Pretty-print LambdaParams

Customized printer instead of falling back to toString

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4039,7 +4039,7 @@ object Types {
   // ----- Type application: LambdaParam, AppliedType ---------------------
 
   /** The parameter of a type lambda */
-  case class LambdaParam(tl: TypeLambda, n: Int) extends ParamInfo {
+  case class LambdaParam(tl: TypeLambda, n: Int) extends ParamInfo, printing.Showable {
     type ThisName = TypeName
 
     def isTypeParam(using Context): Boolean = tl.paramNames.head.isTypeName
@@ -4084,6 +4084,8 @@ object Types {
           case _ =>
             myVariance = Invariant
       myVariance
+
+    def toText(printer: Printer): Text = printer.toText(this)
   }
 
   /** A type application `C[T_1, ..., T_n]` */

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -540,6 +540,12 @@ class PlainPrinter(_ctx: Context) extends Printer {
 
   def toText(annot: Annotation): Text = s"@${annot.symbol.name}" // for now
 
+  def toText(param: LambdaParam): Text =
+    varianceSign(param.paramVariance)
+    ~ toText(param.paramName)
+    ~ (if param.isTypeParam then "" else ": ")
+    ~ toText(param.paramInfo)
+
   protected def escapedString(str: String): String = str flatMap escapedChar
 
   def dclsText(syms: List[Symbol], sep: String): Text = Text(syms map dclText, sep)

compiler/src/dotty/tools/dotc/printing/Printer.scala

@@ -4,7 +4,8 @@ package printing
 
 import core._
 import Texts._, ast.Trees._
-import Types.{Type, SingletonType}, Symbols.Symbol, Scopes.Scope, Constants.Constant,
+import Types.{Type, SingletonType, LambdaParam},
+       Symbols.Symbol, Scopes.Scope, Constants.Constant,
        Names.Name, Denotations._, Annotations.Annotation
 import typer.Implicits.SearchResult
 import util.SourcePosition
@@ -130,6 +131,9 @@ abstract class Printer {
   /** Textual representation of type */
   def toText(tp: Type): Text
 
+  /** Textual representation of lambda param */
+  def toText(tree: LambdaParam): Text
+
   /** Textual representation of all symbols in given list,
    *  using `dclText` for displaying each.
    */