Comment by throwaway_pdp09

I get the "Make illegal states unrepresentable" in theory but how do you do it actually for nontrivial preconditions like "this list must be sorted"? (as a precond for a binary search on a vector, for example).

OOP is fine with functional if you make it immutable too, so I don't see the problem (I've not really got a problem with mutable OOP, or state generally, if it's done carefully).