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?