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public interface Function0<out R> : Function<R> {
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    /** Invokes the function. */
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    public operator fun invoke(): R
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}

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//kotlin
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fun main() {
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    foo {
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        println("foo")
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    }
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}
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//java
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public static void main(String[] args) {
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    foo(new Function0<Unit>() {
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        @Override
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        public Unit invoke() {
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            System.out.println("foo");
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            return Unit.INSTANCE;
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        }
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    });
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}