Why use null function instead of == [] to check for empty list in Haskell?

Question

I am reading through the "Starting Out" chapter of Learn You a Haskell for Great Good!. It says:

null checks if a list is empty. If it is, it returns True, otherwise it returns False. Use this function instead of xs == [] (if you have a list called xs)

I tried in ghci:

xs = []      -- and then,

xs == []
null xs

Both of them are True.

I wonder what's the difference.

Should I use the null function instead of == [] and why?

Answer 1

You should use null. In most cases it doesn't matter, but it is a good habit to get into anyway, because occasionally you may want to check if a list of non-comparable things is empty. Here is a short, crisp example showing this difference:

> null [id]
False
> [id] == []
<interactive>:1:1: error:
    • No instance for (Eq (a0 -> a0)) arising from a use of ‘==’
        (maybe you haven't applied a function to enough arguments?)
    • In the expression: [id] == []
      In an equation for ‘it’: it = [id] == []