Re: Question about use of | in a class declaration

Simon Guest asked:

Please could someone explain the meaning of | in this class declaration (from Andrew's example):

class (Ord k) => Map m k v | m -> k v where lookupM :: m -> k -> Maybe v

I couldn't find reference to this in any of my standard Haskell tutorials, nor the Haskell 98 report. Any references?

It might not have become clear from the previous answers: this construction is not Haskell 98, but an extension. That's why it's not in the report. BTW: Already multi parameter type classes (such as "class (Ord k) => Map m k v where ...") ^ ^ ^ only one type variable allowed here aren't Haskell 98. All the best Christian Sievers