[Haskell-cafe] Type without a data constructor?

Most examples for defining algebraic types include data constructors like so: data Tree a = Tip | Node a (Tree a) (Tree a) I by mistake defined a type which did not specify a data constructor : data SearchCondition = Term Bool | SearchCondition :||: (Term Bool) data Term a = Constant a sc :: SearchCondition sc = Term True is ok, but sc :: SearchCondition sc = Constant True is not (though this is what I intended to capture!). So the question is what are types with no constructors good for? A simple example would be appreciated. Rahul