[Haskell-beginners] Why must inflix constructors begin with a colon?

[Olumide]

I'm new to Haskell and wondering why this restriction?

For example (from LYH):

infixr 5 :-:
data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord)

Thanks,

- Olumide