Package groovy.transform

Annotation Interface Reducer

@Documented @Incubating @Target(METHOD) @Retention(RUNTIME) public @interface Reducer
Declares that a two-argument combining method is associative and has an identity element — together, the algebraic contract a seeded parallel reduction relies on. Equivalent to Associative plus a declared zero such that combine(zero, a) == a == combine(a, zero).

The zero() member names the identity element as a constant expression (for example "0", "''", or a fully-qualified static field reference). When supplied, tooling such as groovy.typecheckers.CombinerChecker can additionally check that the seed passed to injectParallel matches the declared identity where both are statically determinable. An empty value asserts associativity and the existence of an identity without naming it.

As with Associative, this annotation asserts the laws; it does not prove them. The assertion is intended to be backed by tests.

Since:
6.0.0

  • Optional Element Summary

    Optional Elements
    Modifier and Type
    Optional Element
    Description
    String
    zero
    The identity element, as a constant expression.

  • Element Details

    • zero

      String zero
      The identity element, as a constant expression. Empty means the identity exists but is not named (associativity is still asserted).
      Default:
      ""