Strategic Scala Style: Designing Datatypes

When programming in Scala, there are two main ways of avoiding repetition: you can define functions to represent commonly-used procedures or computations, and you can define data-types, e.g. using class es or case class es, to represent commonly-used bundles of data that you tend to pass around or use together.

Lots of people have opinions about functions: they should be "pure", not too long, not be indented more than this much, etc. etc. etc.. Much less has been written about what a good data-type looks like, even though they play just as important a role in your Scala codebase. This post will explore some of the considerations and guidelines I follow when designing the class es and case class es that make up my Scala programs, and how you can apply them to your own Scala code.

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy the Author's book Hands-on Scala Programming

In Scala, as in Java, everything has to live in a class or object . However, there is a qualitative difference between the kind of class whose only purpose is a place to "put code", and the kind of class which represents a value that you instantiate and pass around your program. The latter is what I'm going to call a Datatype.

Examples of data-types include:

java.io.File

scala.Tuple2

scala.concurrent.Future

java.awt.Point2D

Versus classes which aren't data-types are things such as:

java.lang.System

scala.Predef

java.lang.Math

You instantiate data-types, pass them around, store them in values or fields and use them later. Just as defining functions lets you avoid copy-pasting the same logic all over your code, defining data-types lets you avoid copy-pasting the same groups of parameters or groups of values all over your code. Rather than passing around xPosition: Double and yPosition: Double everywhere, you can instead pass around a position: scala.Tuple2[Double, Double] or position: java.awt.Point2D .

Apart from the built in data-types, you will end up using data-types defined in libraries, and find yourself defining your own as your program grows. This post will explore some considerations and guidelines to remember when designing your own data-types, and help you sort through the over-abundance of ways that Scala lets you model your data:

This post is fourth in the Strategic Scala Style series, after Principle of Least Power, Conciseness & Names, and Practical Type Safety. Like the others, it focuses on Vanilla Scala, without any libraries or frameworks that would likely have their own conventions to follow. It's an intermediate-level post, and expects the audience to be familiar with Scala's language features, but doesn't need you to be familiar with Scala's fancy design patterns or fancy frameworks.

Many people who have been using Scala for a while might find much of this "obvious". Nevertheless, hopefully this post will still be useful in codifying this "obvious" knowledge for anyone who doesn't find it obvious, and form a basis for future discussion.

And on to the first consideration: should you make your data-type opaque or transparent?

Opaque or Transparent?

The first decision you have to make when designing a data type is: how opaque should the data-type be?

Should it hide/encapsulate it's internals?

Should it expose all its internals for external code to see and use?

The first case corresponds to "traditional" Java-style object oriented programming, while the second case corresponds to more a "functional" Scala style. Both cases have their uses in a Scala program.

For example, let's say we are trying to design a ParseError data-type to represent a syntax error when trying to parse some configuration file. Let's imagine we want the following things to be available:

The line and col the error occurred at

and the error occurred at Some kind of human readable message to explain what went wrong

We could imagine defining it as an opaque data-type, with a constructor, initialization logic, and the necessary methods exposed:

class ParseError(index: Int, input: String){ ... some computation ... def line: Int = ... def col: Int = ... def message: String = ... }

Here, we take an index and an input when constructing the ParseError , but neither of them are public. Rather, we only expose the line , col , and message as def s for people to use. I call this Opaque, because someone using ParseError has no visibility into how it actually works: they can only see the few def s it exposes. Whatever computation is happening is totally hidden within the body of the ParseError

Another way we could decide to do it is as a case class :

case class ParseError(line: Int, col: Int, message: String)

As a case class , all its constructor parameters line , col and message are automatically public. We do not do any computation in the body of ParseError ; computing the line and col and message from the index and the input will have to happen before the ParseError is created, outside of it. I call this Transparent, because someone using ParseError can from-the-outside can see all there is to know about it. There is no hidden data being stored, or hidden computation being performed. It is simply a bundle of two integers ( line and col , and a string msg .

In general, more internally-complex data-types with more internal invariants are better treated as opaque class es, while simpler data-types are better treated as transparent case class es. The following reasons illustrate why:

It is worth noting that Transparency and Opacity is a Spectrum: it's not an entirely binary decision to choose between either side, and you can choose a point on the transparency-opacity spectrum with the set of tradeoffs you want.

Opacity enforces Invariants

It's easier to enforce invariants with an opaque data-type: since construction of the computed def s from the "original" index / input data is done inside the constructor, there's no way for someone to construct a "bad" ParseError with nonsensical values for line , col or message . If message is always in some particular style, I can't accidentally make it return "I am Cow" instead. If line or col are always going to be greater than zero, I can be certain (if I trust the internals of ParseError itself) that they're never going to suddenly return negative numbers.

On the other hand, with a transparent data-type, I could easily pass in negative integers to line or col . I can pass in weird strings into message , and the transparent data-type will happily expose them.

Even in transparent data-types, some invariants can be enforced by the structure of your data-type. Other invariants can be validated using Self Checks. Nevertheless, in the case where the invariants are not easily enforced structurally, and adding lots and lots of self-checks is tedious, having the data-type simply be Opaque could be the right answer.

Opacity can save on Defensiveness

It is possible to enforce invariants in a transparent data-structure such as the case class above, but it requires additional steps. For example:

assert s to ensure that the message is not null

assert s to ensure that the line and col are not negative

(Possibly) assert s to ensure that the message obeys some format

In a transparent data-type, asserts are necessary if we want to enforce these things: who knows what random downstream code is going to try stuffing into our ParseError(line: Int, col: Int, message: String) constructor! However, in an opaque data-type, asserts are not as necessary: only the data-type itself is able to compute these values. So we can trust - without asserts - that it will do the right thing and no external code can interfere with it.

Transparency reduces Complexity

While encapsulating your logic within opaque data-types seems tempting, it can also be a curse: opaque data-types may be complex, but the complexity is hidden, so you often don't notice it until it causes subtle problems.

For example, from the outside we might not know whether the opaque ParseError holds on to a reference to the input: String after it is constructed. Given that the input to a parse is often far larger than any data included in the error message, that could cause a considerable memory leak if we unwittingly keep a number of ParseError s hanging around!

Another bit of uncertainty is how the line , col , and message are populated.

Are they pre-computed once during construction?

Are they re-computed every time you access them?

Are they lazy, meaning the first time you access them they may be slow to compute, but subsequent times become fast? Depending

How much memory they take and how long they take to compute?

These could be important questions! Certainly we could figure out the answer by digging through the code, but it may be non-trivial to do so.

With the transparent ParseError modeled as a case class , these questions are all answers from a glance at the signature. We can see at a glance that line , col and message are all eagerly pre-computed the first time. We can see at a glance that we don't keep any possibly-massive input: String hanging around using memory. While opaque data-types help you hide complexity and maintain invariants, transparent data-types help you remove complexity altogether. With a case class , there's data sitting in it's fields, and that's all there is to know about it.

Transparency and Opacity is a Spectrum

As with many things, Transparency and Opacity is not a binary choice. You can go half-way, by having part of your data-type's fields be "dumb" constructor arguments, while other "smart" fields are computed in the body of the class. You can even go beyond the amount of opacity in the above "opaque" example, e.g. by hiding the constructor as private and only letting people create instances through a factory method (e.g. apply on the companion object):

object ParseError{ def apply(...): ParseError = { ... some computation ... new ParseError(...) } } class ParseError private(index: Int, input: String){ ... some computation ... def line: Int = ... def col: Int = ... def message: String = ... }

In general, more internally-complex data-types with more internal invariants are better treated as opaque class es, while simpler data-types are better treated as transparent case class es, though there's no hard-and-fast rule. Next time you are picking a data-type, it's worth considering where on the transparency-opacity spectrum to place it!

Data-Types should be Total

When people say functions are Total, they usually mean that no matter what arguments you pass in to the function and satisfy the compiler, it will never give an "invalid" result at runtime: e.g. not blowing up with an exception. That's a handy property to have when reasoning about your code (e.g. "this code will never throw") and though hard to reach in reality, is still worth striving toward.

When I say a data-type is Total, I mean that no matter how the data-type is constructed, it cannot be "invalid": that is to say, I shouldn't be able to construct an instance of your class or case class that doesn't make any sense according to what you are going to use it for. For example, here are some example data-types which can possibly be invalid

case class URL(value: String) case class EmailAddress(value: String) /** * Represents a folder full of text files, without sub-folders, storing * the name of each file together with it's text content in a tuple */ case class FolderContents(value: Seq[(String, String))

While these may seem like plausible ways to model a URL , an EmailAddress , and a FolderContents , it is possible to construct invalid instances of them:

new URL("http:/www.google.com") is invalid; it needs a double // after the http

new EmailAddress("haoyi.com") as a email address, is invalid, as email addresses must contain an @ somewhere in them

new FolderContents(Seq("file.txt" -> "Hello", "file.txt" -> "World")) , is invalid: you cannot have two files of the same name

For example, if someone was trying to make a HTTP request to http:/www.google.com , it would fail as a malformed URL. And since we don't find out until the URL is used, it could easily be stored in our program for minutes or hours before blowing up later on when we're not paying attention, maybe past midnight on Saturday at 1am. Not great!

If they were total we would be sure that if we had a URL object, it would be "well-formed" and not blow up due to its own malformed-ness when you try to use it. Sure it could still fail due to run-time problems (Wifi down?) but at least it won't fail due to "internal" problems, or if it does fail it'll fail early while we're trying to construct it, so we can fix the bug early and go home worry-free.

Now that definitely sounds like a nice property to have, but how can we achieve it? It turns out there are two main techniques to make data-types Total:

Self Checks

A simple way of enforcing that the data-types are never invalid is to add assertions to their constructors to ensure that if someone tries to make an invalid instance that violates whatever rules we have in mind, we throw an exception:

case class URL(value: String){ assert(value.contains("//")) } case class EmailAddress(value: String){ assert(value.contains("@")) } /** * Represents a folder full of text files, without sub-folders, storing * the name of each file together with it's text content in a tuple */ class FolderContents(value: Vector[(String, String)){ assert(value.map(_._1).distinct == value.map(_._1)) }

Now if we try to instantiate an invalid instance, it fails before we get our hands on it:

@ new EmailAddress("haoyi.com") java.lang.AssertionError: assertion failed scala.Predef$.assert(Predef.scala:156)

And we can be sure that if someone passes us EmailAddress as a function argument, or we're reading it from some field on some object, that EmailAddress satisfies at least some basic properties we expect all email addresses to have: in this case it must contain an @ . If it didn't contain an @ , it would have thrown an exception during construction, and we wouldn't be able to get our hands on it. That's a nice property to have!

Implementing totality in this way has some benefits:

It's really easy: just assert the things you know must be true in the body of your class, and you're done

It requires minimal changes to existing code: everyone can construct EmailAddress s and use their .value as they always did, except now if you try to construct a bad one it'll blow up on you.

It also has some problems:

These self-checks can be expensive to perform each time! For example, the self-check on the FolderContents class creates three brand new Vector s and compares them before throwing them away.

They can be incomplete; there are likely other constraints on a URL apart from the fact that they contain a // , but this won't catch them.

As your checks get more complex, they become both slower and easier to get wrong.

Thus, it's often worth considering the other way we can ensure totality: Structural Enforcement.

Structural Enforcement

Structural enforcement of totality is when you make sure a data-type cannot contain invalid data purely by how it is defined. For example, you may define the above cases as

case class URL(protocol: String, host: String, path: String){ def value = protocol + "://" + host + "/" + path } case class EmailAddress(prefix: String, suffix: String) def value = prefix + "@" + suffix } /** * Represents a folder full of text files, without sub-folders */ case class FolderContents(value: Map[String, String])

In the case of URL and EmailAddress , instead of starting with a "raw" String and trying to assert components of it exist, we instead start off with the components, and then only convert it to a String on-demand.

In the case of FolderContents , we pick a better data-structure that fits what we know of the data: a Map can only have one value for each key, no matter what you try to do to it. Here we may not even need to provide an alternate def value , as the Map[String, String] is as easy to work with as Seq[(String, String)] was.

Note we may still want to put asserts in here: for example, we may assert that the prefix and suffix of an EmailAddress does not contain @ :

case class EmailAddress(prefix: String, suffix: String) assert(!prefix.contains('@')) assert(!suffix.contains('@')) def value = prefix + "@" + suffix }

This would give us additional checks that we did not have above: that there is exactly one @ in an email address. No more, no less. While we could have added the same check via Self Checks, doing so makes the asserts more complicated, and thus slower and more error-prone.

Structural Enforcement has many advantages over Self Checks:

If anyone wants to construct an instance of your data-type from the base components, they can skip the asserts and do so directly, which can save a lot of unnecessary computation.

The constraints of "what it contains" are usually much more clear to future maintainers when expressed as the fields of a class, rather than some ad-hoc assert s in the class body

Even when using Structural Enforcement, it could still be worthwhile having a "parse from unsafe input" method for creating these data-types. e.g. throwing exceptions if the input is invalid:

object EmailAddress{ def parse(input: String): EmailAddress = { input.split('@') match{ case Array(prefix, suffix) => new EmailAddress(prefix, suffix) case _ => throw new IllegalArgumentException("Invalid email address: " + input) } } case class EmailAddress(prefix: String, suffix: String) assert(!prefix.contains('@')) assert(!suffix.contains('@')) def value = prefix + "@" + suffix }

Or returning Option s

object EmailAddress{ def parse(input: String): Option[EmailAddress] = { input.split('@') match{ case Array(prefix, suffix) => Some(new EmailAddress(prefix, suffix)) case _ => None } } case class EmailAddress(prefix: String, suffix: String) assert(!prefix.contains('@')) assert(!suffix.contains('@')) def value = prefix + "@" + suffix }

After all, at the edges of your program you are definitely going to need to be interfacing with external APIs which would often use String s rather than your own special EmailAddress class.

Even though there's an escape hatch, having structural enforcement of totality is valuable. Often, using escape hatches like EmailAddress.parse are relegated to the edges of your program. You won't find yourself "accidentally" parsing email addresses from invalid strings at random spots in your program!. But if you pass around plain String s, or even wrapped class EmailAddress(value: String) as shown above, it's very easy to confuse an email-address String with any other String , or even accidentally use string methods like .substring without noticing it might leave you with an invalid EmailAddress .

Data-Types should be Normalized

Ideally, data-types should be Normalized: this means they should not allow multiple instances with different contents that are "equal". Just like how "invalid" is defined above, "equal" is subjective and depends on what you are trying to do. For example:

The absolute filesystem path /home/haoyi/./.ssh is equal to the path /home/haoyi/.ssh , which is equal to the path /home/haoyi/.ssh/config/..

The relative filesystem path ./.ssh/../.. is equal to the relatieve path ..

The Ansi-colored string "\u001b[31mHello\u001b[0mWorld" is equal to the Ansi-colored string "\u001b[31mHello\u001b[0m\u001b[0mWorld" when displayed at the terminal; the "Red" at the start ( \u001b[31m ) is reverted by the "Reset" in the middle ( \u001b[0m ) but it doesn't matter if there's two resets in a row or one; it still resets the color the same way and renders the same way.

If we did not care about Canonicity, and only about Totality, we might define the two filesystem paths as:

case class AbsPath(value: String){ assert(value(0) == '/') } case class RelPath(value: String){ assert(value(0) != '/') }

With the assert enforcing what we know of the two types (absolute paths start with / , relative paths don't). We can see this take effect if we try to create invalid relative or absolute paths:

@ AbsPath("foo/bar/..") java.lang.AssertionError: assertion failed @ RelPath("/foo/bar/..") java.lang.AssertionError: assertion failed

Although this is great for ensuring our data is valid, it does not help us ensure our data is normalized. For example, if we wanted to compare two paths, the default comparison wouldn't work:

@ AbsPath("/foo/bar/..") == AbsPath("/foo") res3: Boolean = false

What do we do about that?

Manual Normalization

One option is to provide a .normalize() or .canonicalize() method that people have to call to make the data-type into it's "normalized form" before doing comparisons and e.g. java.io.File provides it as a getCanonicalFile method, and we could write our own version for AbsPath :

case class AbsPath(value: String){ assert(value(0) == '/') def normalized() = { val parts = value.split('/') val output = collection.mutable.Buffer.empty[String] parts.foreach{ case "." => // do nothing case ".." => output.remove(output.length - 1) case segment => output.append(segment) } AbsPath(output.mkString("/")) } }

Something similar could be written for RelPath , but the idea is the same: you give the user an option to normalize it, if necessary.

This works, and if you remember to call .normalized() before comparing AbsPath s for equality, you get the right answer:

@ AbsPath("/foo/bar/..") == AbsPath("/foo") res11: Boolean = false @ AbsPath("/foo/bar/..").normalized() res12: AbsPath = AbsPath("/foo") @ AbsPath("/foo").normalized() res13: AbsPath = AbsPath("/foo") @ AbsPath("/foo/bar/..").normalized() == AbsPath("/foo").normalized() res14: Boolean = true

This has been the way things are in programming for decades: Java, Python, and countless other languages provide filesystem paths as basically strings with some kind of normalize method. However, it's not ideal: you can easily forget, and normalizing is expensive so you don't want to unnecessarily normalize things either! You're thus left in a tight spot where you have to normalize things exactly as much as you need them, else suffer bugs or wasted-performance.

We can do better, and that's via Automatic Normalization

Automatic Normalization

Automatic Normalization is a similar principle to Structural Enforcement, and is often used together: you enforce that a data-type is normalized not by trying to remember when to call the right method on it, but by ensuring that every time the data-type is created it is in a normalized state. This has several advantages over manually normalizing things:

It's easier to find things! If you see a normalized path printed in your logs, for example, you can be sure that you can grep for it to find any other times it's printed, rather than grepping for a dozen different variants of the same path. The same applies to data stored in your databases, caches, in the console output, etc.

You cannot forget to normalize something before comparing it with == , or doing other things that require normalization (putting it into a Map , Set , etc.), which would result in subtle bugs where how a path was constructed affects the behavior of your program, even for "equal" paths.

You cannot accidentally double-normalize things. If you get an unknown AbsPath or java.io.File from somewhere else, and you're not sure where (codebases can be big places...) it is often tempting to normalize it again "just in case". This can end up being wasteful if you do it over and over unnecessarily, and with Automatic Normalization that does not happen.

It's more efficient! Manipulating data-types when in their normalized form is almost always easier than manipulating them "normally". For example, comparing two paths to see if one is a sub-path of the other is much easier if both paths are already normalized. The cost of keeping already-normalized data normalized is usually not more expensive than normalizing things manually when-necessary, and is often actually cheaper.

For example, rather than defining filesystem paths as a case class with a normalize method:

case class AbsPath(value: String){ assert(value(0) == '/') def normalized() = { val parts = value.split('/') val output = collection.mutable.Buffer.empty[String] parts.foreach{ case "." => // do nothing case ".." => output.remove(output.length - 1) case segment => output.append(segment) } AbsPath(output.mkString("/")) } } case class RelPath(value: String){ assert(value(0) != '/') def normalized() = { ??? } }

We can define them as a case class whose parse method performs the normalization before creating the case class es:

object AbsPath{ def parse(input: String) = { val output = collection.mutable.Buffer.empty[String] input.drop(1).split('/').foreach{ case "." => // do nothing case ".." => output.remove(output.length - 1) case segment => output.append(segment) } AbsPath(output) } } case class AbsPath(segments: Seq[String]){ assert(!segments.contains("..") && !segments.contains(".")) def value = "/" + segments.mkString("/") } object RelPath{ def parse(input: String) = { var ups = 0 val output = collection.mutable.Buffer.empty[String] input.split('/').foreach{ case "." => // do nothing case ".." => if (output.nonEmpty) output.remove(output.length - 1) else ups += 1 case segment => output.append(segment) } RelPath(ups, output) } } case class RelPath(ups: Int, segments: Seq[String]){ assert(!segments.contains("..") && !segments.contains(".")) def value = (Array.fill(ups)("..") ++ segments).mkString("/") }

What did we do?

Move the body of the normalize method in each class into their respective parse functions

Changed the representation of both paths from a dumb String to a Seq[String] , representing the individual path segments

Enforced with asserts that the segments cannot be . or .. !

Made RelPath keep a count of ups: Int

The first three points should be relatively straighforward: after all, paths are just sequences of path segments, and in an absolute path the normalized version of a file never has . or .. segments:

@ new java.io.File("/foo/../bar/././baz/..").getCanonicalPath res24: String = "/bar"

The last point is a bit subtle: the normalized version of an absolute path can have .. s, but they can only be at the start of a path! For example, the relative path

foo/../../bar/../../././baz

Can be reduced step by step, removing the . s:

foo/../../bar/../../baz

And collapsing the .. s to the left:

../bar/../../baz

../../baz

Until you have all the .. s in the left-most segments, and the non- .. s in the segments on the right. Thus, in this normalized form, a relative path is simply a count of .. s on the left, and a Seq of non- .. segments on the right! Anyway, we've made the AbsPath.parse and RelPath.parse methods in the above defifition perform this normalization, and once everything is defined, it works, and both AbsPath s:

@ AbsPath.parse("/foo/bar/..") res19: AbsPath = AbsPath(ArrayBuffer("foo")) @ AbsPath.parse("/foo") res20: AbsPath = AbsPath(ArrayBuffer("foo")) @ AbsPath.parse("/foo/bar/..") == AbsPath.parse("/foo") res21: Boolean = true

And RelPath s:

@ RelPath.parse("../../baz") res38: RelPath = RelPath(2, ArrayBuffer("baz")) @ RelPath.parse("foo/../../bar/../../././baz") res39: RelPath = RelPath(2, ArrayBuffer("baz")) @ RelPath.parse("foo/../../bar/../../././baz") == RelPath.parse("../../baz") res40: Boolean = true

Are in an always-normalized state.

In a real data-type, there will be more operations that just .parse and == , and you will have to ensure that all those operations keep the data-type in its normalized form. Nevertheless, doing so is often relatively cheap (at runtime) and easy (to implement), so it's not a terrible cost.

You can see that in the automatically-normalized version, == equality works. We can be free to put things in Set s, Map s. You get all the good things mentioned at the beginning of this section, without having to worry about it forever more!

Data-types should have Instant Initialization

Data-types should ideally be totally initialized and ready to use as soon as the constructor or factory function returns. You should not construct "half baked" instances and hope people fill them in later. When I call a constructor:

val myFoo = new Foo(...)

myFoo should be complete and immediately ready to use. Two common cases where this is important are when:

Initializing Classes

In Scala, you should use constructor parameters to initialize your classes; if you need to given them defaults, etc., do so:

class MyAction(text: String, image: String, tooltip: String = "") val action = new MyAction("My Action Text", "Some Image")

Do not design your data-type so that you instantiate the class, in a half-baked un-usable state, and "later" come along and fill in all the missing fields. This is less common in Scala, but very common in Java libraries, e.g. the following Java definition of class MyAction :

public class MyAction { private String _text = ""; private String _tooltip = ""; private String _imageUrl = ""; public MyAction() { // nothing to do here. } public MyAction text(string value) { this._text = value; return this; } public MyAction tooltip(string value) { this._tooltip = value; return this; } public MyAction image(string value) { this._imageUrl = value; return this; } }

Lets you construct a MyAction via

val action = new MyAction() action.text("My Action Text") action.tooltip("My Action Tool tip") action.image("Some Image");

This has a problem: if you accidentally use action somewhere between new MyAction() and action.image , anywhere in the NOT READY area below:

// Not available... val action = new MyAction() // NOT READY action.text("My Action Text") // NOT READY action.tooltip("My Action Tool tip") // NOT READY action.image("Some Image"); // ...Ready

You will find yourself with a half-constructed MyAction with some fields mysteriously set to "" . Not good!

Even worse, is the fact that someone may be trying to use your MyAction class and forget one of the initialization steps altogether:

val action = new MyAction() action.text("My Action Text") action.tooltip("My Action Tool tip") // Oops

And find themselves wondering why, sometime much later, somewhere far away in their program, their image is becoming "" . Quite likely "much later" means 1am past Saturday midnight, and "far away" means in your employer's data-center where it needs to be fixed ASAP... by you.

These APIs also lend themselves to be called "fluently", e.g.

val action = new MyAction() .text("My Action Text") .tooltip("My Action Tool tip") .image("Some Image");

Which lets you chain the initialization calls one after another. This helps fix the NOT READY problem:

// Not available... val action = new MyAction() .text("My Action Text") .tooltip("My Action Tool tip") .image("Some Image"); // ...Ready

Where you can see it jumps immediately from Not available , where the compiler won't let you make use of the val action yet, and Ready , where it's fully initialized. At no point does it go through the NOT READY phase where the compiler will let you use it but it's not ready, and will either misbehave or throw an exception at runtime.

That's great, but it doesn't help in case someone forgets one of the initialization calls:

// Not available... val action = new MyAction() .text("My Action Text") .tooltip("My Action Tool tip") // Oops // NOT READY

There are ways to work around this by using a Builder Class and making the different stages of your fluent constructor a Builder and not a real MyAction . That stops you from accidentally using an incomplete action, but is a huge amount of boilerplate to write!

Luckily, in Scala, you do not need special fluent-builder-classes; you can simply use the constructor, possibly with defaults for some of the arguments (something you can't do in Java, which is why the fluent/Builder patterns emerged). In Scala, it is much simpler to define your class as:

class MyAction(text: String, image: String, tooltip: String = "")

Called via

val action = new MyAction("My Action Text", "Some Image")

Or

val action = new MyAction( text = "My Action Text", image = "Some Image" )

And not worry about anyone mis-using your half-complete class, since there's no point around the instantiation side where the action instance is available but not ready for use:

// Not available... val action = new MyAction( text = "My Action Text", image = "Some Image" ) // ...Ready

And the compiler helpfully tells you if you forget a required argument:

val action = new MyAction( text = "My Action Text", tooltip = "My Action Tool tip" ) // not enough arguments for constructor MyAction: (text: String, image: String, tooltip: String)ammonite.session.cmd18.MyAction. // Unspecified value parameter image. // val action = new MyAction( // ^ // Compilation Failed

In this case it's simple, but you should try to maintain this style even if your constructor is getting large and unwieldy. Perhaps your constructor has 11 different arguments:

class Interpreter(prompt0: Ref[String], frontEnd0: Ref[FrontEnd], width: => Int, height: => Int, colors0: Ref[Colors], printer: Printer, storage: Storage, history: => History, predef: String, wd: Path, replArgs: Seq[Bind[_]]){ ... }

Even so, it's better to just pass everything in and initialize the instance all-at-once rather than trying to do it piecemeal and risk someone will make use of your half-baked instance and end up fighting weird bugs.

Initializing Collections

In many existing libraries in the Java ecosystem, it is common to create something, spend time initializing it, and hope that nobody tries to use your thing until you're done initializing.

One common offender is the Java collections library:

val listA = new java.util.ArrayList[String](); listA.add("element 1"); listA.add("element 2"); listA.add("element 3");

Although ArrayList is a mutable collection, it often isn't used in a mutable way: people will create it empty, insert content to initialize it, and never change it again after that. However, if we look at the places where our listA is not ready to use, we can see that there is a lot of space where someone writing code can get their hands on a not-ready ArrayList :

// Not available... val listA = new java.util.ArrayList[String](); // NOT READY listA.add("element 1"); // NOT READY listA.add("element 2"); // NOT READY listA.add("element 3"); // ...Ready

While this is a small example and perhaps making a mistake is unlikely, in larger real-world codebases all these NOT READY spots are bugs waiting to happen when someone adds code to call some initialization method, which finds listA in it's NOT READY state and uses it anyway, resulting in odd bugs.

On the other hand, when you consider initializing a list in Scala:

// Not available... val listB = List[String]( "element 1", "element 2", "element 3" ) // ...Ready

You can see that it jumps immediately from Not available to Ready , without any intermediate states being exposed to the outside world. Obviously, inside the List(...) constructor it must be taking time assembling the list piece by piece, but to outside code, they cannot see any of that. Again, outside code can only see listB "instantly" jumping from Not available to Ready , and there is no chance to inadvertently use a partially-incomplete list in your other code.

While the Scala collections library is already written, remember this principle if you ever end up writing your own collection. If you ever find yourself writing some custom collection data-structure, make sure you can initialize "instantly" in one go.

Conclusion

In this post, we've covered the following guidelines:

These are just a small number of guidelines that I follow when deciding how to represent my data as class es or case class es in my Scala programs, together with the reasoning behind them. None of these guidelines are "novel", or of any academic interest, and in fact may well be "obvious" for people who have been writing Scala for a while. Nevertheless, hopefully they will be useful to beginning or intermediate Scala programmers trying to build a foundation for talking about Scala code and evaluating the plethora of solutions that the Scala programming language offers to someone designing their own data-types.

What are your favorite tips for how to design the data-types holding your Scala application together? Let us know in the comments below!

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy the Author's book Hands-on Scala Programming