Haskell has this simply for historical reasons. It's a wart and we wish it weren't so but changing it would break a lot of legacy code (and probably have adverse performance impacts in some cases) so it remains. So as it is today, [a] indeed means it can be empty or not. And a function signature of "[a] -> a" is essentially an unsafe partial function.

That isn't to say you _have_ to write it this way for new things and there are packages like https://hackage.haskell.org/package/safe which provide non-partial/safe versions of the various unsafe base functions (like head, maximum and friends).

The base package in Haskell also includes nonempty which is probably what you want in a lot of cases, anyway.