On Sat, 22 Dec 2007 22:41:37 -0500, Neil Mitchell
Hi
data (Ord a) => BST a = Empty | BST (BST a) a (BST a)
Experience has taught me to _never_ put class contexts on data definitions. Now you can't write something as simple as "Empty" - you have to give it a class context. This is just plain annoying.
With the class context in the BST definition, ghc gives no complaints when I evaluate "Empty": *BST> Empty Empty *BST> :t Empty Empty :: BST a I assume I misunderstand you.
I would accept this pain if it meant I could write:
insert :: a -> BST a -> BST a
and have the Ord silently inserted - but you can't. You still have to write the Ord a => explicitly.
I see what you mean here. To get ghc to accept my code (where BST is Foldable), I omit the class context from the data definition, but add the Ord class context to the signatures of the BST functions that require it (insert, for example).
As a result, I see no advantage to adding the class constraint, and plenty of disadvantages. If there are some advantages to this context I would be interested to know what they are.
Thanks
Neil
Thanks! Brad