On Wed, Sep 05, 2007 at 03:35:03PM +1200, ok wrote:
I've been thinking about making a data type an instance of MonadPlus. From the Haddock documentation at haskell.org, I see that any such instance should satisfy
mzero `mplus` x = x x `mplus` mzero = x mzero >>= f = mzero v >> mzero = mzero
but is that all there is to it? Are there no other requirements for MonadPlus to make sense?
I also wondered why, once MonadPlus was added to the language, the definition of ++ wasn't changed to (++) = MonadPlus (with the MonadPlus instance for [] defined directly).
It was. They changed it back in Haskell 98, as part of a grand program of dumbing down the langauge... Cale Gibbard says {{{I get the impression there were a lot of people on the Haskell 98 committee who really really hated polymorphism.}}}. Stefan