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

Mauricio wrote:

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.