[Haskell-cafe] A Foldable binary search tree

[Neil Mitchell]

Hi Brad,

> > 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":

In some circumstances, you need to give a type sig. For example using Hugs:

Main> Empty
ERROR - Cannot find "show" function for:
*** Expression : Empty
*** Of type    : BST a

I guess GHC has enough defaulting to display this anyway.

You'll also have the dreaded-evil-horrid monomorhpism restriction if you type.

empty = Empty

Thanks

Neil