On Monday 06 August 2007 19:23, 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 :

In this example, you have two different uses of the lexical name 'Term', and I think it is confusing you. One kind of use is as a data constructor; the other is as a type constructor. I've annotated the uses below:

data SearchCondition = Term Bool -- data constructor | SearchCondition :||: (Term Bool) -- type constructor

data Term a = Constant a -- defn of type constr 'Term'

sc :: SearchCondition sc = Term True -- data constructor

is ok, but

sc :: SearchCondition sc = Constant True

Now, here you have an expression of type 'Term Bool'. These can only appear on the right-hand side of :||: . This probably isn't what you want. Likely what you actually want is:

data SearchCondition = SearchTerm (Term Bool) | SearchCondition :||: (Term Bool)

Here, both uses of 'Term' refer to the type constructor.

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.

There are some situations where an explicitly empty type is useful. Type-level programming voodoo is one. Other times the void type is nice because it can be used as a parameter to a type constructor. For example, if you use the nested datatype representation of de Bruijn lambda terms [1], you can use the void type to create the type of closed terms (terms with no free variables). Here's the short version: data Void data Succ a = Zero | Incr a data Term a = Var a | App (Term a) (Term a) | Lam (Term (Succ a)) type ClosedTerm = Term Void [1] Richard Bird and Ross Patterson, _de Brujin Notation as a Nested Datatype_, Journal of Functional Programming 9(1):77-91, January 1999. Rob Dockins

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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe