When the implementation detail involves whether or not the function will perform I/O, it is better to let that leak. Excessive hiding of implementation detail

When the implementation detail involves whether or not the function will perform I/O, it is better to let that leak. Excessive hiding of implementation detail is what leads to things like fetching a collection of user IDs from a database and then fetching each user from an ID separately (the 1+N problem). Excessive hiding of implementation detail is what leads to accidental O(N^2) algorithms. Excessive hiding of implementation detail is what leads to most performance problems.