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class (Monoid w, Monad m) => MonadWriter w m | m -> w where 
   pass   :: m (a,w -> w) -> m a 
   listen :: m a -> m (a,w) 
   tell   :: w -> m ()

What is the meaning of the pipe above? The snippet comes from here.

Volker Stolz
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luntain
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    possible duplicate of [What's the "|" for in a Haskell class definition?](http://stackoverflow.com/questions/2675655/whats-the-for-in-a-haskell-class-definition) – Don Stewart Apr 19 '11 at 01:36

1 Answers1

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Actually, it's a "functional dependency". In this case that means that m uniquely identifies w -- the type m determines the type w. (This may be a better link.)

luqui
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Andrew Jaffe
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