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):

> partest-only-no-bootstrap --show-diff --verbose tests/partest-generated/x/y.scala

Currently this will re-run some unit tests and do some preprocessing because of the way partest has been set up.

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