[Haskell-cafe] Type without a data constructor?

[Dan Weston]

The answer would be phantom types, but your example doesn't use them. 
Each of your types has at least one constructor:

Possibly you overlooked the infix constructor :||: ?

Tree a          has 2 constructors: Tip  and Node
SearchCondition has 2 constructors: Term and (:||:)
Term a          has 1 constructor : Constant

*Prelude> :t Tip
Tip :: Tree a

*Prelude> :t Node
Node :: a -> Tree a -> Tree a -> Tree a

*Prelude> :t Term True
Term True :: SearchCondition

*Prelude> :t (:||:)
(:||:) :: SearchCondition -> Term Bool -> SearchCondition

*Prelude> :t Constant True
Constant True :: Term Bool

Dan Weston

Rahul Kapoor wrote:
> 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
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