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. The output documentation will include the module dependencies (import), any top-level documentation comments (##), and exported symbols (*), including procedures, types, and variables.

commandoutput format
nim doc.html HTML
nim doc2tex.tex LaTeX
nim jsondoc.json JSON

Nim can generate HTML and LaTeX from input Markdown and RST (reStructuredText) files as well, which is intended for writing standalone documents like user's guides and technical specifications. See Nim-flavored Markdown and reStructuredText document for the description of this feature and particularly section Nim-flavored Markdown and reStructuredText: Command line usage for the full list of supported commands.

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.
# Use `--showNonExports` to show non-exported fields of an exported type.

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.


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.

Structuring output directories

Basic directory for output is set by --outdir:OUTDIR switch, by default OUTDIR is htmldocs sub-directory in the directory of the processed file.

There are 2 basic options as to how generated HTML output files are stored:

  1. complex hierarchy when docgen-compiling with --project, which follows directory structure of the project itself. So nim doc replicates project's directory structure inside --outdir:OUTDIR directory. --project is well suited for projects that have 1 main module. File name clashes are impossible in this case.
  2. flattened structure, where user-provided script goes through all needed input files and calls commands like nim doc with --outdir:OUTDIR switch, thus putting all HTML (and .idx) files into 1 directory.
    Important: Make sure that you don't have files with same base name like x.nim and in the same package, otherwise you'll have name conflict for x.html.

    To structure your output directories and avoid file name clashes you can split your project into different packages -- parts of your repository that are docgen-compiled with different --outdir:OUTDIR options.

    An example of such strategy is Nim repository itself which has:

    • its stdlib .nim files from different directories and .md documentation from doc/ directory are all docgen-compiled into --outdir:web/upload/<version>/ directory
    • its .nim files from compiler/ directory are docgen-compiled into --outdir:web/upload/<version>/compiler/ directory. Interestingly, it's compiled with complex hierarchy using --project switch.

    Contents of web/upload/<version> are then deployed into Nim's Web server.

    This output directory structure allows to work correctly with files like compiler/docgen.nim (implementation) and doc/ (user documentation) in 1 repository.

Index files

Index (.idx) files are used for 2 different purposes:

  1. easy cross-referencing between different .nim and/or .md / .rst files described in Nim external referencing
  2. creating a whole-project index for searching of symbols and keywords, see Buildindex command.

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.

Simple documentation links

It's possible to use normal Markdown/RST syntax to manually reference Nim symbols using HTML anchors, however Nim has an automatic facility that makes referencing inside .nim and .md/.rst files and between them easy and seamless. The point is that such links will be resolved automatically by nim doc (or md2html, or jsondoc, or doc2tex, ...). And, unlike manual links, such automatic links check that their target exists -- a warning is emitted for any broken link, so you avoid broken links in your project.

Nim treats both .md/.rst files and .nim modules (their doc comment part) as documents uniformly. Hence all directions of referencing are equally possible having the same syntax:

  1. .md/rst -> itself (internal). See Nim-flavored Markdown and reStructuredText: Markup local referencing.
  2. .md/rst -> external .md/rst. See Nim-flavored Markdown and reStructuredText: Markup external referencing. To summarize, referencing in .md/.rst files was already described in Nim-flavored Markdown and reStructuredText (particularly it described usage of index files for referencing), while in this document we focus on Nim-specific details.
  3. .md/rst -> external .nim. See Nim external referencing.
  4. .nim -> itself (internal). See Nim local referencing.
  5. .nim -> external .md/rst. See Nim-flavored Markdown and reStructuredText: Markup external referencing.
  6. .nim -> external .nim. See Nim external referencing.

To put it shortly, local referencing always works out of the box, external referencing requires to use .. importdoc:: <file> directive to import file and to ensure that the corresponding .idx file was generated.

Syntax for referencing is basically the same as for normal markup. Recall from Nim-flavored Markdown and reStructuredText: Referencing that our parser supports two equivalent syntaxes for referencing, Markdown and RST one. So to reference proc f one should use something like that, depending on markup type:

Markdown                    RST

Ref. [proc f] or [f]        Ref. `proc f`_ or just f_ for a one-word case

Nim local referencing

You can reference Nim identifiers from Nim documentation comments inside their .nim file (or inside a .rst file included from a .nim). This pertains to any exported symbol like proc, const, iterator, etc. Link text is either one word or a group of words enclosed by delimiters (brackets [...] for Markdown or backticks `...`_ for RST). Link text will be displayed as is while link target will be set to the anchor [*] of Nim symbol that corresponds to link text.

[*] anchors' format is described in HTML anchor generation section below.

If you have a constant:

const pi* = 3.14

then it should be referenced in one of the 2 forms:

  1. non-qualified (no symbol kind specification):
  2. qualified (with symbol kind specification):
    `const pi`_

For routine kinds there are more options. Consider this definition:

proc foo*(a: int, b: float): string

Generally following syntax is allowed for referencing foo:

  • short (without parameters):
    1. non-qualified:
    2. qualified:
      `proc foo`_
  • longer variants (with parameters):
    1. non-qualified:
      1. specifying parameters names:
        `foo(a, b)`_
      2. specifying parameters types:
        `foo(int, float)`_
      3. specifying both names and types:
        `foo(a: int, b: float)`_
      4. output parameter can also be specified if you wish:
        `foo(a: int, b: float): string`_
    2. qualified: all 4 options above are valid. Particularly you can use the full format:
      `proc foo(a: int, b: float): string`_
Tip: Avoid cluttering your text with extraneous information by using one of shorter forms:
`binarySearch(a, key, cmp)`_

Brevity is better for reading! If you use a short form and have an ambiguity problem (see below) then just add some additional info.

Symbol kind like proc can also be specified in the postfix form:

`foo proc`_
`walkDir(d: string) iterator`_
Warning: An ambiguity in resolving documentation links may arise because of:
  1. clash with other RST anchors
    • manually setup anchors
    • automatically set up, e.g. section names
  2. collision with other Nim symbols:
    • routines with different parameters can exist e.g. for proc and template. In this case they are split between their corresponding sections in output file. Qualified references are useful in this case -- just disambiguate by referring to these sections explicitly:
      See `foo proc`_ and `foo template`_.
    • because in Nim proc and iterator belong to different namespaces, so there can be a collision even if parameters are the same. Use `proc foo`_ or `iterator foo`_ then.

Any ambiguity is always reported with Nim compiler warnings and an anchor with higher priority is selected. Manual anchors have highest priority, then go automatic RST anchors; then Nim-generated anchors (while procs have higher priority than other Nim symbol kinds).

Generic parameters can also be used. All in all, this long form will be recognized fine:

`proc binarySearch*[T; K](a: openArray[T], key: K, cmp: proc(T, K)): int`_


  1. The parameters of a nested routine type can be specified only with types (without parameter names, see form A.2 above). E.g. for this signature:

    proc binarySearch*[T, K](a: openArray[T]; key: K;
                             cmp: proc (x: T; y: K): int {.closure.}): int
                                        ~~    ~~   ~~~~~

    you cannot use names underlined by ~~ so it must be referenced with cmp: proc(T, K). Hence these forms are valid:

    `binarySearch(a: openArray[T], key: K, cmp: proc(T, K))`_
    `binarySearch(openArray[T], K, proc(T, K))`_
    `binarySearch(a, key, cmp)`_
  2. Default values in routine parameters are not recognized, one needs to specify the type and/or name instead. E.g. for referencing proc f(x = 7) use one of the mentioned forms:
    `f(int)`_ or `f(x)`_ or `f(x: int)`_.
  3. Generic parameters must be given the same way as in the definition of referenced symbol.
    • their names should be the same
    • parameters list should be given the same way, e.g. without substitutions between commas (,) and semicolons (;).

A bit special case is operators (as their signature is also defined with `):

func `$`(x: MyType): string
func `[]`*[T](x: openArray[T]): T

A short form works without additional backticks:


However for fully-qualified reference copy-pasting backticks (`) into other backticks will not work in our RST parser (because we use Markdown-like inline markup rules). You need either to delete backticks or keep them and escape with backslash \:

no backticks: `func $`_
escaped:  `func \`$\``_
no backticks: `func [][T](x: openArray[T]): T`_
escaped:  `func \`[]\`[T](x: openArray[T]): T`_
Note: Types that defined as enum, or object, or tuple can also be referenced with those names directly (instead of type):
type CopyFlag = enum
## Ref. `CopyFlag enum`_

Nim external referencing

Just like for Nim-flavored Markdown and reStructuredText: Markup external referencing, which saves markup anchors, the Nim symbols are also saved in .idx files, so one needs to generate them beforehand, and they should be loaded by an .. importdoc:: directive. Arguments to .. importdoc:: is a comma-separated list of Nim modules or Markdown/RST documents.

--index:only tells Nim to only generate .idx file and do not attempt to generate HTML/LaTeX output. For .nim modules there are 2 alternatives to work with .idx files:

  1. using Project switch implies generation of .idx files, however, if importdoc is called on upper modules as its arguments, their .idx are not yet created. Thus one should generate all required .idx first:

    nim doc --project --index:only <main>.nim
    nim doc --project <main>.nim

  2. or run nim doc --index:only <module.nim> command for all (used) Nim modules in your project. Then run nim doc <module.nim> on them for output HTML generation.
    Warning: A mere nim doc --index:on may fail on an attempt to do importdoc from another module (for which .idx was not yet generated), that's why --index:only shall be used instead.

    For .md/.rst markup documents point 2 is the only option.

Then, you can freely use something like this in your_module.nim:

## .. importdoc::, another_module.nim

## Ref. [some section from User Manual].

## Ref. [proc f]
## (assuming you have a proc `f` in ``another_module``).

and compile it by nim doc. Note that link text will be automatically prefixed by the module name of symbol, so you will see something like "Ref. another_module: proc f" in the generated output.

It's also possible to reference a whole module by prefixing or suffixing full canonical module name with "module":

Ref. [module subdir/name] or [subdir/name module].

Markup documents as a whole can be referenced just by their title (or by their file name if the title was not set) without any prefix.

Tip: During development process the stage of .idx files generation can be done only once, after that you use already generated .idx files while working with a document being developed (unless you do incompatible changes to referenced documents).
Hint: After changing a referenced document file one may need to regenerate its corresponding .idx file to get correct results. Of course, when referencing internally inside any given .nim file, it's not needed, one can even immediately use any freshly added anchor (a document's own .idx file is not used for resolving its internal links).

If an importdoc directive fails to find a .idx, then an error is emitted.

In case of such compilation failures please note that:

  • all relative paths, given to importdoc, relate to insides of OUTDIR, and not project's directory structure.
  • importdoc searches for .idx in --outdir:OUTDIR directory (htmldocs by default) and not around original modules, so:
    Tip: look into OUTDIR to understand what's going on.
  • also keep in mind that .html and .idx files should always be output to the same directory, so check this and, if it's not true, check that both runs with and without --index:only have all other options the same.

To summarize, for 2 basic options of Structuring output directories compilation options are different:

  1. complex hierarchy with --project switch.

    As the original project's directory structure is replicated in OUTDIR, all passed paths are related to this structure also.

    E.g. if a module path1/module.nim does .. importdoc:: path2/another.nim then docgen tries to load file OUTDIR/path1/path2/another.idx.


    markup documents are just placed into the specified directory OUTDIR by default (i.e. they are not affected by --project), so if you have PROJECT/doc/ document and want to use complex hirearchy (with doc/), compile it with --docroot:

    # 1st stage
    nim md2html --outdir:OUTDIR --docroot:/absolute/path/to/PROJECT \
         --index:only PROJECT/doc/
    # 2nd stage
    nim md2html --outdir:OUTDIR --docroot:/absolute/path/to/PROJECT \

    Then the output file will be placed as OUTDIR/doc/manual.idx. So if you have PROJECT/path1/module.nim, then can be referenced as ../doc/

  2. flattened structure.

    E.g. if a module path1/module.nim does .. importdoc:: path2/another.nim then docgen tries to load OUTDIR/path2/another.idx, so the path path1 does not matter and providing path2 can be useful only in the case it contains another package that was placed there using --outdir:OUTDIR/path2.

    The links' text will be prefixed as another: ... in both cases.

    Warning: Again, the same --outdir:OUTDIR option should be provided to both doc --index:only / md2html --index:only and final generation by doc/md2html inside 1 package.

To temporarily disable importdoc, e.g. if you don't need correct link resolution at the moment, use a --noImportdoc switch (only warnings about unresolved links will be generated for external references).

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.

Note: --index switch only affects creation of .idx index files, while user-searchable Index HTML file is created by buildIndex commmand.

Buildindex command

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

nim buildIndex -o:path/to/htmldocs/theindex.html path/to/htmldocs

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

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.

See Markdown and RST markup languages for usage of those commands.

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 6 fields. The content of these columns is:

  1. Discriminator tag denoting type of the index entry, allowed values are:
    a title for .md/.rst document
    a title of .nim module
    heading of sections, can be both in Nim and markup files
    terms marked with :idx: role
    a Nim symbol
    a Nim group for overloadable symbols like procs
  2. Mandatory term being indexed. Terms can include quoting according to Nim's rules (e.g. `^`).
  3. Base filename plus anchor hyperlink (e.g. algorithm.html#*,int,SortOrder).
  4. 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.
  5. Optional title or description of the hyperlink. Browsers usually display this as a tooltip after hovering a moment over the hyperlink.
  6. A line number of file where the entry was defined.

The index generation tools differentiate between documentation generated from .nim files and documentation generated from .md or .rst files by tag nimTitle or markupTitle in the 1st line of the .idx file.

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.