thrown types cannot be inferred from lambda body because of cyclic inference

[maurizio cimadamore]

On 30/10/2010 17:39, Thomas Jung wrote:
> Hi,
>
> the following does not compile:
>
> import com.google.common.base.*;
> class X{
> 	private Predicate<Integer>  x = new Predicate<Integer>(){
> 		public boolean apply(Integer v) { return  v % 3 == 0; }
> 	};
> 	Predicate<Integer>  y = #{Integer v ->  v % 3 == 0 };
> 	Predicate<Integer>  z = #{v ->  v % 3 == 0 }; //does not compile
> }
>
> X.java:11: warning: thrown types cannot be inferred from lambda body
> because of cyclic inference
> 	Predicate<Integer>  z = #{v ->  v % 3 == 0 }; //does not compile
> 	                       ^
>    explicit type required for the following parameter: v
> 1 warning
>
> The error message is a bit strange. I can't see a cycle. Could someone
> explain it?
The message means that the body of the lambda expression cannot be 
attributed before the target type (Predicate<Integer>) is known. In this 
particular example, this has no effect; if you were calling a generic 
method with some 'throws' type-variable there could have been problems 
though. I think that in examples like this, where the target type is 
provided in an assignment context, the compiler could do better job than 
this.

Maurizio
> Thomas
>