Nim DocGen Tools Guide

Author: Erik O'Leary
Version: 0.16.0


This document describes the documentation generation tools built into the Nim compiler, which can generate HTML and JSON output from input .nim files and projects, as well as HTML and LaTeX from input RST (reStructuredText) files. The output documentation will include module dependencies (import), any top-level documentation comments (##), and exported symbols (*), including procedures, types, and variables.

Documentation Comments

Any comments which are preceded by a double-hash (##), are interpreted as documentation. Comments are parsed as RST (see reference), providing Nim module authors the ability to easily generate richly formatted documentation with only their well-documented code.


type Person* = object
  ## This type contains a description of a person
  name: string
  age: int


Person* = object
  name: string
  age: int

This type contains a description of a person

Field documentation comments can be added to fields like so:

var numValues: int ## \
  ## `numValues` stores the number of values

Note that without the * following the name of the type, the documentation for this type would not be generated. Documentation will only be generated for exported types/procedures/etc.

Nim file input

The following examples will generate documentation for the below contrived Nim module, aptly named 'sample.nim'


## This module is a sample.

import strutils

proc helloWorld*(times: int) =
  ## Takes an integer and outputs
  ## as many "hello world!"s
  for i in 0 .. times-1:
    echo "hello world!"


Document Types


Generation of HTML documents is done via both the doc and doc2 commands. These command take either a single .nim file, outputting a single .html file with the same base filename, or multiple .nim files, outputting multiple .html files and, optionally, an index file.

The doc command:

nim doc sample

Partial Output:

proc helloWorld*(times: int)

Output can be viewed in full here: docgen_sample.html. The next command, called doc2, is very similar to the doc command, but will be run after the compiler performs semantic checking on the input nim module(s), which allows it to process macros.

The doc2 command:

nim doc2 sample

Partial Output:

proc helloWorld(times: int) {.raises: [], tags: [].}

The full output can be seen here: docgen_sample2.html. As you can see, the tool has extracted additional information provided to it by the compiler beyond what the doc command provides, such as pragmas attached implicitly by the compiler. This type of information is not available from looking at the AST (Abstract Syntax Tree) prior to semantic checking, as the doc command does.


Generation of JSON documents is done via the jsondoc command. This command takes in a .nim file, and outputs a .json file with the same base filename. Note that this tool is built off of the doc command, and therefore is performed before semantic checking.

The jsondoc command:

nim jsondoc sample


    "comment": "This module is a sample."
    "name": "helloWorld",
    "type": "skProc",
    "description": "Takes an integer and outputs as many "hello world!"s",
    "code": "proc helloWorld*(times: int)"

Related Options

Project switch

nim doc2 --project filename.nim

This will recursively generate documentation of all nim modules imported into the input module, including system modules. Be careful with this command, as it may end up sprinkling html files all over your filesystem!

Index switch

nim doc2 --index:on filename.nim

This will generate an index of all the exported symbols in the input Nim module, and put it into a neighboring file with the extension of .idx. The index file is line oriented (newlines have to be escaped). Each line represents a tab separated record of several columns, the first two mandatory, the rest optional. See the Index (idx) file format section for details.

Once index files have been generated for one or more modules, the Nim compiler command buildIndex directory can be run to go over all the index files in the specified directory to generate a theindex.html file.

See source switch

nim doc2 --docSeeSrcUrl:txt filename.nim

When you pass the docSeeSrcUrl switch to docgen, after each documented item in your source code the hyper link See source will appear pointing to the implementation of that item on a GitHub repository. You can click the link to see the implementation of the item.

If you want to reuse this feature in your own documentation you will have to modify config/nimdoc.cfg to contain a doc.item.seesrc value with a hyper link to your own code repository. As you will see by the comments in that file, the value txt passed on the command line will be used in the HTML template along others like $path and $line.

In the case of Nim's own documentation, the txt value is just a commit hash to append to a formatted URL to The tools/nimweb.nim helper queries the current git commit hash during doc generation, but since you might be working on an unpublished repository, it also allows specifying a githash value in web/website.ini to force a specific commit in the output.

Other Input Formats

The Nim compiler also has support for RST (reStructuredText) files with the rst2html and rst2tex commands. Documents like this one are initially written in a dialect of RST which adds support for nim sourcecode highlighting with the .. code-block:: nim prefix. code-block also supports highlighting of C++ and some other c-like languages.


nim rst2html docgen.txt


You're reading it!

The input can be viewed here docgen.txt. The rst2tex command is invoked identically to rst2html, but outputs a .tex file instead of .html.

HTML anchor generation

When you run the rst2html command, all sections in the RST document will get an anchor you can hyper link to. Usually you can guess the anchor lower casing the section title and replacing spaces with dashes, and in any case you can get it from the table of contents. But when you run the doc or doc2 commands to generate API documentation, some symbol get one or two anchors at the same time: a numerical identifier, or a plain name plus a complex name.

The numerical identifier is just a random number. The number gets assigned according to the section and position of the symbol in the file being processed and you should not rely on it being constant: if you add or remove a symbol the numbers may shuffle around.

The plain name of a symbol is a simplified version of its fully exported signature. Variables or constants have the same plain name symbol as their complex name. The plain name for procs, templates, and other callable types will be their unquoted value after removing parameters, return types and pragmas. The plain name allows short and nice linking of symbols which works unless you have a module with collisions due to overloading.

If you hyper link a plain name symbol and there are other matches on the same HTML file, most browsers will go to the first one. To differentiate the rest, you will need to use the complex name. A complex name for a callable type is made up from several parts:

(plain symbol)(.type),(first param)?(,param type)*

The first thing to note is that all callable types have at least a comma, even if they don't have any parameters. If there are parameters, they are represented by their types and will be comma separated. To the plain symbol a suffix may be added depending on the type of the callable:

Callable typeSuffix
procempty string

The relationship of type to suffix is made by the proc complexName in the compiler/docgen.nim file. Here are some examples of complex names for symbols in the system module.

Index (idx) file format

Files with the .idx extension are generated when you use the Index switch along with commands to generate documentation from source or text files. You can programatically generate indices with the setIndexTerm() and writeIndexFile() procs. The purpose of idx files is to hold the interesting symbols and their HTML references so they can be later concatenated into a big index file with mergeIndexes(). This section documents the file format in detail.

Index files are line oriented and tab separated (newline and tab characters have to be escaped). Each line represents a record with at least two fields, but can have up to four (additional columns are ignored). The content of these columns is:

  1. Mandatory term being indexed. Terms can include quoting according to Nim's rules (eg. `^` like in the actors module).
  2. Base filename plus anchor hyper link (eg. algorithm.html#*,int,SortOrder).
  3. Optional human readable string to display as hyper link. If the value is not present or is the empty string, the hyper link will be rendered using the term. Prefix whitespace indicates that this entry is not for an API symbol but for a TOC entry.
  4. Optional title or description of the hyper link. Browsers usually display this as a tooltip after hovering a moment over the hyper link.

The index generation tools try to differentiate between documentation generated from .nim files and documentation generated from .txt or .rst files. The former are always closely related to source code and consist mainly of API entries. The latter are generic documents meant for human reading.

To differentiate both types (documents and APIs), the index generator will add to the index of documents an entry with the title of the document. Since the title is the topmost element, it will be added with a second field containing just the filename without any HTML anchor. By convention this entry without anchor is the title entry, and since entries in the index file are added as they are scanned, the title entry will be the first line. The title for APIs is not present because it can be generated concatenating the name of the file to the word Module.

Normal symbols are added to the index with surrounding whitespaces removed. An exception to this are table of content (TOC) entries. TOC entries are added to the index file with their third column having as much prefix spaces as their level is in the TOC (at least 1 character). The prefix whitespace helps to filter TOC entries from API or text symbols. This is important because the amount of spaces is used to replicate the hiearchy for document TOCs in the final index, and TOC entries found in .nim files are discarded.

Additional resources

Nim Compiler User Guide

RST Quick Reference

The output for HTML and LaTeX comes from the config/nimdoc.cfg and config/nimdoc.tex.cfg configuration files. You can add and modify these files to your project to change the look of docgen output.

You can import the packages/docutils/rstgen module in your programs if you want to reuse the compiler's documentation generation procs.