Tuesday, December 27, 2011

Using Scala API Documentation

One link that was missing from my post about Scala on the Web was the link to the Scala API documentation, often called "scaladoc" for short, after the tool that generates it. I tried to put it in, but it kind of broke the narrative and, to be honest, I have a lot to talk about the subject. So I decided to do a whole blog just about it.

First of all, there are two main links that you can use to access it:

Most of the time, I prefer the second link: it contains improvements to the Scaladoc tool, and the documentation often contains improvements. On the other hand, being a nightly release, it is subject to regresssions in the Scaladoc tool, and the documentation contains information that is not correct for the latest release. For that reason, I keep both links in my bookmarks.

So, what else is there to say? Well, if you are new to Scala, a lot. Those familiar with the Java equivalent probably find it both familiar and strange: the screen is divided into left and right sections, the right section having description of classes and packages, while the left contains a list of them. Whereas Java splits package and class lists, however, Scala does not. And, of course, the look is completely different. These, however, are skin-deep differences, and I want to get to the bones of the matter.

One Doc, Two Frames

The first thing I want you to notice are the two search boxes: one on the left, at the very top of the page, and one on the right, a bit lower. Scaladoc is search-oriented! That means you usually don't browse it, looking for stuff, but just type what you want. You can still browse, of course, which is useful when you don't know what you are looking for.

On the left side you have the package hierarchy and classes, traits and objects belonging to the packages. Note that classes, traits and objects can be members of other classes traits and objects, in which case they won't appear on the left.

The right side contains information about a selected package, trait, class or object. The URL will change to match what is being displayed on the right side of the screen, so that you can easily bookmark or share links to specific class or object. At the present, there's no way to further refer to a member of a class, such as a specific method. This improvement will be added at some point in the future.

Typically, you'll search or browse for a class on the left side of the screen, and then browse its contents on the left, or further search for a particular member of that class.

The Left Frame

Let's drill down the left side, then, to get a better understanding of what's there and how to use it. At the top of the left frame, you'll see this:

Topmost is the search box, with a little "x" icon on the right hand to clear the search. Searching for something will hide any traits, objects and classes that don't match, as well as any packages that don't have any matching members.

Right below that is an index of all existing methods. Click on a letter and you'll get all methods starting with that letter, and the places they are defined at. Click on the hash symbol (#), and you'll get a page with all methods starting with a symbol instead of a letter. For example:

Finally, you have a small caption saying "display packages only". Surprisingly, that's a clickable option. In fact, ScalaDoc is full of clickable parts, so if you are getting familiar with it, my advice is to try to click stuff, just to see what happens. Back to that caption, though, it switches the package hierarchy from displaying all entities to displaying packages only. Clicking it will give you something like this:

If you click on "show" on one of these packages, it will open up that particular package. If you click on "display all entities", it will revert to the initial mode of display.

Speaking of "show", let's now look at the various parts of the entities list:

The darker background indicates package or package objects, and the entities on the lighter background right below it are the classes, objects and traits belonging to that package, as indicated by the icons on their left. Note that, though package objects have other members such as methods, they do not appear on the left frame of Scaladoc.

The icons for "o", "c" and "t", in dark blue, green and light blue respectively, indicate objects, classes and traits. A name can be shared between an object and a trait or class, as seen for Regex above. Traits and classes cannot share the same name, however, so each name will have at most two icons beside it.

If you are not familiar with Scala, an object is a singleton, containing what, in Java, would be represented as static.

One thing to realize here is that everything in that image is clickable, except whitespace. You can click on "scala.util.matching" and "scala.util.parsing.combinator", on "hide" and "focus", on "Regex" and "RegexParsers", and on the icons.

Clicking on a package name will show its traits, classes and objects on the right frame, unless the package is a package object, in which case the right frame will show the other members it might have. This is important because many package objects contain implicit definitions used as helpers with that package. Check, for instance, scala.sys.process.

Clicking on "hide" will hide the entities belonging to that particular package, but not any subpackage that might exist. The text will then change to "show", which will revert this action upon being clicked.

Clicking on "focus" will hide all other packages from view, like this:

Clicking on the "x" icon that appears will revert this action.

Clicking on an icon for object, class or trait will show information for it on the right frame, as will clicking on the text itself. If an object shares a name with a trait or class, however, clicking on the text will show not the object, but the class or trait, following the assumption that this is what people want most of the time.

On recent ScalaDoc versions, clicking on an entity will also move the focus to the search box on the right frame, so that you can instantly type

The Right Frame

To begin the discussion of the right, I picked GenSeq, which is rich enough in ScalaDoc UI components, but (relatively) poor enough in actual content to fit in here.

You may have wondered why the search box is not at the top on the right frame, like it is on the left. The reason for it is our starting point in explaining the right frame.

The right frame is divided in three parts, the topmost two being shown above. The first part contains general information about the selected entity, and it comprises everything from the top until right before the search box. The second part starts with the search box, and is comprised of everything in that gray background. This part contains display options that affect the third part of the right frame. The third part contains all members of the selected entity, with individual information for each one.

From the top, then, we have a green or blue background (the former for traits and classes, the latter for objects, packages and package objects) on which the name of the entity is prominently displayed. A big icon beside it indicates what kind of entity it is: t for traits, c for classes, o for objects and p for packages and package objects alike.

If the icon is slightly folded on the bottom, like the one in the example, clicking on it (or the entity name) will switch the right frame to the companion of that entity. If you are not familiar with Scala, objects, traits and classes that share a name are said to be companions to each other. Clicking on the GenSeq trait above, then, will display the GenSeq object.

In a smaller font above the entity name is the full path to that entity. Clicking on a path component will display that component.

After that, in a gray background, comes a description of all classes, traits and type parameters used in the definition of that entity. It does not list classes or traits that are inherited -- that is available further below. Not to sound too repetitive, but clicking on any of the classes and traits will display it... Also, moving the mouse over one of these names will show the full path for that name.

What follows is the full description and attributes associated with that entity. I deliberately choose one extremely poor in those, so that I could concentrate on what ScalaDoc provides. One of those things is the link to the source code in which that entity is defined. Not all Scala projects provide that attribute, but Scala API itself does. The link currently points to the web interface to the old Subversion repository -- I presume this will be switched to the new repository on Github in the near future. At any rate, all niceties of source version control systems are available on that link. For the Scala API, anyway.

"Linear Supertypes" and "Known Subclasses" at the bottom of this part can be expanded upon click, to display the exact linearization of an entity's supertypes -- the inheritance precedence -- and all known subclasses. For example, for List it will show this:

Also shown above is the tool tip indicating the full path of one of the supertypes being pointed at, just like mentioned above for entity declaration.

Let's now look at the second part of the right frame:

The search box works pretty much like the one on the left frame, hiding anything that isn't a match, but it includes descriptions on the search as well. Search for a verb, therefore, will often yield good results, unless the verb is too general.

One particularly nice feature, available on recent versions, is that typing multiple words will search for matches of any one of the words, which makes it easy to display two methods close together on the screen.

All other options below the search box also change the way things are shown in the third part. The default Ordering mode, Alphabetic, will display all non-private members of an entity, separated in categories which we'll shown below, in alphabetical order. This is different than Java, which only shows full information for members defined or overridden on that class/interface.

Clicking "By inheritance" will change the display mode to separate the members according to where it was last defined/overridden. Full information will still be displayed, and the members will still be shown in alphabetical order in their own section.

The Inherited options let you easily filter out inherited methods. Clicking on "Hide All" will toggle off all supertypes, leaving only the entity itself selected. This will hide methods that are not abstract, defined or overridden on the entity. For GenSeq, for example only two methods will be displayed: seq and companion.

Note that clicking on "Show all" will not select AnyValAnyRef or Any. Because the methods defined on these are available on pretty much everything, one rarely needs to see these methods.

Because many times you might be interested in a particular aspect of an entity, you can also toggle each supertype individually. You can do so to display the methods on Any, for example, or you could look into what Function1 has to offer to List.

The last option, Visibility, will toggle between displaying only public members and everything except private members.

The last part of the right frame, as mentioned, contains all members of that entity. These are divided into type members and value members. A type member is a trait, a class or a type, and value members are everything else. For example, the object Regex contains this:

Note that anything that shows up as a type member of anything but a package will not be displayed on the left frame, even though you can display it on the right frame by clicking on a link to it.

Value members are actually divided in three separate sections: abstract value members, concrete value members and deprecated value members. These are shown in that precise order, so that one can easily see all members that must be implemented to make a concrete class, and don't get the screen polluted by methods they shouldn't be using -- deprecated methods -- when browsing.

Let's look at some important points of value member definitions. The snapshot below was taken from a List with members filtered by "map":

By default, only a member's definition and the first sentence of its description are shown, like method groupBy above. Clicking on either the small arrow on the left or on the definition itself will show the full information for that member, as seen in the two definitions for map.

Since Scala supports method overload, methods can have multiple definitions. In this particular case, however, the first definition is not a real one -- this is indicated by the [use case] tag. It is important to understand what use cases are, for two reasons: first, they represent the most common way to use the method, and, second, they are lies. Well-meaning lies, but lies.

Compare the definition of the two map methods shown. Clearly, the second definition has a lot going on, whereas the first definition is pretty clear: on a List[A] (see definition for List), the map method takes a function that converts an A into a B, and returns a List[B].

Though that definition works very fine for List, the map method is not defined on List, but much  higher on the hierarchy. And what works for List won't work for a BitSet, for example: since a BitSet is a Set[Int], if you map those Int into String, you won't be able to return a BitSet! After all, a BitSet  cannot be a Set[String]. The same thing happens in other cases: a WrappedString is a Seq[Char], a Map[A, B] is a Iterable[Tuple2[A, B]] (aka Iterable[(A, B)]), etc. In any of these cases, a map definition like in the use case won't work.

So the actual definition of map is the second one, which can handle all of these cases. If you need precise information about map -- for example, if you are extended Scala collections -- you can look it up. On the other hand, if you just want to know how a method is used, the use case should be much easier to understand.

Most of the remaining information is pretty obvious: a description follows, and then various attributes describing how parameters are supposed to be used, what the return type is, etc.

The final interesting thing here is the Definition Classes shown below each method. This only appears when a method has been inherited from elsewhere, and it indicates both where it was originally declared (which might have been as abstract), and all places where it was overridden, ie, the implementation has been changed.

And this concludes the tutorial on using the Scala API documentation, as well as the docs for any other library written in Scala. Looking back, it is much longer (and took much more time to write) than I first thought. Yet, rest assured that using ScalaDoc becomes second nature very fast!


  1. Haven't read the article yet, but right when I read "Manual for ScalaDoc" I thought a welcome page. What you think?

  2. Thanks for such a good article. This will definitely help new programmers in community.

  3. Good article highlighting important topic. However I'm bit surprised that no one has complained that one can not read Scaladoc with a ipad.

  4. Many thanks for this. I've been programming Scala for a year and had assumed that the method index feature in JavaDoc just didn't exist in scaladoc. Never would have guessed that the *method* index and the *type* search were completely unrelated to each other. Glossed right over those letters, assuming they were a quick way to filter *types* by first letter. The ScalaDoc team might get some good milage out of some strategically placed tool-tips.

  5. Thank You Daniel...

    It is a great article.

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  7. Thank You Daniel, This is what exactly i was looking for and You really helped many people like me. Thanks again!!

  8. Thanks for writing this Daniel. I too found there to be quite a lot of features that were not documented in the scaladoc interface and wrote a similar article just last week! Unfortunately I didn't know this article existed until just today! It looks like I am almost 4-years late to the party. My attempt is here and covers a few extra features: http://sanj.ink/posts/2016-10-20-how-to-navigate-scaladoc.html

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  10. nice blog for beginners.thank you.
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