Combining IO and state monads

[Derek Elkins]

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

> This may be a very dumb question, but...
> 
> I've been digging around Monad transformers, trying to get the feel of
> how to use them.  In part, I'm referring to the library source code,
> and come across the following in Control.Monad.State, which I'm having
> trouble figuring out:
> 
> [[
> -- MonadState class
> --
> --  get: returns the state from the internals of the monad.
> --  put: changes (replaces) the state inside the monad.
> 
> class (Monad m) => MonadState s m | m -> s where
>          get :: m s
>          put :: s -> m ()
> ]]
> 
> 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.

It's mentioned in GHC's type extensions, but only as a reference to a
paper.  The '| m -> s' is a functional dependency.  You could certainly
use MonadState without even knowing about it, all you really need to
know is what get and put do and that's fairly straightforward.