If you create a Scala 3 source code file named Hello.scala:
@main def hello = println("Hello, world")
and then compile that file with
$ scalac Hello.scala
you’ll notice that amongst other resulting files,
scalac creates files with the .tasty extension:
$ ls -1 Hello$package$.class Hello$package.class Hello$package.tasty Hello.scala hello.class hello.tasty
This leads to the natural question, “What is tasty?”
What is TASTy?
TASTy is an acronym that comes from the term, Typed Abstract Syntax Trees. It’s a high-level interchange format for Scala 3, and in this document we’ll refer to it as Tasty.
A first important thing to know is that Tasty files are generated by the
scalac compiler, and contain all of the information about your source code, including the syntactic structure of your program, and complete information about types, positions, and even documentation. Tasty files contain much more information than .class files, which are generated to run on the JVM. (More on this shortly.)
In Scala 3, the compilation process looks like this:
+-------------+ +-------------+ +-------------+ $ scalac | Hello.scala | -> | Hello.tasty | -> | Hello.class | +-------------+ +-------------+ +-------------+ ^ ^ ^ | | | Your source TASTy file Class file code for scalac for the JVM (contains (incomplete complete information) information)
The issue with .class files
Because of issues such as type erasure, .class files are actually an incomplete representation of your code.
A simple way to demonstrate this is with a
Type erasure means that when you write Scala code like this that’s supposed to run on the JVM:
val xs: List[Int] = List(1, 2, 3)
that code is compiled to a .class file that needs to be compatible with the JVM. As a result of that compatibility requirement, the code inside that class file — which you can see with a
javap command — ends up looking like this:
public scala.collection.immutable.List<java.lang.Object> xs();
javap command output shows a Java representation of what’s contained in the class file. Notice in this output that
xs is not defined as a
List[Int] any more; it’s essentially represented as a
List[java.lang.Object]. For your Scala code to work with the JVM, the
Int type has been erased.
Later, when you access an element of your
List[Int] in your Scala code, like this:
val x = xs(0)
the resulting class file will have a casting operation for this line of code, which you can think of being like this:
int x = (Int) xs.get(0) // Java-ish val x = xs.get(0).asInstanceOf[Int] // more Scala-like
Again, this is done for compatibility, so your Scala code can run on the JVM. However, the information that we already had a list of integers is lost in the class files. This poses problems when trying to compile a Scala program against an already compiled library. For this, we need more information than usually available in class files.
And this discussion only covers the topic of type erasure. There are similar issues for every other Scala construct that the JVM isn’t aware of, including constructs like unions, intersections, traits with parameters, and many more Scala 3 features.
TASTy to the Rescue
So, instead of having no information about the original types in .class files, or only the public API (as with the Scala 2.13 “Pickle” format), the TASTy format stores the complete abstract syntax tree (AST), after type checking. Storing the whole AST has a lot of advantages: it enables separate compilation, recompilation for a different JVM version, static analysis of programs, and many more.
So that’s the first takeaway from this section: The types you specify in your Scala code aren’t represented completely accurately in .class files.
A second key point is to understand that there are differences between the information that’s available at compile time and run time:
- At compile time, as
scalacreads and analyzes your code, it knows that
- When the compiler writes your code to a class file, it writes that
List[Object], and it adds casting information everywhere else
- Then at run time — with your code running inside the JVM — the JVM doesn’t know that your list is a
With Scala 3 and Tasty, here’s another important note about compile time:
- When you write Scala 3 code that uses other Scala 3 libraries,
scalacdoesn’t have to read their .class files any more; it can read their .tasty files, which, as mentioned, are an exact representation of your code. This is important to enable separate compilation and compatiblity between Scala 2.13 and Scala 3.
As you can imagine, having a complete representation of your code has many benefits:
- The compiler uses it to support separate compilation.
- The Scala Language Server Protocol-based language server uses it to support hyperlinking, command completion, documentation, and also for global operations such as find-references and renaming.
- Tasty makes an excellent foundation for a new generation of reflection-based macros.
- Optimizers and analyzers can use it for deep code analysis and advanced code generation.
In a related note, Scala 2.13.4 has a TASTy reader, and the Scala 3 compiler can also read the 2.13 “Pickle” format. The Compatibility Reference in the Scala 3 Migration Guide summarizes the benefits of this cross-compiling capability:
“You can have a Scala
2.13.4module that depends on a Scala
3.0.0-M1module, and the latter can even depend on another Scala
2.13.4module. >Cross-compatibility will not restrain you from using the exciting new features of Scala 3.0. In short, we have backward and forward compatibility and so migration can happen gradually and in any order.”
In summary, Tasty is a high-level interchange format for Scala 3, and .tasty files contain a complete representation of your source code, leading to the benefits outlined in the previous section.
For more details, see these resources:
- In this this video, Jamie Thompson of the Scala Center provides a thorough discussion of how Tasty works, and its benefits
- Binary Compatibility for library authors discusses binary compatibility, source compatibility, and the JVM execution model
- Forward Compatibility for the Scala 3 Transition demonstrates techniques for using Scala 2.13 and Scala 3 in the same project
These articles provide more information about Scala 3 macros: