This is a Scala compiler plugin to emulate the "typed holes" feature of Haskell, Idris, Agda, etc.

Whenever you use ??? in your code, the compiler plugin will generate a compiler warning containing useful information about it.

For example, given the Scala code

package example

object Example {

  def foo(x: Int, y: String): Boolean = {
    if (y.length == x) {
      ??? // TODO implement!
    } else {

  def bar(x: Int): String = x match {
    case 0 => "zero"
    case 1 => "one"
    case _ => ???


you'll get warnings that look something like this:

[warn] /Users/chris/code/scala-typed-holes/src/test/scala/example/Example.scala:7:7:
[warn] Found hole with type: Boolean
[warn] Relevant bindings include
[warn]   x: Int (bound at Example.scala:5:11)
[warn]   y: String (bound at Example.scala:5:19)
[warn]       ??? // TODO implement!
[warn]       ^
[warn] /Users/chris/code/scala-typed-holes/src/test/scala/example/Example.scala:16:15:
[warn] Found hole with type: String
[warn] Relevant bindings include
[warn]   x: Int (bound at Example.scala:13:11)
[warn]     case _ => ???
[warn]               ^

Named holes

The plugin also supports named holes. Instead of using ???, you can give custom names to your holes.

For example, code like this

def hello(args: Array[String]): Option[Result] = Foo.doStuff(args) match {
  case Left(error)  => __left
  case Right(x)     => __right

will result in warnings like this

Found hole 'left' with type: Option[Result]
Relevant bindings include
  args: Array[String] (bound at input.scala:11:13)
  error: String (bound at input.scala:12:15)

    case Left(error)  => __left

Named holes must start with a double underscore.

Warning: if you happen to use a naming convention that includes double underscores (which is pretty rare in Scala), this plugin will probably trash your code!

How to use

In sbt:

addCompilerPlugin("com.github.cb372" % "scala-typed-holes" % "0.1.11" cross CrossVersion.full)

The plugin is published for the following Scala versions:

  • 2.11.12
  • 2.12.15
  • 2.13.{0, 1, 2, 3, 4, 5, 6, 7, 8}

Changing the log level

By passing a compiler option -P:typed-holes:log-level:<level>, you can control the severity with which holes are logged.

  • info means holes will be logged as informational messages
  • warn means holes will be logged as compiler warnings
  • error means holes will be logged as compiler errors, so your program will fail to compile if it contains any holes.

The default behaviour is to log holes as warnings.

If you are using sbt, you can pass the option like this:

scalacOptions += "-P:typed-holes:log-level:info"