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

2008-11-02 Thread 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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Re: [Haskell-cafe] Syntax question: class (Monad m) = MonadReader r m | m - r where

2008-11-02 Thread Andrew Coppin 

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.

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