There is actually an interesting technical reason for having short names in generic Haskell functions. Because of parametricity, the behavior of the function doesn't depend on what the values actually are. The shortness of the names really is meant to convey "don't think about what this is doing, because it's not important for this function". In the traditional example for map,

  map f [] = []
  map f (x:xs) = f x : map f xs

You're supposed to infer from the short function names that f and x could be anything. The only important bit is that you can apply one argument to f (so, for example, f could take two parameters, and then map is just doing a single partial application). In that context, x and xs is actually a better convention than "first" and "rest", because they indicate the adherence to the type system. The naming here is saying that x is of the type of elements of xs, and that this is the only important information for map. This seriously helps in more complicated functions like zip, etc.