The parser AST describes the high-level syntactic structure of Enso, as well as containing robust and descriptive parser errors directly in the AST.


The parser AST needs to account for the following:

  • A single Name type, removing the distinction between different names found in the lexer. This should provide functions is_var, is_opr, and is_ref.
  • It should contain all of the language constructs that may appear in Enso’s source.
  • It should contain Invalid nodes, but these should be given a descriptive error as to why the construct is invalid.
  • It should also contain Ambiguous nodes, where a macro cannot be resolved in an unambiguous fashion.

Each node should contain:

  • An identifier, attributed to it from the ID map.
  • The start source position of the node, and the length (span) of the node.

The actionables for this section are:

  • Flesh out the design for the AST based on the requirements of the various parser phases.


The single source of truth for the AST is its Rust implementation. Therefore, in order to not get out of sync, the Scala AST implementation is generated during compilation directly from the Rust ast source file.

The command for generating the Scala ast and storing it in the file foo/ast.scala is following: cargo run -p ast -- --generate-scala-ast foo/ast.scala.

Since there isn’t 1:1 mapping between Rust and Scala, only a subset of Rust’s structures is supported. These are follows.

u32   | i32   | u16 | i16 | i8 => Int,
usize | isize | u64 | i64      => Long,
u8                             => Byte,
char                           => Char,
Vec                            => Vector,
Uuid                           => UUID,

Note: It is assumed, that Enso runs on 64bit platforms. Therefore, usize and isize are converted to Long.

Structures With Named Fields
struct Foo<X> { x: X<z::Y>, .. }

Is converted into:

case class Foo[X](x: X[Z.Y], ..)
Enums With Named Fields
enum Foo{ Bar{x:X}, Baz{y:Y, z:Z} }

Is converted into:

sealed trait Foo
case class Bar(x:X) extends Foo
case class Baz(y:Y, z:Z) extends Foo
Enums With One Unnamed Qualified Field
enum Enum { Foo(x::Foo), Bar(x::Bar), Baz(y::Baz) }

mod x {
    pub struct Foo { .. }
    pub struct Bar { .. }

mod y {
    pub struct Baz { .. }

Is converted into:

sealed trait Enum

object X {
    sealed trait X extends Enum
    case class Foo(..) extends X
    case class Bar(..) extends X

object Y {
    sealed trait Y extends Enum
    case class Baz(..) extends Y
mod foo { .. }

Is converted into:

object Foo { .. }

Furthermore, the content of is wrapped inside additional object Ast, in order to support top level type aliases.

Type Aliases
type A<X> = B<X,Y>;

Is converted into:

type A[X] = B[X, Y]

Note that in contrast to Rust, Scala doesn’t support types outside objects.