This is a really useful pattern when you want to have rigidly defined/enforced state transitions to ensure that the data is never in an invalid state for a give

This is a really useful pattern when you want to have rigidly defined/enforced state transitions to ensure that the data is never in an invalid state for a given operation. It's pretty awful to deal with when you're unsure of what the state machine should look like, or if there needs to be a lot more flexibility in how the data is accessed. Maintainability nightmare. An example of this I ran into is a data processing pipeline architecture where each vertex of the processing graph had a processing function called in a loop on its own dedicated thread. Using the type state pattern helped clearly define the "life cycle" of each vertex and enforce it, which provided for some powerful synchronization guarantees (e.g. we could provide some elements of memory safety even when loading things through shared libraries). If you dug into it you could break things, but that would be more work than just following the pattern.