You still run into the problem with other functions, though. For example, in Haskell, 'tail' is a partial function - it's undefined if the list is nil. If, in your code, you write:

  xs = if p x then concat [[x, "bar"], foos] else "baz" : foos
  ys = tail xs

Then you know that the call to tail is not going to fail in your code, because the input has a guaranteed minimum length of either 1 or 2. You can't make that same guarantee about tail in isolation, though.