E099: Only Functions Can Be Followed By Underscore

This error is emitted when the eta-expansion syntax x _ is used on an expression that is not a method.

The syntax x _ for converting a method to a function value is deprecated and only works with methods. For non-method expressions, you need to explicitly write () => x to create a function value.

Example

scala

val x = 42
val f = x _

Error

scala

-- [E099] Syntax Error: example.scala:2:10 -------------------------------------
2 |val f = x _
  |        ^^^
  |Only function types can be followed by _ but the current expression has type Int
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | The syntax x _ is no longer supported if x is not a function.
  | To convert to a function value, you need to explicitly write () => x
   -----------------------------------------------------------------------------

Solution

scala

// Use explicit function syntax
val x = 42
val f = () => x

<aside class="warning"> This reference page was created with LLM assistance - the description of the error code may not be accurate or cover all possible scenarios. </aside>