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

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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.

The documentation says "There should be more documentation, but there isn't (yet). Yell if you need it." :) But I think I was able to understand everything from the examples. Thanks.