Controlling mutation with types

A couple of weeks ago I spent some time pair-programming with Penelope Phippen, on her Ruby auto-formatter rubyfmt. This codebase is largely written in Rust and does a lot of work transforming abstract syntax trees to determine how your Ruby code should be formatted. As such, it deals with recursion and stack structures, and this presented an interesting opportunity to get the type system to enforce some business rules for us....

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