[Haskell-cafe] Re: Syntax question: class (Monad m) => MonadReader r m | m -> r where

[Mauricio]

>> Hi,
>>
>> I've reading Control.Monad.Reader source code and
>> arrived here:
>>
>> class (Monad m) => MonadReader r m | m -> r where
>>
>> I can't understand that syntax. Since this is not a
>> 'data' line, what is the '|' supposed mean?
> 
> It's called a "functional dependency". This is not part of the 
> Haskell-98 language standard; check the GHC manual.

The documentation says "There should be more documentation, but there 
isn't (yet). Yell if you need it." :)

But I think I was able to understand everything from
the examples.

Thanks.