Looks like :-> is just a fancy constructor name:
Prelude> data Rel a b = a :-> b
Prelude> :t (5 :-> 6)
(5 :-> 6) :: (Num a, Num b) => Rel a b
In this code, 5 :-> 6 produces a Rel value.
One could've used data Rel a b = a :-% b instead, for example, where :-% would be the constructor name.
One could also have used a non-operator such as
data Rel a b = MkRel a band:t MkRel 5 6.