Do you really never need to see what a passed-in function actually does in your FP code? Never need to see what type an argument is in our fancy Haskell or Hindley-Milner-typed language?
In FP, I tend to look at implementations to understand what they'll produce
In OO, I do this, but I also need to know _how_ since they might be changing the current context I'm working in. It's not enough to know that X method produces Y, I also need to remember that calling X implicitly modifies Z when I'm referencing Z
In an FP environment X can't modify Z outside of his context, so I don't have to care about how he produces Y. The cost of this is that someone needs to own Z and functions need to additionally return the new Z, but I think that explicit hierarchy helps in guiding towards a more understandable architecture
I'm hesitant to say always, but I can say:
- in imperative code I never feel comfortable without inspecting the passed in function
- in pure functional code, I rarely feel uncomfortable taking a pure function at it's word