Sam Danielson wrote:
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.
Or, in Monad terms: "[()]" and "Just ()" are both "return ()" in their respective Monads. "[]" and "Nothing" are both "mzero" in their respective MonadsPluses. (Both are also "fail" in their respective Monads, but I find "fail"'s presence in Monad a bit inelegant, though handy.) -- src/