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From Cédric Champeau <cedric.champ...@gmail.com>
Subject Re: SAM type closure coercion
Date Wed, 18 Jan 2017 11:04:01 GMT
This is something that we can improve. We knew it when we implemented SAM
type coercion, and decided to wait until someone complains, to see how
often this use case happens :)

2017-01-18 11:59 GMT+01:00 Andres Almiray <aalmiray@gmail.com>:

> Hello everyone,
>
> Just yesterday Greg L. Turnquist blogged about an usage pattern of
> Spinnaker, Cloud Foundry, and Groovy. See http://greglturnquist.com/
> 2017/01/reactively-talking-to-cloud-foundry-with-groovy.html
>
> Basically he complains that Groovy can't coerce a Closure to a given SAM
> type. In his own words
>
> "Groovy has this nice feature where it can coerce objects. However, with
> all the overloading, Groovy gets lost and can’t tell which TupleUtils
> function to target."
>
> Now I know that based on historical reasons Groovy did not coerce closures
> into SAMs until very "recently" (was it 2.2?). We also gained @DelegatesTo
> in order to supply additional hints to the compiler (@CompileStatic and
> @TypeChecked) and IDEs. Despite all this Groovy does not offer a "clean"
> solution to automatically coerce a closure into a SAM.
>
> Take for example the following Java code:
>
> ----
> public interface Function1 { void call(String arg0); }
>
> public interface Function2 { void call(String arg0, String arg1); }
>
> import groovy.lang.DelegatesTo;
> public class API {
>     public void doit(@DelegatesTo Function1 func) {
>         System.out.println("Invoking "+ func.toString());
>         func.call("arg0");
>     }
>
>     public void doit(@DelegatesTo Function2 func) {
>         System.out.println("Invoking "+ func.toString());
>         func.call("arg0", "arg1");
>     }
> }
> -----
>
> Invoking an instance of API from Groovy
>
> ----
> class Main {
>     static void main(String[] args) {
>         API api = new API()
>         api.doit({ String arg0 -> println "Received $arg0" })
>         api.doit({ String arg0, String arg1 -> println "Received $arg0
> $arg1" })
>     }
> }
> ----
>
> Results in a runtime exception such as
>
> Exception in thread "main" groovy.lang.GroovyRuntimeException: Ambiguous
> method overloading for method sample.API#doit.
> Cannot resolve which method to invoke for [class
> sample.Main$_main_closure1] due to overlapping prototypes between:
>         [interface sample.Function1]
>         [interface sample.Function2]
>         at groovy.lang.MetaClassImpl.chooseMostSpecificParams(
> MetaClassImpl.java:3263)
>         at groovy.lang.MetaClassImpl.chooseMethodInternal(
> MetaClassImpl.java:3216)
>         at groovy.lang.MetaClassImpl.chooseMethod(MetaClassImpl.java:3159)
>         at groovy.lang.MetaClassImpl.getMethodWithCachingInternal(
> MetaClassImpl.java:1336)
>         at groovy.lang.MetaClassImpl.createPojoCallSite(
> MetaClassImpl.java:3391)
>         at org.codehaus.groovy.runtime.callsite.CallSiteArray.
> createPojoSite(CallSiteArray.java:132)
>         at org.codehaus.groovy.runtime.callsite.CallSiteArray.
> createCallSite(CallSiteArray.java:166)
>         at org.codehaus.groovy.runtime.callsite.CallSiteArray.
> defaultCall(CallSiteArray.java:48)
>         at org.codehaus.groovy.runtime.callsite.AbstractCallSite.
> call(AbstractCallSite.java:113)
>         at org.codehaus.groovy.runtime.callsite.AbstractCallSite.
> call(AbstractCallSite.java:125)
>         at sample.Main.main(Main.groovy:6)
>
> Activating static type checking yields
>
> /private/tmp/foo/src/main/groovy/sample/Main.groovy: 7: [Static type
> checking] - Reference to method is ambiguous. Cannot choose between [void
> sample.API#doit(sample.Function1), void sample.API#doit(sample.Function2)]
>  @ line 7, column 9.
>            api.doit({ String arg0 -> println "Received $arg0" })
>            ^
>
> /private/tmp/foo/src/main/groovy/sample/Main.groovy: 8: [Static type
> checking] - Reference to method is ambiguous. Cannot choose between [void
> sample.API#doit(sample.Function1), void sample.API#doit(sample.Function2)]
>  @ line 8, column 9.
>            api.doit({ String arg0, String arg1 -> println "Received $arg0
> $arg1" })
>
> Clearly Groovy requires some hints to determine the first closure must be
> coerced to Function1 and the second to Function2. Not even @DelegatesTo
> helps in this case.
>
> The current "workaround" (from a Java dev POV anyway) is to sprinkle the
> code with usages of the 'as' keyword to explicitly coerce a closure into a
> target type. And this is exactly where the Java interop history breaks due
> to Java 8, because as we know Java 8 lambda expressions are automatically
> coerced into a matching SAM type.
>
> Does the parrot parser offer an alternative to this problem?
>
> Can the compiler be aware of additional arg/type information provided by
> the Closure to figure out the right SAM type to use?
>
> Cheers,
> Andres
>
> -------------------------------------------
> Java Champion; Groovy Enthusiast
> http://jroller.com/aalmiray
> http://www.linkedin.com/in/aalmiray
> --
> What goes up, must come down. Ask any system administrator.
> There are 10 types of people in the world: Those who understand binary,
> and those who don't.
> To understand recursion, we must first understand recursion.
>

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