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From "Till Rohrmann (JIRA)" <j...@apache.org>
Subject [jira] [Closed] (FLINK-3681) CEP library does not support Java 8 lambdas as select function
Date Thu, 31 Mar 2016 08:34:25 GMT

     [ https://issues.apache.org/jira/browse/FLINK-3681?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Till Rohrmann closed FLINK-3681.
    Resolution: Fixed

Fixed via 833339f3a0faafbab9af90433488393421ae7171

> CEP library does not support Java 8 lambdas as select function
> --------------------------------------------------------------
>                 Key: FLINK-3681
>                 URL: https://issues.apache.org/jira/browse/FLINK-3681
>             Project: Flink
>          Issue Type: Bug
>          Components: CEP
>    Affects Versions: 1.0.0
>            Reporter: Till Rohrmann
>            Assignee: Till Rohrmann
>            Priority: Minor
>             Fix For: 1.1.0, 1.0.1
> Currently, the CEP library does not support Java 8 lambdas to be used as {{select}} or
{{flatSelect}} function. The problem is that the {{TypeExtractor}} has different semantics
when calling {{TypeExtractor.getUnaryOperatorReturnType}} either with a Java 8 lambda or 
an instance of an UDF function.
> To illustrate the problem assume we have the following UDF function 
> {code}
> public interface MyFunction[T, O] {
>     O foobar(Map<String, T> inputElements);
> }
> {code}
> When calling the {{TypeExtractor}} with an anonymous class which implements this interface,
the first type parameter is considered being the input type of the function, namely {{T}}.

> In contrast, when providing a Java 8 lambda for this interface, the {{TypeExtractor}}
will see an input type of {{Map<String, T>}}. 
> This problem also occurs with a {{FlatMapFunction}} whose first type argument is {{T}}
but whose first parameter of a Java 8 lambda is {{Iterable<T>}}. In order to solve the
problem here, the {{TypeExtractor.getUnaryOperatorReturnType}} method has the parameters {{hasIterable}}
and {{hasCollector}}. If these values are {{true}}, then a special code path is taken (in
case of a Java 8 lambda), where the input type is compared to the first type argument of the
first input parameter of the lambda (here an {{Iterable<T>}}). This hand-knitted solution
does not generalize well, as it will fail for all parameterized types which have the input
type at a different position (e.g. {{Map<String, T>}}.
>  In order to solve the problem, I propose to generalize the {{getUnaryOperatorReturnType}}
a little bit so that one can specify at which position the input type is specified by a parameterized

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