Скачать или смотреть Understanding Anonymous Subclasses in Scala: Differences Between Scala 2 and Scala 3

Discover the nuances of using `anonymous subclasses` in `Scala 2` and `Scala 3`. Learn why the behavior differs and how to resolve compilation issues effectively.
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This video is based on the question https://stackoverflow.com/q/78009792/ asked by the user 'bwbecker' ( https://stackoverflow.com/u/1714801/ ) and on the answer https://stackoverflow.com/a/78010040/ provided by the user 'Andrey Tyukin' ( https://stackoverflow.com/u/2707792/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Anonymous Subclasses in Scala: Differences Between Scala 2 and Scala 3

When transitioning from Scala 2 to Scala 3, developers may notice subtle yet impactful changes in how the language handles certain features. One such area is the treatment of anonymous subclasses. If you've encountered the error regarding anonymous subclasses with extra methods while trying to compile your code in Scala 3, you're not alone. This issue has raised questions among seasoned Scala developers about the core differences between these two versions.

The Problem: Compilation Error

Consider the following piece of code that runs seamlessly in Scala 2 but throws a compilation error in Scala 3:

[[See Video to Reveal this Text or Code Snippet]]

When you attempt to print bar.triple, Scala 3 throws a compile-time error indicating that triple is not a member of Foo:

[[See Video to Reveal this Text or Code Snippet]]

So, what’s going wrong?

Scala 3's stricter type inference is at play here.

The Solution: Explicitly Defined Classes

Upon examination with a type printer (scala -Xprint:typer), you will see a significant difference in type inference between the two versions.

Type Inference in Scala 3 Vs Scala 2

In Scala 3, the type for bar explicitly becomes Foo:

[[See Video to Reveal this Text or Code Snippet]]

In Scala 2, however, bar gets more specificity:

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Why This Change?

The reason behind these differing behaviors is that the Scala language designers wanted to ensure that type inference plays better with other language features.

Resolution

To replicate the behavior of Scala 2 in Scala 3, you can explicitly define a subclass instead of using an anonymous class. Here’s how you can achieve this:

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Full Compilable Code Example

Here’s the modified complete code to run successfully in Scala 3:

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Conclusion

Transitioning from Scala 2 to Scala 3 can present challenges, particularly regarding type inference and the introduction of stricter rules. By understanding the differences in how anonymous subclasses are treated, you can avoid common pitfalls and write code that leverages the best of what Scala offers.

Always remember that with progress in programming languages, such discrepancies can arise, but with a little patience and understanding, solutions are often straightforward!