13 Feb
2006
13 Feb
'06
4:39 a.m.
On Sun, Feb 12, 2006 at 09:32:14PM +0100, Niklas Broberg wrote:
Irrelevant to the discussion above, but I wonder whether the Monoid constraint should really be there on MonadWriter. I could imagine lots of interesting applications of writer monads that don't output a monoid, an example would be a monad that iteratively computes a result of better and better precision and "tells" the result of each step until told to stop. In this case the merging would not be mplus but to always throw away the old argument (flip const).
You also need a value for return. Then the monad laws will ensure that you have another monoid.