Introduce a compiler option to store generic type information about a lambda expression using the Signature Attribute

Remi Forax forax at
Mon Jan 19 15:01:29 UTC 2015

On 01/19/2015 03:39 PM, Joel Borggrén-Franck wrote:
>> On 7 jan 2015, at 21:18, Alex Buckley <alex.buckley at> wrote:
>> On 1/7/2015 7:22 AM, Timo Walther wrote:
>>> the Java Reflection API allows to access the generic signature of
>>> methods through the java.lang.reflect.Method#getGenericReturnType()
>>> method. However, this is not yet supported for Lambda expressions.
>>> Given the following classes:
>>> class Tuple2<F0,F1> {
>>>    F0 field0;
>>>    F1 field1;
>>>    public Tuple2(F0 f0, F1 f1) {
>>>      this.field0 = f0;
>>>      this.field1 = f1;
>>>    }
>>> }
>>> interface Map<IN, OUT> {
>>>    OUT map(IN in);
>>> }
>>> Currently, there is no solution that allows to get further type
>>> information about expressions like:
>>> Map<String, Tuple2<String, Integer>> map = (str) -> new Tuple2<>(str, 1);
>>> System.out.println(getReturnType(map)) // can only print Tuple2 =>
>>> information about the Tuple2's fields are always lost
>> map is a variable, so it doesn't have a return type.
>> If map is a class variable or an instance variable, then Field#getGenericType should help.
>> Expressions (including lambda expressions) aren't generally exposed through Core Reflection. If an expression (such as a lambda expression) happens to be implemented as a method, then relying on the [generic] signature of that method is dangerous since it may change from release to release.
> One could argue that we should generate more attributes for synthetic methods. We did for example add annotations to bridges [1]. While we do want to it discourage depending on a specific translation strategy, I’m not sure adding Signature will make more code break once/if we change translation strategy.

The problem is not that adding a Signature will break, it's that these 
information will be not available
anymore if we use a strategy based on one lambda proxy shared for 
several lambdas*

> cheers
> /Joel


*I've a prototype of such strategy but it puts more pressure on lambda 
I will expose that at the JFokus VM Summit ;)

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