Scaladoc

Scaladoc for Library Authors

Language

    Scaladoc is a documentation system that lives in the comments of Scala source code and is related to the code structure within which it is written. It is based on other comment based documentation systems like Javadoc, but with some extensions such as:

    • Markup may be used in the comments.
    • Extended @ tags (e.g. @tparam, @see, @note, @example, @usecase, @since, etc.)
    • Macro definitions (defined values to be substituted in scaladoc).
    • Automatic inheritance of comments from a super-class/trait (may be used effectively in combination with macro definitions).

    Where to put Scaladoc

    Scaladoc comments go before the items they pertain to in a special comment block that starts with a /** and ends with a */, like this:

    /** Start the comment here
  * and use the left star followed by a
  * white space on every line.
  *
  * Even on empty paragraph-break lines.
  *
  * Note that the * on each line is aligned
  * with the second * in /** so that the
  * left margin is on the same column on the
  * first line and on subsequent ones.
  *
  * The closing Scaladoc tag goes on its own,
  * separate line. E.g.
  *
  * Calculate the square of the given number
  *
  * @param d the Double to square
  * @return the result of squaring d
  */
 def square(d: Double): Double = d * d

    In the example above, this Scaladoc comment is associated with the method square since it is right before it in the source code.

    Scaladoc comments can go before fields, methods, classes, traits, objects and even (especially) package objects. Scaladoc comments for package objects make a great place to put an overview of a specific package or API.

    For class primary constructors which in Scala coincide with the definition of the class itself, a @constructor tag is used to target a comment to be put on the primary constructor documentation rather than the class overview.

    Tags

    Scaladoc uses @ tags to provide specific detail fields in the comments. These include:

    Class specific tags

    • @constructor placed in the class comment will describe the primary constructor.

    Method specific tags

    • @return detail the return value from a method (one per method).

    Method, Constructor and/or Class tags

    • @throws what exceptions (if any) the method or constructor may throw.
    • @param detail a value parameter for a method or constructor, provide one per parameter to the method/constructor.
    • @tparam detail a type parameter for a method, constructor or class. Provide one per type parameter.

    Usage tags

    • @see reference other sources of information like external document links or related entities in the documentation.
    • @note add a note for pre- or post-conditions, or any other notable restrictions or expectations.
    • @example for providing example code or related example documentation.
    • @usecase provide a simplified method definition for when the full method definition is too complex or noisy. An example is (in the collections API), providing documentation for methods that omit the implicit canBuildFrom.

    Member grouping tags

    These tags are well-suited to larger types or packages, with many members. They allow you to organize the Scaladoc page into distinct sections, with each one shown separately, in the order that you choose.

    These tags are not enabled by default! You must pass the -groups flag to Scaladoc in order to turn them on. Typically, the sbt for this will look something like:

    scalacOptions in (Compile, doc) ++= Seq(
  "-groups"
)

    Each section should have a single-word identifier that is used in all of these tags, shown as <group> below. By default, that identifier is shown as the title of that documentation section, but you can use @groupname to provide a longer title.

    Typically, you should put @groupprio (and optionally @groupname and @groupdesc) in the Scaladoc for the package/trait/class/object itself, describing what all the groups are, and their order. Then put @group in the Scaladoc for each member, saying which group it is in.

    Members that do not have a @group tag will be listed as “Ungrouped” in the resulting documentation.

    • @group <group> - mark the entity as a member of the <group> group.
    • @groupname <group> <name> - provide an optional name for the group. <name> is displayed as the group header before the group description.
    • @groupdesc <group> <description> - add optional descriptive text to display under the group name. Supports multiline formatted text.
    • @groupprio <group> <priority> - control the order of the group on the page. Defaults to 0. Ungrouped elements have an implicit priority of 1000. Use a value between 0 and 999 to set a relative position to other groups. Low values will appear before high values.

    Diagram tags

    • @contentDiagram - use with traits and classes to include a content hierarchy diagram showing included types. The diagram content can be fine-tuned with additional specifiers taken from hideNodes, hideOutgoingImplicits, hideSubclasses, hideEdges, hideIncomingImplicits, hideSuperclasses and hideInheritedNode. hideDiagram can be supplied to prevent a diagram from being created if it would be created by default. Packages and objects have content diagrams by default.
    • @inheritanceDiagram - TODO

    Other tags

    • @author provide author information for the following entity
    • @version the version of the system or API that this entity is a part of.
    • @since like @version but defines the system or API that this entity was first defined in.
    • @todo for documenting unimplemented features or unimplemented aspects of an entity.
    • @deprecated marks the entity as deprecated, providing both the replacement implementation that should be used and the version/date at which this entity was deprecated.
    • @inheritdoc take comments from a superclass as defaults if comments are not provided locally.
    • @documentable Expand a type alias and abstract type into a full template page. - TODO: Test the “abstract type” claim - no examples of this in the Scala code base

    Macros

    • @define <name> <definition> allows use of $name in other Scaladoc comments within the same source file which will be expanded to the contents of <definition>.

    2.12 tags - TODO: Move these into the above groups with a 2.12 note

    • @shortDescription ???
    • @hideImplicitConversion ???

    Comment Inheritance

    Implicit

    If a comment is not provided for an entity at the current inheritance level, but is supplied for the overridden entity at a higher level in the inheritance hierarchy, the comment from the super-class will be used.

    Likewise, if @param, @tparam, @return and other entity tags are omitted but available from a superclass, those comments will be used.

    Explicit

    For explicit comment inheritance, use the @inheritdoc tag.

    Markup

    It is still possible to embed HTML tags in Scaladoc (like with Javadoc), but not necessary most of the time as markup may be used instead.

    Some types of markup available:

    `monospace`
''italic text''
'''bold text'''
__underline__
^superscript^
,,subscript,,
[[entity link]], e.g. [[scala.collection.Seq]]
[[https://external.link External Link]],
  e.g. [[https://scala-lang.org Scala Language Site]]

    Other formatting notes

    • Paragraphs are started with one (or more) blank lines. * in the margin for the comment is valid (and should be included) but the line should be blank otherwise.
    • Code blocks are contained within {{{ this }}} and may be multi-line. Indentation is relative to the starting * for the comment.
    • Headings are defined with surrounding = characters, with more = denoting subheadings. E.g. =Heading=, ==Sub-Heading==, etc.
    • Tables are defined using | to separate elements in a row, as described in the blog.
    • List blocks are a sequence of list items with the same style and level, with no interruptions from other block styles. Unordered lists can be bulleted using -; numbered lists can be denoted using 1., i., I., or a. for the various numbering styles. In both cases, you must have extra space in front, and more space makes a sub-level.

    The markup for list blocks looks like:

    /** Here is an unordered list:
  *
  *   - First item
  *   - Second item
  *     - Sub-item to the second
  *     - Another sub-item
  *   - Third item
  *
  * Here is an ordered list:
  *
  *   1. First numbered item
  *   1. Second numbered item
  *     i. Sub-item to the second
  *     i. Another sub-item
  *   1. Third item
  */

    General Notes for Writing Scaladoc Comments

    • Concise is nice! Get to the point quickly, people have limited time to spend on your documentation, use it wisely.
    • Omit unnecessary words. Prefer returns X rather than this method returns X, and does X,Y & Z rather than this method does X, Y and Z.
    • DRY - don’t repeat yourself. Resist duplicating the method description in the @return tag and other forms of repetitive commenting.

    When two methods are indistinguishable from each other lexically, it can cause Scaladoc to report that there are ambiguous methods. As an example:

    import scala.collection.mutable.ListBuffer
class bar {
    def foo(x: Int): Boolean = ???
    def foo(x: ListBuffer[Int], y: String): Int = ???
}

    If one references foo via [[foo]], then the Scaladoc will complain and offer both alternatives. Fixing this means elaborating the signature enough so that it becomes unambiguous. There are a few things to be aware of in general:

    • You must not use a space in the description of the signature: this will cause Scaladoc to think the link has ended and move onto its description.
    • You must fully qualify any types you are using: assume that you have written your program without any import statements!

    Then, to disambiguate between objects and types, append $ to designate a term name and ! for a type name. Term names include members which are not types, such as val, def, and object definitions. For example:

    • [[scala.collection.immutable.List!.apply class List's apply method]] and
    • [[scala.collection.immutable.List$.apply object List's apply method]]

    When dealing with ambiguous overloads, however, it gets a bit more complex:

    • You must finish the signature, complete or otherwise, with a *, which serves as a wildcard that allows you to cut off the signature when it is umambiguous.
    • You must specify the names of the arguments and they must be exactly as written in the function definition:
      • [[bar.foo(Int)*]] is illegal (no name)
      • [[bar.foo(y:Int)*]] is illegal (wrong name)
      • [[bar.foo(x: Int)*]] is illegal (space! Scaladoc sees this as bar.foo(x:)
      • [[bar.foo(x:Int):Boolean]] is illegal (no *)
      • [[bar.foo(x:Int)*]] is legal and unambiguous
      • [[bar.foo(x:Int*]] is legal, the Int is enough to disambiguate so no closing paren needed
    • The enclosing scope (package/class/object etc) of the method must use ., but within the arguments and return type \. must be used instead to fully qualify types:
      • [[bar.foo(x:ListBuffer[Int],y:String)*]] is illegal (no qualification on ListBuffer)
      • [[bar.foo(x:scala.collection.mutable.ListBuffer[Int],y:String)*]] is illegal (non-escaped dots!)
      • [[bar\.foo(x:scala\.collection\.mutable\.ListBuffer[Int],y:String)*]] is illegal (must not escape dots in the prefix)
      • [[bar.foo(x:scala\.collection\.mutable\.ListBuffer[Int],y:String)*]] is legal
      • [[bar.foo(x:scala\.collection\.mutable\.ListBuffer[Int]*]] is legal, the first argument is enough to disambiguate.
    • When generics are involved, additional square brackets may be used to avoid the signature accidentally closing the link. Essentially, the number of leading left brackets determines the number of right brackets required to complete the link:
      • [[baz(x:List[List[A]])*]] is illegal (it is read as baz(x:List[List[A)
      • [[[baz(x:List[List[A]])*]]] is legal (the ]] is no longer a terminator, ]]] is)

    Known Limitations

    • # syntax does not seem to be supported for parameters and return types.
    • Spaces cannot be escaped with \ , so implicit parameters seem not to be supported either.

    More details on writing Scaladoc

    Further information on the formatting and style recommendations can be found in Scala-lang scaladoc style guide.

    Contributors to this page:

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