nested in another.
  • packages/docutils/rst This module implements a reStructuredText parser. A large subset is implemented. Some features of the markdown wiki syntax are also supported.
  • packages/docutils/rstast This module implements an AST for the reStructuredText parser.
  • packages/docutils/rstgen This module implements a generator of HTML/Latex from reStructuredText.

XML Processing

  • xmltree A simple XML tree. More efficient and simpler than the DOM. It also contains a macro for XML/HTML code generation.
  • xmlparser This module parses an XML document and creates its XML tree representation.


  • htmlgen This module implements a simple XML and HTML code generator. Each commonly used HTML tag has a corresponding macro that generates a string with its HTML representation.


  • base64 This module implements a Base64 encoder and decoder.
  • hashes This module implements efficient computations of hash values for diverse Nim types.
  • md5 This module implements the MD5 checksum algorithm.
  • oids An OID is a global ID that consists of a timestamp, a unique counter, and a random value. This combination should suffice to produce a globally distributed unique ID. This implementation was extracted from the MongoDB interface and it thus binary compatible with a MongoDB OID.
  • sha1 This module implements a sha1 encoder and decoder.


  • browsers This module implements procs for opening URLs with the user's default browser.
  • colors This module implements color handling for Nim.
  • coro This module implements experimental coroutines in Nim.
  • enumerate This module implements enumerate syntactic sugar based on Nim's macro system.
  • logging This module implements a simple logger.
  • segfaults Turns access violations or segfaults into a NilAccessDefect exception.
  • sugar This module implements nice syntactic sugar based on Nim's macro system.
  • unittest Implements a Unit testing DSL.
  • varints Decode variable-length integers that are compatible with SQLite.
  • with This module implements the with macro for easy function chaining.

Modules for the JS backend

  • asyncjs Types and macros for writing asynchronous procedures in JavaScript.
  • dom Declaration of the Document Object Model for the JS backend.
  • jsbigints Arbitrary precision integers.
  • jsconsole Wrapper for the console object.
  • jscore The wrapper of core JavaScript functions. For most purposes, you should be using the math, json, and times stdlib modules instead of this module.
  • jsffi Types and macros for easier interaction with JavaScript.

Impure libraries

Regular expressions

  • re This module contains procedures and operators for handling regular expressions. The current implementation uses PCRE.

Database support

  • db_postgres A higher level PostgreSQL database wrapper. The same interface is implemented for other databases too.
  • db_mysql A higher level MySQL database wrapper. The same interface is implemented for other databases too.
  • db_sqlite A higher level SQLite database wrapper. The same interface is implemented for other databases too.

Generic Operating System Services

  • rdstdin This module contains code for reading from stdin.


The generated HTML for some of these wrappers is so huge that it is not contained in the distribution. You can then find them on the website.


  • winlean Contains a wrapper for a small subset of the Win32 API.
  • registry Windows registry support.

UNIX specific

  • posix Contains a wrapper for the POSIX standard.
  • posix_utils Contains helpers for the POSIX standard or specialized for Linux and BSDs.

Regular expressions

  • pcre Wrapper for the PCRE library.

Database support

  • postgres Contains a wrapper for the PostgreSQL API.
  • mysql Contains a wrapper for the mySQL API.
  • sqlite3 Contains a wrapper for the SQLite 3 API.
  • odbcsql interface to the ODBC driver.

Network Programming and Internet Protocols