This is functional dependency. For this case It means for given monad
m, there is unique possible value of Monoid w.
http://en.wikipedia.org/wiki/Type_class#Functional_dependencies
http://research.microsoft.com/pubs/67435/jfp06.pdf
Thanks
Divyanshu Ranjan
On Mon, Sep 16, 2013 at 10:30 AM, Vinay Emani
Hi,
I'm trying to understand the MonadWriter class definition syntax. Now, I'm using GHC 7.6 and the class is defined as
class (Monoid w, Monad m) => MonadWriter w m | m -> w where ....
I've never seen '|' in the middle of a class definition, could some one explain this? It's used in the guard syntax as well as to separate data constructors in a type definition.
Regards Vinay
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