>>10
A programmer should be able to understand that the function open_file will open a file, and that the function to_array converts whatever arguments it is given to an array.

IMO, the functions names length should reflect how complex the function is, if it just does basic stuff, names should be short, like the examples above - not only because it's what they do is basic enough to be easily understood, but also because they ill probably be called a lot. If it does something rather complex, then names can be longer - it probably isn't called too often anyways.