This document details common workflow patterns when working with Dotty.

Cloning and building

# Start by cloning the repository:
git clone
cd dotty
# Clone dotty-compatible stdlib. Needed for running the test suite.
git clone -b dotty-library scala-scala

Compiling files with dotc

From sbt:

$ sbt
> dotc <OPTIONS> <FILE>

From terminal:

$ ./bin/dotc <OPTIONS> <FILE>

Here are some useful debugging <OPTIONS>:

  • -Xprint:PHASE1,PHASE2,... or -Xprint:all: prints the AST after each specified phase. Phase names can be found by searching compiler/src/dotty/tools/dotc/transform/ for phaseName.
  • -Ylog:PHASE1,PHASE2,... or -Ylog:all: enables ctx.log("") logging for the specified phase.
  • -Ycheck:all verifies the consistency of AST nodes between phases, in particular checks that types do not change. Some phases currently can't be Ychecked, therefore in the tests we run: -Ycheck:tailrec,resolveSuper,mixin,restoreScopes,labelDef.

Additional logging information can be obtained by changes some noPrinter to new Printer in compiler/src/dotty/tools/dotc/config/Printers.scala. This enables the subtyping.println("") and ctx.traceIndented("", subtyping) style logging.

Running tests

$ sbt
> partest --show-diff --verbose

Running single tests

To test a specific test tests/x/y.scala (for example tests/pos/t210.scala):

> filterTest .*pos/t210.scala

The filterTest task takes a regular expression as its argument. For example, you could run a negative and a positive test with:

> filterTest (.*pos/t697.scala)|(.*neg/i2101.scala)

or if they have the same name, the equivalent can be achieved with:

> filterTest .*/i2101.scala

Inspecting Trees with Type Stealer

There is no power mode for the REPL yet, but you can inspect types with the type stealer:

> repl
scala> import; import; import Contexts._,Types._

Now, you can define types and access their representation. For example:

scala> val s = stealType("class O { type X }", "O#X")
scala> implicit val ctx: Context = s._1
scala> val t = s._2(0)
t: = TypeRef(TypeRef(ThisType(TypeRef(NoPrefix,<empty>)),O),X)
scala> val u = t.asInstanceOf[TypeRef].underlying
u: = TypeBounds(TypeRef(ThisType(TypeRef(NoPrefix,scala)),Nothing), TypeRef(ThisType(TypeRef(NoPrefix,scala)),Any))


Many objects in the dotc compiler implement a Showable trait (e.g. Tree, Symbol, Type). These objects may be prettyprinted using the .show method