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https://issues.apache.org/jira/browse/GROOVY-12486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18131291#comment-18131291
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ASF GitHub Bot commented on GROOVY-12486:
-----------------------------------------
testlens-app[bot] commented on PR #3020:
URL: https://github.com/apache/groovy/pull/3020#issuecomment-6379629541
## ✅ All tests passed ✅
🏷️ Commit: e40295c190b6bae0f5bffbf18e82dd57e875d51d
▶️ Tests: 123068 executed
⚪️ Checks: 32/32 completed
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> MonadicChecker types the DO generator variable from the carrier's first type
> argument
> -------------------------------------------------------------------------------------
>
> Key: GROOVY-12486
> URL: https://issues.apache.org/jira/browse/GROOVY-12486
> Project: Groovy
> Issue Type: Bug
> Components: groovy-typecheckers
> Reporter: Paul King
> Priority: Major
>
> h2. Summary
> Under {{MonadicChecker}}, the variable bound by a {{DO}} generator is typed
> as the carrier's *first* type argument. For carriers with more than one type
> parameter that bind on a later one, the variable gets the wrong type.
> Examples are an {{Either<L, R>}} or {{Validation<E, A>}} that is
> right-biased. The result is a spurious type error, or code that compiles only
> by accident.
> This affects carriers already on the 6.0.0 allow-list: Vavr {{Either}} and
> {{Validation}}, and Functional Java {{Validation}}. It also affects
> structural and {{@Monadic}} carriers of the same shape.
> h2. Reproducer
> {code:groovy}
> import groovy.transform.TypeChecked
> import java.util.function.Function
> import static org.apache.groovy.macrolib.MacroLibGroovyMethods.DO
> class Result<E, A> {
> final A value
> Result(A value) { this.value = value }
> static <E, A> Result<E, A> ok(A value) { new Result<E, A>(value) }
> def <B> Result<E, B> flatMap(Function<? super A, ? extends Result<E, B>>
> f) { f.apply(value) }
> def <B> Result<E, B> map(Function<? super A, ? extends B> f) {
> ok(f.apply(value)) }
> String toString() { "Ok($value)" }
> }
> @TypeChecked(extensions = 'groovy.typecheckers.MonadicChecker')
> def f() {
> DO(x in Result.<String, Integer>ok(1)) {
> int n = x
> Result.<String, Integer>ok(n + 1)
> }
> }
> println f()
> {code}
> h2. Actual (6.0.0)
> {noformat}
> [Static type checking] - Cannot assign value of type java.lang.String to
> variable of type int
> {noformat}
> {{x}} is typed as {{E}} ({{String}}). It should be {{A}} ({{Integer}}), the
> parameter type of {{flatMap}}'s function argument.
> The same happens with a real right-biased {{Either}}. With an uninferred left
> type, e.g. {{DO(x in Either.right(1)) \{ Either.right(x + 1) \}}}, the error
> is:
> {noformat}
> [Static type checking] - Cannot find matching method #L#plus(int)
> {noformat}
> When the type arguments are explicit, code such as {{x + 1}} still compiles,
> because it resolves to {{String.plus}}. It then works at runtime only because
> the actual value is an {{Integer}}.
> h2. Expected
> {{x}} is typed as {{Integer}}, the reproducer compiles, and it prints
> {{Ok(2)}}.
> h2. Cause and suggested fix
> {{MonadicChecker.elementType}} returns {{carrier.genericsTypes[0]}}. That is
> right for single-parameter carriers ({{Optional}}, {{Stream}},
> {{CompletionStage}}, {{Awaitable}}) but not in general.
> The element type should come from the carrier's bind method. Find the
> single-argument method named by the carrier's bind entry ({{flatMap}}, or
> {{bind}} for Functional Java, or the {{@Monadic}} value). Take the first type
> argument of its function parameter ({{? super A}}) and resolve it against the
> receiver's type arguments. Fall back to the last type argument, then
> {{Object}}, when that can't be determined, e.g. for a raw receiver.
> h2. Tests
> * A two-parameter structural carrier, as in the reproducer.
> * A {{@Monadic}} carrier whose type parameters are declared in a different
> order from the bound one.
> * A name-keyed carrier, using a test stub in the style of
> {{fj/data/Option.groovy}}, e.g. a {{Validation<E, A>}} stub.
> * Regression check that single-parameter carriers ({{Optional}}, {{Stream}},
> {{CompletableFuture}}) keep their current typing.
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