Haskell: What is the meaning of :.?

Question

A symbol combined (:.) of cons following by a dot.

Taken from here:

-- The custom list type
data List t =
  Nil
  | t :. List t
  deriving (Eq, Ord)

-- Right-associative
infixr 5 :.

Taken from here: https://github.com/tonymorris/fp-course

Answer 1

In short : the code constructs an alterative version of a list, and (:.) is one of the two data constructors .

Well it is a data constructor of the List t data type. For example the standard list [a] has two data constructors [] and (:) , here the code introduces a new data constructor (:.) , but it acts completely the same way as the "cons" of the standard list (:) .

So the code defines it as:

data List t = Nil | (:.) t (List t) deriving (Eq, Ord)

and like any data constructor, we can do pattern matching on it, construct new lists, etc.