[Haskell-cafe] 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?

Thanks,
Maurício

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