I think the coolest aspect about Unison, is that they're trying to reify the concept of a codebase (http://unisonweb.org/2016-09-17/editing.html) into something more than just a collection of lines of source code. You can think of it more like a well-defined object that can be transformed in coherent, strongly-typed ways. Unison identifies everything by a hash, so if you "change" a function definition, you're not actually mutating any state in the codebase, you're introducing a new function with a new hash.