An operator symbol starting with a colon is a constructor

Question

I learn Haskell. From Haskell 2010 documentation:

• An operator symbol starting with a colon is a constructor.
• An operator symbol starting with any other character is an ordinary identifier.

I don't understand first phrase. I know exist data constructors and class type constructors. What constructor this case? Maybe I need a code sample.

Answer 1

You can define stuff like

data Symbolic n
   = Constant n
   | Variable String
   | Symbolic n :+ Symbolic n
   | Symbolic n :* Symbolic n
  deriving (Show)

GHCi> let v = Variable; c = Constant
GHCi> c 2 :* v"a" :+ c 3
    (Constant 2 :* Variable "a") :+ Constant 3

That's what the first phrase refers to.

Answer 2

I know exist data constructors and class type constructors. What constructor this case?

In standard Haskell only data constructors can be symbolic and type names must be alphanumeric. If you enable the GHC extension TypeOperators, type names can be symbolic as well, allowing you to define type constructors that start with :.