Guide to Scala 3 Compiler Contribution

Compiler Types


Common Types and their Representation

Type representations in dotc derive from the class, defined in Types.scala. The toString method on Type will display types in a format corresponding to the backing data structure, e.g. ExprType(...) corresponds to class ExprType, defined in Types.scala.

You can inspect the representation of any type using the script, its usage and integration into your debugging workflow is described here.

Types of Definitions

The following table describes definitions in Scala 3, followed by the dotc representation of two types - a reference to the definition, and then its underlying type.

Note: in the following types, p refers to the self-type of the enclosing scope of the definition, or NoPrefix for local definitions and parameters.

Definition Reference Underlying Type
type Z >: A <: B TypeRef(p, Z) RealTypeBounds(A, B)
type Z = A TypeRef(p, Z) TypeAlias(A)
type F[T] = T match … TypeRef(p, F) MatchAlias([T] =>> T match …)
class C TypeRef(p, C) ClassInfo(p, C, …)
trait T TypeRef(p, T) ClassInfo(p, T, …)
object o TermRef(p, o) TypeRef(p, o$) where o$ is a class
def f(x: A): x.type TermRef(p, f) MethodType(x, A, TermParamRef(x))
def f[T <: A]: T TermRef(p, f) PolyType(T, <: A, TypeParamRef(T))
def f: A TermRef(p, f) ExprType(A)
(x: => A) TermRef(p, x) ExprType(A) where x is a parameter
val x: A TermRef(p, x) A

Types of Values

The following types may appear in part of the type of an expression:

Type Representation
x.y.type TermRef(x, y)
X#T TypeRef(X, T)
x.y.T and x.y.type#T TypeRef(TermRef(x, y), T)
this.type ThisType(C) where C is the enclosing class
"hello" ConstantType(Constant("hello"))
A & B AndType(A, B)
A | B OrType(A, B)
A @foo AnnotatedType(A, @foo)
[T <: A] =>> T HKTypeLambda(T, <: A, TypeParamRef(T))
x.C[A, B] AppliedType(x.C, List(A, B))
C { type A = T } RefinedType(C, A, T)
when T is not a member of C
C { type X = Y } RecType(RefinedType(C, X, z.Y))
when X and Y are members of C
and z is a RecThis over the enclosing RecType
super.x.type TermRef(SuperType(…), x)

Constructing Types

Method Definition Types

You can see above that method definitions can have an underlying type of either PolyType, MethodType, or ExprType. PolyType and MethodType may be mixed recursively however, and either can appear as the result type of the other.

Take this example as given:

def f[A, B <: Seq[A]](x: A, y: B): Unit

it can be constructed by the following code:


given Context =  // contains the definitions of the compiler

val f: Symbol =  // def f[A, B <: Seq[A]](x: A, y: B): Unit = PolyType(
  List("A".toTypeName, "B".toTypeName))(
  pt => List(
    TypeBounds(defn.NothingType, defn.AnyType),
    TypeBounds(defn.NothingType, AppliedType(defn.SeqType, List(pt.newParamRef(0))))
  pt => MethodType(
    List("x".toTermName, "y".toTermName))(
    mt => List(pt.newParamRef(0), pt.newParamRef(1)),
    mt => defn.UnitType

Note that pt.newParamRef(0) and pt.newParamRef(1) refers to the type parameters A and B respectively.

Proxy Types and Ground Types

Types in dotc are divided into two semantic kinds:

  • Ground Types (inheriting from either CachedGroundType or UncachedGroundType)
  • Proxy Types (inheriting from TypeProxy via either CachedProxyType or UncachedProxyType)

A Proxy Type is anything that can be considered to be an abstraction of another type, which can be accessed by the underlying method of the TypeProxy class. It’s dual, the Ground Type has no meaningful underlying type, typically it is the type of method and class definitions, but also union types and intersection types, along with utility types of the compiler.

Here’s a diagram, serving as the mental model of the most important and distinct types available after the typer phase, derived from [dotty/tools/dotc/core/Types.scala][1]:

Type -+- proxy_type --+- NamedType --------+- TypeRef
      |               |                     \
      |               +- SingletonType ----+- TermRef
      |               |                    +- ThisType
      |               |                    +- SuperType
      |               |                    +- ConstantType
      |               |                    +- TermParamRef
      |               |                    +- RecThis
      |               |                    +- SkolemType
      |               +- TypeParamRef
      |               +- RefinedOrRecType -+-- RefinedType
      |               |                   -+-- RecType
      |               +- AppliedType
      |               +- TypeBounds
      |               +- ExprType
      |               +- AnnotatedType
      |               +- TypeVar
      |               +- HKTypeLambda
      |               +- MatchType
      +- ground_type -+- AndType
                      +- OrType
                      +- MethodOrPoly -----+-- PolyType
                      |                    +-- MethodType
                      +- ClassInfo
                      +- NoType
                      +- NoPrefix
                      +- ErrorType
                      +- WildcardType

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