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Dotty Internals 1: Trees & Symbols (Meeting Notes)

These are meeting notes for the Dotty Internals 1: Trees & Symbols talk by Dmitry Petrashko on Mar 21, 2017.

Entry point


The entry point to the compiler contains the list of phases and their order.


Some phases executed independently, but others (miniphases) are grouped for efficiency. See the paper "Miniphases: Compilation using Modular and Efficient Tree Transformation" for details.



Trees represent code written by the user (e.g. methods, classes, expressions). There are two kinds of trees: untyped and typed.

Unlike other compilers (but like scalac), dotty doesn't use multiple intermediate representations (IRs) during the compilation pipeline. Instead, it uses trees for all phases.

Dotty trees are immutable, so they can be shared.

Untyped trees


These are the trees as output by the parser.

Some trees only exist as untyped: e.g. WhileDo and ForDo. These are desugared by the typechecker.

Typed trees


Typed trees contain not only the user-written code, but also semantic information (the types) about the code.

Notes on some tree types

  • RefTree: trees that refer to something. There are multiple subtypes
    • Ident: by-name reference
    • Select: select (e.g. a field) from another tree (e.g. a.foo is represented as Select(Ident(a), foo))
  • This: the this pointer
  • Apply: function application: e.g. a.foo(1, 2)(3, 4) becomes Apply(Apply(Select(Ident(a), foo), List(1, 2)), List(3, 4))
  • TypeApply: type application: def foo[T](a: T) = ??? foo[Int](1) becomes Apply(TypeApply(Ident(foo), List(Int)), List(1))
  • Literal: constants (e.g. integer constant 1)
  • Typed: type ascription (e.g. for widening, as in (1: Any))
  • NamedArg: named arguments (can appear out-of-order in untyped trees, but will appear in-order in typed ones)
  • Assign: assignment. The node has a lhs and a rhs, but the lhs can be arbitrarily complicated (e.g. (new C).f = 0).
  • If: the condition in an if-expression can be arbitrarily complex (e.g. it can contain class definitions)
  • Closure: the free variables are stored in the env field, but are only accessible "around" the LambdaLift phase.
  • Match and CaseDef: pattern-matching trees. The pat field in CaseDef (the pattern) is, in turn, populated with a subset of trees like Bind and Unapply.
  • Return: return from a method. If the from field is empty, then we return from the closest enclosing method. The expr field should have a types that matches the return type of the method, but the Return node itself has type bottom.
  • TypeTree: tree representing a type (e.g. for TypeApply).
  • AndType, OrType, etc.: these are other trees that represent types that can be written by the user. These are a strict subset of all types, since some types cannot be written by the user.
  • ValDef: defines fields or local variables. To differentiate between the two cases, we can look at the denotation. The preRhs field is lazy because sometimes we want to "load" a definition without know what's on the rhs (for example, to look up its type).
  • DefDef: method definition.
  • TypeDef: type definition. Both type A = ??? and class A {} are represented with a TypeDef. To differentiate between the two, look at the type of the node (better), or in the case of classes there should be a Template node in the rhs.
  • Template: describes the "body" of a class, including inheritance information and constructor. The constr field will be populated only after the Constructors phase; before that the constructor lives in the preBody field.
  • Thicket: allows us to return multiple trees when a single one is expected. This kind of tree is not user-visible. For example, transformDefDef in LabelDefs takes in a DefDef and needs to be able to sometimes break up the method into multiple methods, which are then returned as a single tree (via a Thicket). If we return a thicket in a location where multiple trees are expected, the compiler will flatten them, but if only one tree is expected (for example, in the constructor field of a class), then the compiler will throw.


Tree classes have a ThisTree type field which is used to implement functionality that's common for all trees while returning a specific tree type. See withType in the Tree base class, for an example.

Additionally, both Tree and ThisTree are polymorphic so they can represent both untyped and typed trees.

For example, withType has signature def withType(tpe: Type)(implicit ctx: Context): ThisTree[Type]. This means that withType can return the most-specific tree type for the current tree, while at the same time guaranteeing that the returned tree will be typed.

Creating trees

You should use the creation methods in untpd.scala and tpd.scala to instantiate tree objects (as opposed to creating them directly using the case classes in Trees.scala).

Meaning of trees

In general, the best way to know what a tree represents is to look at its type or denotation; pattern matching on the structure of a tree is error-prone.



Sometimes there's an error during compilation, but we want to continue compiling (as opposed to failing outright), to uncover additional errors.

In cases where a tree is expected but there's an error, we can use the errorTree methods in ErrorReporting to create placeholder trees that explicitly mark the presence of errors.

Similarly, there exist ErrorType and ErrorSymbol classes.


The closest in Dotty to what a programming language like C calls an "l-value" is a RefTree (so an Ident or a Select). However, keep in mind that arbitrarily complex expressions can appear in the lhs of an assignment: e.g.

trait T {
  var s = 0
  class T2 extends T
  while (true) 1
  new Bla
}.s = 10

Another caveat, before typechecking there can be some trees where the lhs isn't a RefTree: e.g. (a, b) = (3, 4).



Symbols are references to definitions (e.g. of variables, fields, classes). Symbols can be used to refer to definitions for which we don't have ASTs (for example, from the Java standard library).

NoSymbol is used to indicate the lack of a symbol.

Symbols uniquely identify definitions, but they don't say what the definitions mean. To understand the meaning of a symbol we need to look at its denotation (spefically for symbols, a SymDenotation).

Symbols can not only represent terms, but also types (hence the isTerm/isType methods in the Symbol class).


ClassSymbol represents either a class, or an trait, or an object. For example, an object

object O {
  val s = 1

is represented (after Typer) as

class O$ { this: O.type =>
  val s = 1
val O = new O$

where we have a type symbol for class O$ and a term symbol for val O. Notice the use of the selftype O.type to indicate that this has a singleton type.



Symbols contain SymDenotations. The denotation, in turn, refers to:

  • the source symbol (so the linkage is cyclic)
  • the "owner" of the symbol:
    • if the symbol is a variable, the owner is the enclosing method
    • if it's a field, the owner is the enclosing class
    • if it's a class, then the owner is the enclosing class
  • a set of flags that contain semantic information about the definition (e.g. whether it's a trait or mutable). Flags are defined in Flags.scala.
  • the type of the definition (through the info method)