What is the difference between empty list [] and list with one unit element [()]?
The [] constructor takes no arguments and is like Nothing in the Maybe type. The list ":" (cons) infix constructor takes two arguments, an element of type a and a list of type a, to construct a new list. Compare to Maybe. data [] a = [] | a : [a] data Maybe a = Nothing | Just a Another way of saying [()] is ():[] which, comparing with the Maybe type, is similar to saying Just () but Just only takes one argument where (:) takes two. Both List and Maybe are containers that have a null constructor, namely [] and Nothing. "():[]" contains () similar to how "Just ()" contains (). You can make your own list type and put () in it as follows. data List a = Empty | Cons a (List a) deriving (Show, Eq) () `Cons` Empty -- This is ():[] -Sam PS. I'm new on the list and also to Haskell.