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Scala 3 Language Reference

Automatic Eta Expansion

Language

The conversion of methods into functions has been improved and happens automatically for methods with one or more parameters.

def m(x: Boolean, y: String)(z: Int): List[Int]
val f1 = m
val f2 = m(true, "abc")

This creates two function values:

f1: (Boolean, String) => Int => List[Int]
f2: Int => List[Int]

The syntax m _ is no longer needed and will be deprecated in the future.

Automatic eta-expansion and nullary methods

Automatic eta expansion does not apply to “nullary” methods that take an empty parameter list.

def next(): T

Given a simple reference to next does not auto-convert to a function. One has to write explicitly () => next() to achieve that. Once again since the _ is going to be deprecated it’s better to write it this way rather than next _.

The reason for excluding nullary methods from automatic eta expansion is that Scala implicitly inserts the () argument, which would conflict with eta expansion. Automatic () insertion is limited in Scala 3, but the fundamental ambiguity remains.

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