I'm using functions instead of classes, and I find that I can't tell when another function that it relies on is a dependency that should be individually unit-tested or an internal implementation detail that should not. How can you tell which one it is?
A little context: I'm writing a very simple Lisp interpreter which has an eval() function. It's going to have a lot of responsibilities, too many actually, such as evaluating symbols differently than lists (everything else evaluates to itself). When evaluating symbols, it has its own complex workflow (environment-lookup), and when evaluating lists, it's even more complicated, since the list can be a macro, function, or special-form, each of which have their own complex workflow and set of responsibilities.
I can't tell if my eval_symbol() and eval_list() functions should be considered internal implementation details of eval() which should be tested through eval()'s own unit tests, or genuine dependencies in their own right which should be unit-tested independently of eval()'s unit tests.