Take my current project as an example. I have some samples, each of which are observations along a sequence of non-overlapping segments. My objective is to extract observations over arbitrary intervals for all samples. So I have a segment defined as a pair of start and end position plus its observation, a sample as a vector of segments, and all samples as a dictionary of samples. There are various utility functions to make segments, collect samples, and scan individual samples. The problem is I have to remember all three levels of data structures to use this code. I wonder whether it is better to define an interface for those data structures as well so I just need to remember the interface. My objections to formal definition of interfaces is that everything is so simple and obvious and formal interfaces smack of overengineering.
I got to this point because in my previous projects I put every identifiable thing as a class and found too much coupling in classes and convoluted interfaces.