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https://issues.apache.org/jira/browse/GROOVY-12013?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18081343#comment-18081343
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ASF GitHub Bot commented on GROOVY-12013:
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paulk-asert opened a new pull request, #2536:
URL: https://github.com/apache/groovy/pull/2536
…o verify associative combiners in injectParallel/sumParallel/Stream.reduce
> New optional type checking extension: 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
> Assignee: Paul King
> Priority: Major
>
> 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 and non-goals
> * The checker runs only under {{@TypeChecked}}/{{@CompileStatic}} and treats
> the annotations/carrier types as asserted contracts (not proofs);
> sequential {{inject}} is deliberately never flagged.
> * The inline-closure check is syntactic and assumes operators carry their
> conventional meaning. Resolving overloaded operators is a non-goal: in
> particular, overloading a normally-associative operator (e.g. {{+}}, {{*}})
> to be non-associative is poor style and is not detected by design. The
> declaration paths ({{@Associative}}/{{@Reducer}}, Monoid/Semigroup) are the
> reliable, overloading-immune way to assert the contract.
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