Why is this type inference not working with this Lambda expression scenario?

Question

I have a weird scenario where type inference isn't working as I'd expect when using a lambda expression. Here's an approximation of my real scenario:

static class Value<T> { }  @FunctionalInterface interface Bar<T> {   T apply(Value<T> value); // Change here resolves error }  static class Foo {   public static <T> T foo(Bar<T> callback) {   } }  void test() {   Foo.foo(value -> true).booleanValue(); // Compile error here } 

The compile error I get on the second to last line is

The method booleanValue() is undefined for the type Object

if I cast the lambda to Bar<Boolean>:

Foo.foo((Bar<Boolean>)value -> true).booleanValue(); 

or if I change the method signature of Bar.apply to use raw types:

T apply(Value value); 

then the problem goes away. The way I'd expect this to work is that:

• Foo.foo call should infer a return type of boolean
• value in the lambda should be inferred to Value<Boolean>.

Why doesn't this inference work as expected and how can I change this API to make it work as expected?

Answer 1

Under the Hood

Using some hidden javac features, we can get more information about what's happening:

$ javac -XDverboseResolution=deferred-inference,success,applicable LambdaInference.java  LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Foo.foo(value -> true).booleanValue(); // Compile error here        ^   phase: BASIC   with actuals: <none>   with type-args: no arguments   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Object>)Object)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: Deferred instantiation of method <T>foo(Bar<T>)     Foo.foo(value -> true).booleanValue(); // Compile error here            ^   instantiated signature: (Bar<Object>)Object   target-type: <none>   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: error: cannot find symbol     Foo.foo(value -> true).booleanValue(); // Compile error here                           ^   symbol:   method booleanValue()   location: class Object 1 error 

This is a lot of information, let's break it down.

LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Foo.foo(value -> true).booleanValue(); // Compile error here        ^   phase: BASIC   with actuals: <none>   with type-args: no arguments   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Object>)Object)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) 

phase: method applicability phase
actuals: the actual arguments passed in
type-args: explicit type arguments
candidates: potentially applicable methods

actuals is <none> because our implicitly typed lambda is not pertinent to applicability.

The compiler resolves your invocation of foo to the only method named foo in Foo. It has been partially instantiated to Foo.<Object> foo (since there were no actuals or type-args), but that can change at the deferred-inference stage.

LambdaInference.java:16: Note: Deferred instantiation of method <T>foo(Bar<T>)     Foo.foo(value -> true).booleanValue(); // Compile error here            ^   instantiated signature: (Bar<Object>)Object   target-type: <none>   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) 

instantiated signature: the fully instantiated signature of foo. It is the result of this step (at this point no more type inference will be made on the signature of foo).
target-type: the context the call is being made in. If the method invocation is a part of an assignment, it will be the left hand side. If the method invocation is itself part of a method invocation, it will be the parameter type.

Since your method invocation is dangling, there is no target-type. Since there is no target-type, no more inference can be done on foo and T is inferred to be Object.

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Analysis

The compiler does not use implicitly typed lambdas during inference. To a certain extent, this makes sense. In general, given param -> BODY, you will not be able to compile BODY until you have a type for param. If you did try to infer the type for param from BODY, it might lead to a chicken-and-egg type problem. It's possible that some improvements will be made on this in future releases of Java.

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Solutions

Foo.<Boolean> foo(value -> true)

This solution provides an explicit type argument to foo (note the with type-args section below). This changes the partial instantiation of the method signature to (Bar<Boolean>)Boolean, which is what you want.

LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Foo.<Boolean> foo(value -> true).booleanValue(); // Compile error here        ^   phase: BASIC   with actuals: <none>   with type-args: Boolean   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Boolean>)Boolean)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: resolving method booleanValue in type Boolean to candidate 0     Foo.<Boolean> foo(value -> true).booleanValue(); // Compile error here                                     ^   phase: BASIC   with actuals: no arguments   with type-args: no arguments   candidates:       #0 applicable method found: booleanValue() 

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Foo.foo((Value<Boolean> value) -> true)

This solution explicitly types your lambda, which allows it to be pertinent to applicability (note with actuals below). This changes the partial instantiation of the method signature to (Bar<Boolean>)Boolean, which is what you want.

LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Foo.foo((Value<Boolean> value) -> true).booleanValue(); // Compile error here        ^   phase: BASIC   with actuals: Bar<Boolean>   with type-args: no arguments   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Boolean>)Boolean)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: Deferred instantiation of method <T>foo(Bar<T>)     Foo.foo((Value<Boolean> value) -> true).booleanValue(); // Compile error here            ^   instantiated signature: (Bar<Boolean>)Boolean   target-type: <none>   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: resolving method booleanValue in type Boolean to candidate 0     Foo.foo((Value<Boolean> value) -> true).booleanValue(); // Compile error here                                            ^   phase: BASIC   with actuals: no arguments   with type-args: no arguments   candidates:       #0 applicable method found: booleanValue() 

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Foo.foo((Bar<Boolean>) value -> true)

Same as above, but with a slightly different flavor.

LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Foo.foo((Bar<Boolean>) value -> true).booleanValue(); // Compile error here        ^   phase: BASIC   with actuals: Bar<Boolean>   with type-args: no arguments   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Boolean>)Boolean)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: Deferred instantiation of method <T>foo(Bar<T>)     Foo.foo((Bar<Boolean>) value -> true).booleanValue(); // Compile error here            ^   instantiated signature: (Bar<Boolean>)Boolean   target-type: <none>   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: resolving method booleanValue in type Boolean to candidate 0     Foo.foo((Bar<Boolean>) value -> true).booleanValue(); // Compile error here                                          ^   phase: BASIC   with actuals: no arguments   with type-args: no arguments   candidates:       #0 applicable method found: booleanValue() 

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Boolean b = Foo.foo(value -> true)

This solution provides an explicit target for your method call (see target-type below). This allows the deferred-instantiation to infer that the type parameter should be Boolean instead of Object (see instantiated signature below).

LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Boolean b = Foo.foo(value -> true);                    ^   phase: BASIC   with actuals: <none>   with type-args: no arguments   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Object>)Object)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: Deferred instantiation of method <T>foo(Bar<T>)     Boolean b = Foo.foo(value -> true);                        ^   instantiated signature: (Bar<Boolean>)Boolean   target-type: Boolean   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) 

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Disclaimer

This is the behavior that's occurring. I don't know if this is what is specified in the JLS. I could dig around and see if I could find the exact section that specifies this behavior, but type inference notation gives me a headache.

This also doesn't fully explain why changing Bar to use a raw Value would fix this issue:

LambdaInference.java:16: Note: resolving method foo in type Foo to candidate 0     Foo.foo(value -> true).booleanValue();        ^   phase: BASIC   with actuals: <none>   with type-args: no arguments   candidates:       #0 applicable method found: <T>foo(Bar<T>)         (partially instantiated to: (Bar<Object>)Object)   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: Deferred instantiation of method <T>foo(Bar<T>)     Foo.foo(value -> true).booleanValue();            ^   instantiated signature: (Bar<Boolean>)Boolean   target-type: <none>   where T is a type-variable:     T extends Object declared in method <T>foo(Bar<T>) LambdaInference.java:16: Note: resolving method booleanValue in type Boolean to candidate 0     Foo.foo(value -> true).booleanValue();                           ^   phase: BASIC   with actuals: no arguments   with type-args: no arguments   candidates:       #0 applicable method found: booleanValue() 

For some reason, changing it to use a raw Value allows the deferred instantiation to infer that T is Boolean. If I had to speculate, I would guess that when the compiler tries to fit the lambda to the Bar<T>, it can infer that T is Boolean by looking at the body of the lambda. This implies that my earlier analysis is incorrect. The compiler can perform type inference on the body of a lambda, but only on type variables that only appear in the return type.

Answer 2

Inference on lambda parameter type cannot depend on the lambda body.

The compiler faces a tough job trying to make sense of implicit lambda expressions

    foo( value -> GIBBERISH ) 

The type of value must be inferred first before GIBBERISH can be compiled, because in general the interpretation of GIBBERISH depends on the definition of value.

(In your special case, GIBBERISH happens to be a simple constant independent of value.)

Javac must infer Value<T> first for parameter value; there's no constraints in context, therefore T=Object. Then, lambda body true is compiled and recognized as Boolean, compatible with T.

After you made the change to the functional interface, the lambda parameter type does not require inference; T remains uninfered. Next, the lambda body is compiled, and the return type appears to be Boolean, which is set as a lower bound for T.

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Another example demonstrating the issue

<T> void foo(T v, Function<T,T> f) { ... }  foo("", v->42);  // Error. why can't javac infer T=Object ? 

T is inferred to be String; the body of lambda did not participate in the inference.

In this example, javac's behavior seems very reasonable to us; it likely prevented a programming error. You don't want inference to be too powerful; if everything we write compiles somehow, we'll lose the confidence on compiler finding errors for us.

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There are other examples where lambda body appears to provide unequivocal constraints, yet the compiler cannot use that information. In Java, the lambda parameter types must be fixed first, before the body can be looked at. This is a deliberate decision. In contrast, C# is willing to try different parameter types and see which makes the code compile. Java considers that too risky.

In any case, when implicit lambda fails, which happens rather frequently, provide explicit types for lambda parameters; in your case, (Value<Boolean> value)->true