Скачать или смотреть Understanding with-redefs and Multiple Arities in ClojureScript

Explore why the `with-redefs` function in ClojureScript requires multiple arities for proper functionality and how to resolve associated issues.
---
This video is based on the question https://stackoverflow.com/q/63709716/ asked by the user 'heneryville' ( https://stackoverflow.com/u/879034/ ) and on the answer https://stackoverflow.com/a/63726333/ provided by the user 'Thomas Heller' ( https://stackoverflow.com/u/8009006/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Does with-redefs requires multiple arities?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding with-redefs and Multiple Arities in ClojureScript

In the world of ClojureScript, the with-redefs function is a powerful tool for testing and mocking functions. However, it can lead to frustrating errors if not correctly understood, particularly around the concept of function arities. In this post, we'll explore a specific scenario where an unexpected error occurs due to the arieties defined for functions, and we'll provide a thorough breakdown of the solution.

The Problem

Imagine you have a function called real-func with multiple arities and you intend to replace it temporarily with another function, fake-func, using with-redefs. Your goal is for fake-func to function identically in terms of signature to real-func, allowing you to run some tests without invoking the real logic.

For instance, you might write the following code:

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

You would expect to see Fake 2 a b, but instead, you encounter the following error message:

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

What Went Wrong?

The source of the problem lies in the fact that fake-func is only defined with two arities, whereas real-func has three distinct arities. In ClojureScript, when a function is called, the compiler needs to resolve which version of the function to invoke based on the number of arguments. If they don't match up, it leads to complications, as you encountered.

The Solution

Configuration Adjustment

The first step to forestalling such issues is understanding the compiler settings under which your code is running. In many cases, particularly when using tools like shadow-cljs, you might find that your code is compiled with the :static-fns true option enabled by default. This forces the compiler to create static methods that may not accommodate dynamic function redefinitions well.

To mitigate this problem, you can turn off the :static-fns option in your build configuration:

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

Adding another Arity

An alternative or complementary approach to adjusting the compiler options is to define an additional arity for your fake-func. Even if you don’t have immediate use for it, having an extra arity allows the compiler to function seamlessly without leading to any type errors:

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

Conclusion

Understanding the intricacies of function arities and how they interact with ClojureScript’s compilation settings is essential for successfully using with-redefs. When you find your functions are throwing errors due to arity mismatches in such cases, consider reviewing your compiler options or appropriately defining additional arities.

By following these steps, you will strengthen your testing practices and avoid potential pitfalls in your ClojureScript code. Happy coding!