More type mystery

[Marc Petit-Huguenin]

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Thanks Paul, that's really helpful.

On 02/11/2014 08:52 AM, Paul Govereau wrote:
> Hi Marc,
> 
> Java's type inference isn't always able to fill in missing types. Sometimes
> the programmer must supply some types manually as hints to type inference.
>  Unfortunately, your program is one of these cases. Technically, this error
>  stems from the fact that the method invocation of apply is not a "poly 
> expression" as described in section 15.12 of the Java Language
> Specification version 1.8. This means type inference will not propagate
> type constraints through this expression, and therefore is unable to figure
> out how to instantiate the parameter T of IS_ZERO.
> 
> As you discovered, you can supply type information by using a variable for
> part of your expression with the correct type. Another way is to manually
> supply the type parameter for the generic method using this syntax:
> 
> Test.<Boolean>IS_ZERO()
> 
> Your last example provides the necessary type information by adding an
> argument that requires (T=Boolean) for the generic method to be called.
> 
> My understanding is that this particular constraint may be able to be
> relaxed. However, more work is needed to verify this before any changes can
> be made to Java's type inference.
> 
> Paul
> 
> On 02/08/2014 09:35 AM, Marc Petit-Huguenin wrote: With these static
> methods:
> 
> static<T> UnaryOperator<UnaryOperator<T>> ZERO() { return p -> x -> x; } 
> static<T> UnaryOperator<UnaryOperator<T>> ONE() { return p -> x ->
> p.apply(x); } static<T> Function<T,UnaryOperator<T>> TRUE() { return x -> y
> -> x; } static<T> Function<T,UnaryOperator<T>> FALSE() { return x -> y ->
> y; } static<T> 
> Function<UnaryOperator<UnaryOperator<Function<T,UnaryOperator<T>>>>,Function<T,UnaryOperator<T>>>
>
> 
IS_ZERO() { return n -> n.apply(x -> FALSE()).apply(TRUE()); }
> static<T> 
> Function<UnaryOperator<UnaryOperator<Function<T,UnaryOperator<T>>>>,Function<T,UnaryOperator<T>>>
>
> 
IS_ZERO(Class<T> t) { return n -> n.apply(x ->
> FALSE()).apply(TRUE()); } static boolean
> toBoolean(Function<Boolean,UnaryOperator<Boolean>> f) { return 
> f.apply(true).apply(false); }
> 
> 
> Why is this not compiling:
> 
> System.out.println(toBoolean(IS_ZERO().apply(ZERO()))); 
> System.out.println(toBoolean(IS_ZERO().apply(ONE())));
> 
> Test.java:18: error: method toBoolean in class Test cannot be applied to
> given types; System.out.println(toBoolean(IS_ZERO().apply(ZERO()))); ^ 
> required: Function<Boolean,UnaryOperator<Boolean>> found:
> Function<Object,UnaryOperator<Object>> reason: argument mismatch;
> Function<Object,UnaryOperator<Object>> cannot be converted to
> Function<Boolean,UnaryOperator<Boolean>> Test.java:19: error: method
> toBoolean in class Test cannot be applied to given types; 
> System.out.println(toBoolean(IS_ZERO().apply(ONE()))); ^ required:
> Function<Boolean,UnaryOperator<Boolean>> found:
> Function<Object,UnaryOperator<Object>> reason: argument mismatch;
> Function<Object,UnaryOperator<Object>> cannot be converted to
> Function<Boolean,UnaryOperator<Boolean>> 2 errors
> 
> But this works fine:
> 
> Function<UnaryOperator<UnaryOperator<Function<Boolean,UnaryOperator<Boolean>>>>,Function<Boolean,
>
> 
UnaryOperator<Boolean>>> IZ = IS_ZERO();
> System.out.println(toBoolean(IZ.apply(ZERO()))); 
> System.out.println(toBoolean(IZ.apply(ONE())));
> 
> And this works fine:
> 
> System.out.println(toBoolean(IS_ZERO(Boolean.class).apply(ZERO()))); 
> System.out.println(toBoolean(IS_ZERO(Boolean.class).apply(ONE())));
> 
> 
> Thanks.
> 
>> 
> 

- -- 
Marc Petit-Huguenin
Email: marc at petit-huguenin.org
Blog: http://blog.marc.petit-huguenin.org
Profile: http://www.linkedin.com/in/petithug
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