Re: Combining IO and state monads

On Fri, 16 May 2003 14:51:19 +0100, Graham Klyne <GK@ninebynine.org> wrote:

What does the vertical bar "|" in the class declaration mean? I can't find this use mentioned in the Haskell report or the GHC type system extensions.

As someone else has already noted, this is a functional dependency. I'd just like to note that if you want to stay within Haskell 98, it's still perfectly possible to define the state monad transformer - all you lose is the MonadState class. Also, I strongly recommend the use of a record to keep your state - if you use a tuple, then each time you add something new you'll have to change all the getters and setters (or any other code that acts directly on the structure of the state). Cheers, Ganesh