Functional Exchange 2013 featured an interesting talk by Paul Dale that had been mostly gutted but had a helpful introduction to cognitive bias. That reminded me of something I was trying to articulate about in my own talk when I was talking about the difficulty of evolving typed solutions. I used the analogy of the big ball of mud where small incremental changes to the model of the system result in an increasing warped codebase. Ultimately you need someone to come along and rationalise the changes or you end up with something that is ultimately too difficult to work with and is re-built.

This of course is an example of anchoring where the initial type design for the system tends to remain no matter how far the domain and problem have moved from the initial circumstances of their creation.

Redesigning the type definition of a program is also expensive in terms of having to create a new mental model of what is happening rather than just adapting the existing one. Generally it is easier to adapt and incorporate an exception or set of exceptions to the existing model rather than recreating an entire system.

Dynamic or data-driven systems are more sympathetic to the adaptive approach. However they too have their limits, special cases need to be abstracted periodically and holistic data objects can bloat out of control into documents that are dragging the whole world along with them.

Type-based solutions on the other hand need to be periodically re-engineered and the difficulty is that the whole set of type definitions need to be worked on at the same time. Refactoring patterns of object-orientated code often focus on reorganisation that is easy, such as pulling out traits or extracting new structures. This is still anchoring on the original solution though.

If a type system is to be help rather than a hindrance you need to be rework the overall structure. I think with most type-systems this is actually impossible. Hence the pattern of recreating the application if you want to take a different approach.

The best typed solution currently seems to be type unions where you can make substantial changes and abstractions but not have to work about things like type hierarchies or accidentally polluting the edge cases of the system.

Where these aren’t available then good strongly-typed solutions actually rely heavily on good, proactive technical leaders to regularly drive good change through the system and manage the consequences.