Why does Haskell let me return an empty list where a list inside a list is expected?

Question

I'm really new to Haskell and have been going through the 99 problems translation. This is my solution to number 9:

pack :: (Eq a) => [a] -> [[a]]
pack (xs)
    | null xs = []
    | otherwise = 
        let (matched, unmatched) = span (== head xs) xs
        in [matched] ++ pack unmatched

I don't get how I'm allowed to do | null xs = [] when the type signature says the function returns a [[]]. I've seen other solutions to the same problem do the same thing.

I mean, I'm not complaining, but is this something specifically allowed? Are there any caveats I have to look out for?

I'm using the GHCi on a default Windows 7 Haskell Platform 2013.2.0.0 installation, if that helps.

Answer 1

[] is an empty list. It has the following type:

[] :: [b] -- Note: I'm using b instead of a because your function already uses a

That b can be everything. Can you choose a b such that [b] ~ [[a]]? (~ is equality for types)? Yes, just use b ~ [a] and the type of [] becomes:

[] :: [b] :: [[a]]     -- When b ~ [a]

So [] is also a value of type [[a]]. [] is a valid value for any type of list, be it a list of a's or a list of lists of a's.