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Learning the meaning of " :?: " operator in Haskell

I'm learning Haskell and I just saw this weird data structure in my book, it looks like this: data ListaOrd a = a :?: (ListaOrd a) | Nulo deriving Show. It was said that this supposed to represent an ordered list, but I don't think I really understood it. My question is, what is the meaning of " :?: " symbol in that code?

Solution

  • My question is, what is the meaning of :?:

    You are here defining a data constructor that is the :?: operator. Indeed, in GHCi you can query the type of the (:?:) function:

    :t (:?:)
(:?:) :: a -> ListaOrd a -> ListaOrd

    It is thus exactly the same as if you would have used another name, like:

    data ListaOrd a = Conso a (ListaOrd a) | Nulo deriving Show

    But here we define a data constructor with name (:?:) that can be used with operator syntax as well. In fact you likely already used such data constructor(s). For example (:) is a data constructor for a list.