Paul King created GROOVY-12013:
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Summary: Add CombinerChecker to verify associative combiners in
injectParallel/sumParallel/Stream.reduce
Key: GROOVY-12013
URL: https://issues.apache.org/jira/browse/GROOVY-12013
Project: Groovy
Issue Type: New Feature
Reporter: Paul King
h3. Summary
Adds two incubating annotations and an opt-in type-checking extension that
verify, at compile time, that the combining function passed to an
order-independent reduction is associative — the contract such reductions
silently require but cannot otherwise enforce.
h3. Annotations ({{groovy.transform}})
* {{@Associative}} — declares a two-argument combiner is associative:
{{combine(a, combine(b, c)) == combine(combine(a, b), c)}}.
* {{@Reducer}} — associative *and* has an identity element, named via the
{{zero()}} member (e.g. {{@Reducer(zero = '0')}}).
h3. CombinerChecker ({{groovy.typecheckers}})
A type-checking extension enabled with
{{@TypeChecked(extensions='groovy.typecheckers.CombinerChecker')}}.
It inspects the combiner of:
* Groovy's parallel collection reductions {{injectParallel}} and
{{sumParallel}};
* the JDK stream reductions {{Stream}}, {{IntStream}}, {{LongStream}} and
{{DoubleStream}} {{reduce}} (all overloads).
The combiner is accepted when it is:
* a method reference to an {{@Associative}}/{{@Reducer}} method; or
* a method reference to, or a thin delegating closure over, a {{Monoid}}
or {{Semigroup}} from any library (matched by simple type name —
Functional Java, Palatable Lambda, Purefun, etc. — configurable via the
{{monoids}}/{{semigroups}} options, no dependency added).
It reports a compile-time error when:
* an inline closure applies a non-associative operator ({{-}}, {{/}}, {{%}})
directly to its two arguments;
* the seed of a seeded reduction contradicts a {{@Reducer}}'s declared
identity;
* a {{Semigroup}} (which has no identity) is used with a seeded reduction
that requires one.
Two modes: the default (lenient) flags only high-confidence problems; *strict*
({{CombinerChecker(mode: 'strict')}}) additionally requires the combiner to
carry a declared, verifiable contract.
h3. Notes
* The checker runs only under {{@TypeChecked}}/{{@CompileStatic}} and treats
the annotations/carrier types as asserted contracts (not proofs);
sequential {{inject}} is deliberately never flagged.
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