Feasibility study for transformer plugins

1 files changed, 102 insertions, 0 deletions

diff --git a/src/dotty/tools/dotc/core/PluggableTransformers.scala b/src/dotty/tools/dotc/core/PluggableTransformers.scala
new file mode 100644
index 000000000..b0b2ca134
--- /dev/null
+++ b/src/dotty/tools/dotc/core/PluggableTransformers.scala
@@ -0,0 +1,102 @@
+package dotty.tools.dotc
+package core
+
+
+object PluggableTransformers {
+
+  import Trees._, Contexts._
+
+  abstract class PluggableTransformer[T] extends TreeTransformer[T, Context] {
+    type PluginOp[-N <: Tree[T]] = N => Tree[T]
+
+    private[this] var _ctx: Context = _
+    private[this] var _oldTree: Tree[T] = _
+
+    protected implicit def ctx: Context = _ctx
+    protected def oldTree: Tree[T] = _oldTree
+    protected def thisTransformer: PluggableTransformer[T] = this
+
+    class PluginOps[-N <: Tree[T]](op: PluginOp[N], val next: Plugins) {
+      def apply(tree: N, old: Tree[T], c: Context): Tree[T] = {
+        val savedCtx = _ctx
+        val savedOld = _oldTree
+        try {
+          op(tree)
+        } finally {
+          _oldTree = savedOld
+          _ctx = savedCtx
+        }
+      }
+    }
+
+    val NoOp: PluginOp[Tree[T]] = identity
+    val NoOps = new PluginOps(NoOp, null)
+
+    class Plugins {
+      def next: Plugins = null
+
+      def processIdent: PluginOp[Ident[T]] = NoOp
+      def processSelect: PluginOp[Select[T]] = NoOp
+
+      val IdentOps: PluginOps[Ident[T]] = NoOps
+      val SelectOps: PluginOps[Select[T]] = NoOps
+    }
+
+    val EmptyPlugin = new Plugins
+
+    private[this] var _plugins: Plugins = EmptyPlugin
+
+    override def plugins: Plugins = _plugins
+
+    class Plugin extends Plugins {
+      override val next = _plugins
+      _plugins = this
+
+      private def push[N <: Tree[T]](op: PluginOp[N], ops: => PluginOps[N]): PluginOps[N] =
+        if (op == NoOp) ops else new PluginOps(op, next)
+
+      override val IdentOps: PluginOps[Ident[T]] = push(processIdent, next.IdentOps)
+      override val SelectOps: PluginOps[Select[T]] = push(processSelect, next.SelectOps)
+    }
+
+    def postIdent(tree: Ident[T], old: Tree[T], c: Context, ops: PluginOps[Ident[T]]) =
+      if (ops eq NoOps) tree
+      else finishIdent(ops(tree, old, c), old, c, ops.next)
+
+    override def finishIdent(tree: Tree[T], old: Tree[T], c: Context, plugins: Plugins): Tree[T] = tree match {
+      case tree: Ident[_] => postIdent(tree, old, c, plugins.IdentOps)
+      case _ => postProcess(tree, old, c, plugins)
+    }
+
+    def postSelect(tree: Select[T], old: Tree[T], c: Context, ops: PluginOps[Select[T]]) =
+      if (ops eq NoOps) tree
+      else finishSelect(ops(tree, old, c), old, c, ops.next)
+
+    override def finishSelect(tree: Tree[T], old: Tree[T], c: Context, plugins: Plugins): Tree[T] = tree match {
+      case tree: Select[_] => postSelect(tree, old, c, plugins.SelectOps)
+      case _ => postProcess(tree, old, c, plugins)
+    }
+
+    protected def postProcess(tree: Tree[T], old: Tree[T], c: Context, plugins: Plugins): Tree[T] = tree match {
+      case tree: Ident[_] => finishIdent(tree, old, c, plugins)
+      case tree: Select[_] => finishSelect(tree, old, c, plugins)
+    }
+  }
+}
+
+import PluggableTransformers._, Types._, TypedTrees._
+
+class ExampleTransformer extends PluggableTransformer[Type] {
+
+  class ExamplePlugin extends Plugin {
+    override def processIdent = {
+      case tree @ Ident(x) if x.isTypeName => tree.derivedSelect(tree, x)
+      case tree => tree
+    }
+    override def processSelect = { tree =>
+      if (tree.isType) tree.derivedIdent(tree.name)
+      else EmptyTree
+    }
+  }
+
+}
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