Control.Monad.Writer.Strict .... enough?

15 Jan 2009

      Hi,

In most of the 'mtl' variants on Hackage (with the notable exception  
of 'monadLib'), the Control.Monad.Writer.Strict main instance is:

newtype Writer w a = Writer { runWriter :: (a, w) }

instance (Monoid w) => Monad (Writer w) where
     return a = Writer (a, mempty)
     m >>= k  = Writer $ case runWriter m of
                             (a, w) -> case runWriter (k a) of
                                 (b, w') -> (b, w `mappend` w')

...ie the `mappend` is hidden inside a lazy "(,)" constructor.

...shouldn't the bind really be strict in the written monoid?  (eg by  
replacing "(a,w)" with "(a,!w)" and using Bang Patterns).

...if so, is it worth submitting patch?

--Ben

Control.Monad.Writer.Strict .... enough?

Ben Moseley