Why does Haskell let me return an empty list where a list inside a list is expected?
haskell, typechecking
Solution
`[]` 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.
Problem
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.