From 0b0338721ba7c040204dcb44cfb4bc750acbcc5c Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 18 Dec 2012 15:13:34 +0100
Subject: First rough draft of trees. They use copy on write semantics for the
 tpe attribute, following Paul's suggestion.

---
 src/dotty/tools/dotc/core/Trees.scala | 101 ++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)
 create mode 100644 src/dotty/tools/dotc/core/Trees.scala

(limited to 'src/dotty/tools/dotc')

diff --git a/src/dotty/tools/dotc/core/Trees.scala b/src/dotty/tools/dotc/core/Trees.scala
new file mode 100644
index 000000000..a909e37bc
--- /dev/null
+++ b/src/dotty/tools/dotc/core/Trees.scala
@@ -0,0 +1,101 @@
+package dotty.tools.dotc.core
+
+import Types._, Names._, Flags._, Positions._
+
+object Trees {
+
+  abstract class Modifiers {
+    val flags: FlagSet
+  }
+
+  class MissingType {
+    type Type
+  }
+
+  val missing: MissingType =
+    new MissingType {
+      type Type = Types.Type
+    }
+
+  /** Trees take a parameter indicating what the type of their `tpe` field
+   *  is. Two choices: `Types.Type` or `missing.Type`.
+   *  Untyped trees have type `Tree[missing.Type]`. Because `missing.Type`
+   *  is a completely abstract type, there's nothing one can do with it.
+   *
+   *  Tree typing uses a copy-on-write implementation:
+   *
+   *   - You can never observe a `tpe` which is `null` (throws an exception)
+   *   - So when creating a typed tree with `withType` we can re-use
+   *     the existing tree transparently, assigning its `tpe` field,
+   *     provided it was `null` before.
+   *   - It is impossible to embed untyped trees in typed ones.
+   *   - It is possible to embed typed trees in untyped ones. In fact
+   *     there is an implicit conversion from `Tree[Types.Type]` to
+   *     `Tree[missing.Type]` which wraps the typed tree in a
+   *     `TypedSplice` node.
+   *   - Type checking an untyped tree will remove all embedded `TypedSplice`
+   *     nodes.
+   */
+  abstract class Tree[T] {
+    def pos: Position
+
+    private var _tpe: T = _
+
+    def tpe: T = {
+      if (_tpe == null) throw new UnAssignedTypeException(this)
+      _tpe
+    }
+
+    def withType(tpe: Type): Tree[Type] = {
+      val tree =
+        (if (_tpe == null ||
+            (_tpe.asInstanceOf[AnyRef] eq tpe.asInstanceOf[AnyRef])) this
+         else shallowCopy).asInstanceOf[Tree[Type]]
+      tree._tpe = tpe
+      tree
+    }
+
+    def shallowCopy: Tree[T]
+
+  }
+
+  case class Ident[T](name: Name)(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class Select[T](qualifier: Tree[T], name: Name)(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class Apply[T](fun: Tree[T], arg: Tree[T])(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class Pair[T](left: Tree[T], right: Tree[T])(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class ValDef[T](mods: Modifiers, name: Name, rtpe: Tree[T], rhs: Tree[T])(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class TypeDef[T](mods: Modifiers, name: Name, rhs: Tree[T])(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class DefDef[T](mods: Modifiers, name: Name, tparams: List[TypeDef[T]], vparamss: List[List[ValDef[T]]], rtpe: Tree[T], rhs: Tree[T])(implicit val pos: Position) extends Tree[T] {
+    def shallowCopy = copy()
+  }
+
+  case class TypedSplice(tree: Tree[Type]) extends Tree[missing.Type] {
+    def pos = tree.pos
+    def shallowCopy = copy()
+  }
+
+  implicit def embedTyped(tree: Tree[Type]): Tree[missing.Type] = TypedSplice(tree)
+
+  class UnAssignedTypeException[T](tree: Tree[T]) extends Exception {
+    override def getMessage: String = s"type of $tree is not assigned"
+  }
+
+}
\ No newline at end of file
-- 
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