On: How to keep lambda calculus simple

[tikhonj]

It's a constructor defined with infix syntax. You can make constructors with custom infix names by starting the name with the : character. Alternatively, you could call that constructor App or something and have the same effect:

    data ITerm
      = Ann CTerm Type
      | Bound Int
      | Free Name
      | App ITerm CTerm
      deriving (Show, Eq)