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This chapter is from the book

Mutable Data

Changes to data can often lead to unexpected consequences and tricky bugs. I can update some data here, not realizing that another part of the software expects something different and now fails—a failure that’s particularly hard to spot if it only happens under rare conditions. For this reason, an entire school of software development—functional programming—is based on the notion that data should never change and that updating a data structure should always return a new copy of the structure with the change, leaving the old data pristine.

These kinds of languages, however, are still a relatively small part of programming; many of us work in languages that allow variables to vary. But this doesn’t mean we should ignore the advantages of immutability—there are still many things we can do to limit the risks on unrestricted data updates.

You can use Encapsulate Variable (132) to ensure that all updates occur through narrow functions that can be easier to monitor and evolve. If a variable is being updated to store different things, use Split Variable (240) both to keep them separate and avoid the risky update. Try as much as possible to move logic out of code that processes the update by using Slide Statements (223) and Extract Function (106) to separate the side-effect-free code from anything that performs the update. In APIs, use Separate Query from Modifier (306) to ensure callers don’t need to call code that has side effects unless they really need to. We like to use Remove Setting Method (331) as soon as we can—sometimes, just trying to find clients of a setter helps spot opportunities to reduce the scope of a variable.

Mutable data that can be calculated elsewhere is particularly pungent. It’s not just a rich source of confusion, bugs, and missed dinners at home—it’s also unnecessary. We spray it with a concentrated solution of vinegar and Replace Derived Variable with Query (248).

Mutable data isn’t a big problem when it’s a variable whose scope is just a couple of lines—but its risk increases as its scope grows. Use Combine Functions into Class (144) or Combine Functions into Transform (149) to limit how much code needs to update a variable. If a variable contains some data with internal structure, it’s usually better to replace the entire structure rather than modify it in place, using Change Reference to Value (252).

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