[Haskell-beginners] Handling a MonadError

[Stephen Tetley]

On 30 May 2011 20:35, Christopher Howard
<christopher.howard at frigidcode.com> wrote:

> class (Monad m) => MonadError e m | m -> e where
>    throwError :: e -> m a
>    catchError :: m a -> (e -> m a) -> m a
>
> More specifically: Why is there a pipe symbol followed by what looks
> like a function definition in the "class" line?

It is a functional dependency.

Generally you can read it as "m uniquely determines e".