In Haskell, the :: operator stands for "has type", and is used in the type signatures of functions. For example, the signature of length looks like this:
length :: [a] -> Integer
This specifies that the
function "
length" has type: "map a
list of
type a onto an
integer"; that is, it takes a
list of a generic
type (call it 'a') and
returns an
integer.