Nim DocGen Tools Guide

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Author:Erik O'Leary


This document describes the documentation generation tools built into the Nim compiler, which can generate HTML, Latex 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 the module dependencies (import), any top-level documentation comments (##), and exported symbols (*), including procedures, types, and variables.

commandruns on...input formatoutput format
nim docdocumentation comments.nim.html HTML
nim doc2tex.tex LaTeX
nim jsondoc.json JSON
nim rst2htmlstandalone rst files.rst.html HTML
nim rst2tex.tex LaTeX

Quick start

Generate HTML documentation for a file:

nim doc <filename>.nim

Generate HTML documentation for a whole project:

# delete any htmldocs/*.idx file before starting
nim doc --project --index:on --git.url:<url> --git.commit:<tag> --outdir:htmldocs <main_filename>.nim
# this will generate html files, a theindex.html index, css and js under `htmldocs`
# See also `--docroot` to specify a relative root.
# to get search (dochacks.js) to work locally, you need a server otherwise
# CORS will prevent opening file:// urls; this works:
python3 -m http.server 7029 --directory htmldocs
# When --outdir is omitted it defaults to $projectPath/htmldocs,
# or `$nimcache/htmldocs` with `--usenimcache` which avoids clobbering your sources;
# and likewise without `--project`.
# Adding `-r` will open in a browser directly.

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! Basic Markdown syntax is also supported inside the doc comments.


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.

It's recommended to always add exactly one space after ## for readability of comments — this extra space will be cropped from the parsed comments and won't influence RST formatting.

Note: Generally, this baseline indentation level inside a documentation comment may not be 1: it can be any since it is determined by the offset of the first non-whitespace character in the comment. After that indentation must be consistent on the following lines of the same comment. If you still need to add an additional indentation at the very beginning (for RST block quote syntax) use backslash \ before it:
## \
##    Block quote at the first line.
## Paragraph.

Document Types

Example of Nim file input

The following examples will generate documentation for this sample Nim module, aptly named doc/docgen_sample.nim:

## This module is a sample.

import strutils

proc helloWorld*(times: int) =
  ## Takes an integer and outputs
  ## as many indented "hello world!"s

  for i in 0 .. times-1:
    echo "hello world!".indent(2) # using indent to avoid `UnusedImport`


All the below commands save their output to htmldocs directory relative to the directory of file; hence the output for this sample will be in doc/htmldocs.


The generation of HTML documents is done via the doc command. This command takes 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 docgen_sample.nim

Partial Output:

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

The full output can be seen here: docgen_sample.html. It runs after semantic checking and includes pragmas attached implicitly by the compiler.


LaTeX files are intended to be converted to PDF, especially for offline reading or making hard copies. (LaTeX output is oftentimes better than HTML -> PDF conversion).

The doc2tex command:

nim doc2tex docgen_sample.nim
cd htmldocs
xelatex docgen_sample.tex
xelatex docgen_sample.tex
# It is usually necessary to run `xelatex` 2 times (or even 3 times for
# large documents) to get all labels generated.
# That depends on this warning in the end of `xelatex` output:
#   LaTeX Warning: Label(s) may have changed. Rerun to get cross-references right.

The output is docgen_sample.pdf.


The 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 (previously doc2), and contains the same information.

The jsondoc command:

nim jsondoc docgen_sample.nim


  "orig": "docgen_sample.nim",
  "nimble": "",
  "moduleDescription": "This module is a sample",
  "entries": [
      "name": "helloWorld",
      "type": "skProc",
      "line": 5,
      "col": 0,
      "description": "Takes an integer and outputs as many &quot;hello world!&quot;s",
      "code": "proc helloWorld(times: int) {.raises: [], tags: [].}"

Similarly to the old doc command, the old jsondoc command has been renamed to jsondoc0.

The jsondoc0 command:

nim jsondoc0 docgen_sample.nim


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

Note that the jsondoc command outputs its JSON without pretty-printing it, while jsondoc0 outputs pretty-printed JSON.

Related Options

Project switch

nim doc --project filename.nim

This will recursively generate documentation of all Nim modules imported into the input module that belong to the Nimble package that filename.nim belongs to. The index files and the corresponding theindex.html will also be generated.

Index switch

nim doc --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 doc --git.url:<url> filename.nim

With the git.url switch the See source hyperlink will appear below each documented item in your source code pointing to the implementation of that item on a GitHub repository. You can click the link to see the implementation of the item.

The git.commit switch overrides the hardcoded devel branch in config/nimdoc.cfg. This is useful to link to a different branch e.g. --git.commit:master, or to a tag e.g. --git.commit:1.2.3 or a commit.

Source URLs are generated as href="${url}/tree/${commit}/${path}#L${line}" by default and thus compatible with GitHub but not with GitLab.

Similarly, git.devel switch overrides the hardcoded devel branch for the Edit link which is also useful if you have a different working branch than devel e.g. --git.devel:master.

Edit URLs are generated as href="${url}/tree/${devel}/${path}#L${line}" by default.

You can edit config/nimdoc.cfg and modify the doc.item.seesrc value with a hyperlink to your own code repository.

In the case of Nim's own documentation, the commit 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 the 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 source code highlighting with the .. code-block:: nim prefix. code-block also supports highlighting of a few other languages supported by the packages/docutils/highlite module.


nim rst2html docgen.rst


You're reading it!

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 hyperlink 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 command 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 that works unless you have a module with collisions due to overloading.

If you hyperlink 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 of 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
proc, funcempty 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 programmatically 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 (e.g. `^`).
  2. Base filename plus anchor hyperlink (e.g. algorithm.html#*,int,SortOrder).
  3. Optional human-readable string to display as a hyperlink. If the value is not present or is the empty string, the hyperlink 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 hyperlink. Browsers usually display this as a tooltip after hovering a moment over the hyperlink.

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 the 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 hierarchy for document TOCs in the final index, and TOC entries found in .nim files are discarded.

Additional resources

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 the docgen output.

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