Stricter WriterT (Part II)

[Ross Paterson]

On Sun, Mar 17, 2013 at 09:18:13AM -0700, Gabriel Gonzalez wrote:
> So I propose that we add an additional stricter WriterT (under say,
> "Control.Monad.Trans.Writer.Stricter") which is internally
> implemented as StateT, but hide the constructor so we don't expose
> the implementation:
> 
> newtype WriterT w m a = WriterT { unWriterT :: w -> m (a, w) }
> 
> instance (Monad m, Monoid w) => Monad (WriterT w m) where
>     return a = WriterT $ \w -> return (a, w)
>     m >>= f  = WriterT $ \w -> do
>         (a, w') <- unWriterT m w
>         unWriterT (f a) w'
> 
> And define `tell` and `runWriterT` as follows:
> 
> tell :: (Monad m, Monoid w) => w -> WriterT w m ()
> tell w = WriterT $ \w' ->
>     let wt = w `mappend` w'
>      in wt `seq` return ((), w `mappend` w')
> 
> runWriterT :: (Monoid w) => WriterT w m a -> m (a, w)
> runWriterT m = unWriterT m mempty
> 
> If we do that, then WriterT becomes not only usable, but actually
> competitive with expertly tuned code.

Presumably we'll also need

writerT :: m (a, w) -> WriterT w m a

Is there any reason to keep Control.Monad.Trans.Writer.Strict, or
should this replace it?