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Comment by wvenable | Hacker News Reader

Tests fossilize a design. This is generally a good thing and allow you to focus on refactoring and bug fixes with some confidence that you won't break anything. It's also especially good for products that must adhere to specific interface or API.

But when you want to re-write the last thing you want is to fossilize your design. You explicitly don't want the same system you started with otherwise it wouldn't be a rewrite, It would be a refactoring.

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This isn’t necessarily the case. In my experience, the teams that want rewrites have built big monoliths with no design, lots of coupling, and lots of copy-and-paste code (basically a ‘big ball of mud’). So, what the rewrite does is introduce an architecture that decouples (often valuable) functionality into small, targeted functions and libraries. This enables easier maintenance and modification. It also fixes the brittleness and fragility.

So, the goal isn’t to keep the monolith as is. Instead, it’s to break up its functionality into small parts. Then, the parts may be joined so that the old interfaces work as they did before. But, they may also be used to build entirely new interfaces.

Thanks for putting a name to this. Happens way too often when I start rewriting old code.

I usually rename the test class of the old implementation to OldXTest and try to keep the interface of the new code similar enough to enable a reasonably quick transition into the new code with proper unit testing.

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