[Haskell-cafe] Usage of | in classes

[Arthur van Leeuwen]

On Tue, Jun 07, 2005 at 03:54:06PM +0200, Frank-Andre Riess wrote:
> Hi,
> 
> Gracjan's question led me to another question:
> 
> > class Monad m => Ref m r | m -> r where
> >      newRef :: a -> m (r a)
> >      readRef :: r a -> m a
> >      writeRef :: r a -> a -> m ()
> 
> What's the meaning of the bar and the function type in this declaration. I've 
> seen something like that before (with state monads, I think), but couldn't 
> find an explanation in the Haskell grammar (or anywhere else on haskell.org).

It is a functional dependency. That is, in this case, the choice of type r 
is uniquely determined by the choice for the type m. See also
http://haskell.org/hawiki/FunDeps and
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#functional-dependencies

> Thanks in advance,

My pleasure.

Doei, Arthur.

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