Differences between Scalac and Dotty

Overview explanation how symbols, named types and denotations hang together: Denotations1

Some background

Dotc is different from most other compilers in that it is centered around the idea of maintaining views of various artifacts associated with code. These views are indexed by tne.

A symbol refers to a definition in a source program. Traditionally, compilers store context-dependent data in a symbol table. The symbol then is the central reference to address context-dependent data. But for scalac's requirements it turns out that symbols are both too little and too much for this task.

Too little

The attributes of a symbol depend on the phase. Examples: Types are gradually simplified by several phases. Owners are changed in phases LambdaLift (when methods are lifted out to an enclosing class) and Flatten (when all classes are moved to top level). Names are changed when private members need to be accessed from outside their class (for instance from a nested class or a class implementing a trait). So a functional compiler, a Symbol by itself met mean much. Instead we are more interested in the attributes of a symbol at a given phase.

Too much

If a symbol is used to refer to a definition in another compilation unit, we get problems for incremental recompilation. The unit containing the symbol might be changed and recompiled, which might mean that the definition referred to by the symbol is deleted or changed. This leads to the problem of stale symbols that refer to definitions that no longer exist in this form. Scala 2 compiler tried to address this problem by rebinding symbols appearing in certain cross module references, but it turned out to be too difficult to do this reliably for all kinds of references. Scala 3 compiler attacks the problem at the root instead. The fundamental problem is that symbols are too specific to serve as a cross-module reference in a system with incremental compilation. They refer to a particular definition, but that definition may not persist unchanged after an edit.

scalac uses instead a different approach: A cross module reference is always type, either a TermRef or TypeRef. A reference type contains a prefix type and a name. The definition the type refers to is established dynamically based on these fields.

Denotation

Comment with a few details: Denotations2

A Denotation is the result of a name lookup during a given period

• Most properties of symbols are now in the denotation (name, type, owner, etc.)
• Denotations usually have a reference to the selected symbol
• Denotations may be overloaded (MultiDenotation). In this case the symbol may be NoSymbol (the two variants have symbols).
• Non-overloaded denotations have an info

Denotations of methods have a signature (Signature1), which uniquely identifies overloaded methods.

Denotation vs. SymDenotation

A SymDenotation is an extended denotation that has symbol-specific properties (that may change over phases)

• flags
• annotations
• info

SymDenotation implements lazy types (similar to scalac). The type completer assigns the denotation's info.

Implicit Conversion

There is an implicit conversion:

core.Symbols.toDenot(sym: Symbol)(implicit ctx: Context): SymDenotation

Because the class Symbol is defined in the object core.Symbols, the implicit conversion does not need to be imported, it is part of the implicit scope of the type Symbol (check the Scala spec). However, it can only be applied if an implicit Context is in scope.

Symbol

• Symbol instances have a SymDenotation
• Most symbol properties in the Scala 2 compiler are now in the denotation (in the Scala 3 compiler).

Most of the isFooBar properties in scalac don't exist anymore in dotc. Use flag tests instead, for example:

if (sym.isPackageClass)         // Scala 2
if (sym is Flags.PackageClass)  // Scala 3 (*)

(*) Symbols are implicitly converted to their denotation, see above. Each SymDenotation has flags that can be queried using the is method.

Flags

• Flags are instances of the value class FlagSet, which encapsulates a Long
• Each flag is either valid for types, terms, or both

000..0001000..01
        ^     ^^
        flag  | \
              |  valid for term
              valid for type

• Example: Module is valid for both module values and module classes, ModuleVal / ModuleClass for either of the two.
• flags.is(Method | Param): true if flags has either of the two

Tree

• Trees don't have symbols
  • tree.symbol is tree.denot.symbol
  • tree.denot is tree.tpe.denot where the tpe is a NamdedType (see next point)
• Subclasses of DenotingTree (Template, ValDef, DefDef, Select, Ident, etc.) have a NamedType, which has a denot field. The denotation has a symbol.
  • The denot of a NamedType (prefix + name) for the current period is obtained from the symbol that the type refers to. This symbol is searched using prefix.member(name).

Type

• MethodType(paramSyms, resultType) from scalac => mt @ MethodType(paramNames, paramTypes). Result type is mt.resultType