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Guide to Scala 3 Compiler Contribution

How to Inspect Values


In this section, you will find out how to debug the contents of certain objects while the compiler is running, and inspect produced artifacts of the compiler.

Inspecting variables in-place

Frequently you will need to inspect the content of a particular variable. Often, it is sufficient to use println.

When printing a variable, it’s always a good idea to call show on that variable: println( Many objects of the compiler define show, returning a human-readable string. e.g. if called on a tree, the output will be the tree’s representation as source code, rather than the underlying raw data.

Sometimes you need to print flags. Flags are metadata attached to symbols containing information such as whether a class is abstract, comes from Java, what modifiers a variable has (private, protected etc) and so on. Flags are stored in a single Long value, each bit of which represents whether a particular flag is set.

To print flags, you can use the flagsString method, e.g. println(x.flagsString).

Pretty Printing with a String Interpolator

You can also pretty print objects with string interpolators, these default to call .show when possible, avoiding boilerplate and also helping format error messages.

Import them with the following:


Here is a table of explanations for their use:

Usage Description
i"" General purpose string formatting. It calls .show on objects
mixing in Showable, String.valueOf otherwise
em"" Formatting for error messages: Like i but suppress
follow-on, error messages after the first one if some
of their arguments are “non-sensical”.
ex"" Formatting with added explanations: Like em, but add
explanations to give more info about type variables
and to disambiguate where needed.

Obtaining debug output from the compiler

As explained in navigation, we can debug the code being generated as it is transformed through the compiler. As well as plain tree output, there are many compiler options that add extra debug information to trees when compiling a file; you can find the full list in ScalaSettings.

Stopping the compiler early

Sometimes you may want to stop the compiler after a certain phase, for example to prevent knock-on errors from occurring from a bug in an earlier phase. Use the flag -Ystop-after:<phase-name> to prevent any phases executing afterwards.

e.g. -Xprint:<phase> where phase is a miniphase, will print after the whole phase group is complete, which may be several miniphases after phase. Instead you can use -Ystop-after:<phase> -Xprint:<phase> to stop immediately after the miniphase and see the trees that you intended.

Printing TASTy of a Class

If you are working on an issue related to TASTy, it is good to know how to inspect the contents of a TASTy file, produced from compilation of Scala files.

The next example uses an issue directory to compile a class and print its TASTy. In the directory, you should create a file tasty/Foo.scala (with contents of class Foo), and create a file tasty/launch.iss with the following contents:

$ (rm -rv out || true) && mkdir out # clean up compiler output, create `out` dir.

scala3/scalac -d $here/out $here/Foo.scala

scala3/scalac -print-tasty $here/out/Foo.tasty

With sbt command issue tasty you will see output such as the following:

   0: ASTs
   1: <empty>
   2: Foo
   3: <init>

and so on.

Inspecting The Representation of Types

learn more about types in dotc.

If you are curious about the representation of a type, say [T] =>> List[T], you can use a helper program, it prints the internal representation of types, along with their class. It can be invoked from the sbt shell with three arguments as follows:

sbt:scala3> scala3-compiler/Test/runMain
  • The first argument, source, is an arbitrary string that introduces some Scala definitions. It may be the empty string "".
  • The second argument, kind, determines the format of the following arguments, accepting one of the following options:
    • rhs - accept return types of definitions
    • class - accept signatures for classes
    • method - accept signatures for methods
    • type - accept signatures for type definitions
    • The empty string "", in which case rhs will be assumed.
  • The remaining arguments are type signature strings, accepted in the format determined by kind, and collected into a sequence typeStrings. Signatures are the part of a definition that comes after its name, (or a simple type in the case of rhs) and may reference definitions introduced by the source argument.

Each one of typeStrings is then printed, displaying their internal structure, alongside their class.


Here, given a previously defined class Box { type X }, you can inspect the return type Box#X:

sbt:scala3> scala3-compiler/Test/runMain
> "class Box { type X }"
> "rhs"
> "Box#X"
[info] running (fork) "class Box { type X }" rhs Box#X
TypeRef(TypeRef(ThisType(TypeRef(NoPrefix,module class <empty>)),class Box),type X) [class$CachedTypeRef]

Here are some other examples you can try:

  • ...printTypes "" "class" "[T] extends Seq[T] {}"
  • ...printTypes "" "method" "(x: Int): x.type"
  • ...printTypes "" "type" "<: Int" "= [T] =>> List[T]"

Don’t just print: extracting further information is useful to to see the representation of a type at a glance, but sometimes you want to extract more. Instead, you can use the method With the same inputs as printTypes, it returns both a Context containing the definitions passed, along with the list of types.

As a worked example let’s create a test case to verify the structure of Box#X that you saw earlier:


import org.junit.Test

import, DottyTypeStealer.Kind

class StealBox:

  def stealBox: Unit =
    val (ictx, List(rhs)) =
      DottyTypeStealer.stealType("class Box { type X }", Kind.rhs, "Box#X")

    given Context = ictx

    rhs match
      case X @ TypeRef(Box @ TypeRef(ThisType(empty), _), _) =>
        assert( == "Box")
        assert( == "X")
        assert( == "<empty>")

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