Documenting code

Crystal documentation comments use a subset of Markdown.

  • Documentation should be positioned right above definitions of classes, modules, and methods. Leave no blanks between them.
# A unicorn is a **legendary animal** (see the `Legendary` module) that has been
# described since antiquity as a beast with a large, spiraling horn projecting
# from its forehead.
class Unicorn
end

# Bad: This is not attached to any class.

class Legendary
end
  • The documentation of a method is included into the method summary and the method details. The former includes only the first line, the latter includes the entire documentation. In short, it is preferred to:

    1. State a method's purpose or functionality in the first line.
    2. Supplement it with details and usages after that.

For instance:

# Returns the number of horns this unicorn has.
#
# ```
# Unicorn.new.horns # => 1
# ```
def horns
  @horns
end
  • Use the third person: Returns the number of horns this unicorn has instead of Return the number of horns this unicorn has.

  • Parameter names should be italicized (surrounded with single asterisks * or underscores _):

# Creates a unicorn with the specified number of *horns*.
def initialize(@horns = 1)
  raise "Not a unicorn" if @horns != 1
end
  • Code blocks that have Crystal code can be surrounded with triple backticks or indented with four spaces.
# ```
# unicorn = Unicorn.new
# unicorn.speak
# ```

or

#     unicorn = Unicorn.new
#     unicorn.speak
  • Text blocks, for example to show program output, must be surrounded with triple backticks followed by the "text" keyword.
# ```text
# "I'm a unicorn"
# ```
  • To automatically link to other types, enclose them with single backticks.
# the `Legendary` module
  • To automatically link to methods of the currently documented type, use a hash like #horns or #index(char), and enclose it with single backticks.

  • To automatically link to methods in other types, do OtherType#method(arg1, arg2) or just OtherType#method, and enclose it with single backticks.

For example:

# Check the number of horns with `#horns`.
# See what a unicorn would say with `Unicorn#speak`.
  • To show the value of an expression inside code blocks, use #=>.
1 + 2             # => 3
Unicorn.new.speak # => "I'm a unicorn"
  • Use ditto to use the same comment as in the previous declaration.
# ditto
def number_of_horns
  horns
end
  • Use :nodoc: to hide public declarations from the generated documentation. Private and protected methods are always hidden.
class Unicorn
  # :nodoc:
  class Helper
  end
end

Flagging Classes, Modules, and Methods

Given a valid keyword, Crystal will automatically generate visual flags that help highlight problems, notes and/or possible issues.

The supported flag keywords are:

  • BUG
  • DEPRECATED
  • FIXME
  • NOTE
  • OPTIMIZE
  • TODO

Flag keywords must be the first word in their respective line and must be in all caps. An optional trailing colon is preferred for readability.

# Makes the unicorn speak to STDOUT
#
# NOTE: Although unicorns don't normally talk, this one is special
# TODO: Check if unicorn is asleep and raise exception if not able to speak
# TODO: Create another `speak` method that takes and prints a string
def speak
  puts "I'm a unicorn"
end

# Makes the unicorn talk to STDOUT
#
# DEPRECATED: Use `speak`
def talk
  puts "I'm a unicorn"
end

Use Crystal's code formatter

Crystal's built-in code formatter can be used not just to format your code, but also to format code samples included in documentation blocks.

This is done automatically when crystal tool format is invoked, which will automatically format all .cr files in current directory.

To format a single file:

$ crystal tool format file.cr

To format all .cr files within a directory:

$ crystal tool format src/

Use this tool to unify code styles and to submit documentation improvements to Crystal itself.

The formatter is also fast, so very little time is lost if you format the entire project instead of a single file.

A Complete Example

# A unicorn is a **legendary animal** (see the `Legendary` module) that has been
# described since antiquity as a beast with a large, spiraling horn projecting
# from its forehead.
#
# To create a unicorn:
#
# ```
# unicorn = Unicorn.new
# unicorn.speak
# ```
#
# The above produces:
#
# ```text
# "I'm a unicorn"
# ```
#
# Check the number of horns with `#horns`.
class Unicorn
  include Legendary

  # Creates a unicorn with the specified number of *horns*.
  def initialize(@horns = 1)
    raise "Not a unicorn" if @horns != 1
  end

  # Returns the number of horns this unicorn has
  #
  # ```
  # Unicorn.new.horns # => 1
  # ```
  def horns
    @horns
  end

  # ditto
  def number_of_horns
    horns
  end

  # Makes the unicorn speak to STDOUT
  def speak
    puts "I'm a unicorn"
  end

  # :nodoc:
  class Helper
  end
end

Generate Documentation

To generate documentation for a project, invoke crystal doc. This will create a doc directory, with a doc/index.html entry point. All files inside the root src directory will be considered.

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