How Does "20 Seconds" Work in Scala?
This article will show you how a seemingly magical expressions work, and how you can build your own.
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This short article will show you how apparently magical constructs like
20.seconds works in Scala, although the Int type doesn't have such methods natively.
This article will make more sense if you know the basics of implicits, but then again, if you do know how implicits work, there's only one step to understanding how these seemingly magical methods work, so I'll cover everything you need to know.
You can read this article over at
Rock the JVM or watch it on
YouTube or in the video below:
1. The "Problem"
The question we're addressing here is the following: the Int type has a very small set of methods and certainly the
seconds method isn't one of them:
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val womp = 20.seconds // compile error: symbol "seconds" not found
However, once we add a special import, it magically works:
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import scala.concurrent.duration._
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val aDuration = 20.seconds // works!
So how does the magical import work?
2. Enriching Types
The answer is not in the import itself, but in
what's imported — the types and values that are imported might as well be in scope, and methods like
.seconds would work just as fine. It's their structure that provides the magic. To understand how they work, we need to go back to implicits.
I'm not going to talk about all the functionality that the
implicit keyword does in Scala — we'll probably do that in another article — but we are going to focus on one kind of implicits:
implicit classes. Implicit classes are one-argument wrappers, i.e. a class with one constructor argument, with regular methods, fields, etc, except that they have the
implicit keyword in their declaration:
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implicit class MyRichString(string: String) {
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def fullStop: String = string + "."
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}
If we removed the
implicit keyword there, this would be a pretty uninteresting class. Adding
implicit will add some special powers. We can either say
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new MyRichString("This is a sentence").fullStop
or this:
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"This is a sentence".fullStop
This works although the
fullStop method doesn't exist for the String class. Normally, the code would not compile, but the compiler will add an extra step of searching for any implicit wrapping or conversion of a String value that might have the
fullStop method, which in our case it does. So in reality, the compiler will rewrite our last call as
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// "This is a sentence".fullStop beecomes:
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new MyRichString("This is a sentence").fullStop
which is what we (explicitly) wrote earlier. This pattern provides what we call extension methods - libraries like Cats use this all the time.
3. Importing
If an implicit class like this is not written in the scope where we use the "magical" method, the code will not compile until we bring that implicit class into scope. This means an import. Usually, libraries (including the standard library) packs implicits into "Ops"-like objects:
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package mylibrary
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object MyStringOps {
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implicit class MyRichString(string: String) {
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def fullStop: String = string + "."
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}
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}
and then later in our code, when we import it, we'll also have access to the extension method:
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import mylibrary._
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"Starting to get it".fullStop
The Scala
duration package works in the same way: when you import
scala.concurrent.duration._ you gain access to extension methods on the Int type that returns instances of
Duration:
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import scala.concurrent.duration._
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20.seconds
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500.millis
4. A Bit More and Conclusion
Some packages are automatically imported with every Scala code. Some of these packages might include extension methods like the
.seconds one. Ever wondered how things like
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val range = 1 to 100
work? With the tools you now have, it's quite easy to understand that there is an implicit conversion that enriches the Int type:
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// some implicit conversion here { ...
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def to(end: Int): Range
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// ... }
You've now learned how extension methods work and how magical things like
20.seconds work in Scala, making them seem like they're part of the language or standard library. This skill will prove useful as you become more experienced with Scala libraries like Cats.
Scala (programming language)
Published at DZone with permission of Daniel Ciocirlan. See the original article here.
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