Re: [Haskell-cafe] What are the MonadPlus laws?

G'day all. Quoting Iavor Diatchki :

This is not enough, at least in some cases. Consider lists, and m being an infinite list, e.g. [1..] Then we need that the inifinte concatenation of a empty lists gives us the empty list which is not the case.

It also doesn't work for monad transformers (e.g. Ralf Hinze's backtracking transformer) when stacked over monads like IO. I say we remove the "law" altogether. It clearly causes problems, and if you use the Hughes/Hinze method for deriving the monads, it turns out that you don't need it anyway. Cheers, Andrew Bromage