What's the "|" for in a Haskell class definition?

Posted by martingw on Stack Overflow See other posts from Stack Overflow or by martingw
Published on 2010-04-20T13:57:42Z Indexed on 2010/04/20 14:13 UTC
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haskell

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typeclass

I can't figure out what the "| m -> w"-part means in a class definition like this:

class (Monoid w, Monad m) => MonadWriter w m | m -> w

What additional information does this add to the class definition?

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