Edward Kmett
The strictness of Control.Monad.State.Strict has to do with the strictness of the tuple constructor, not of the state per se. The dichotomy between Lazy and Strict comes from the competing desires to fake products as categorical products that don't introduce _|_'s of their own and the lifted products we have in Haskell which do. The former leads you to the .Lazy model, the latter leads you to the .Strict model. The former is nicer for certain lazy knot tying tricks. The latter is easier to reason about when you start throwing fmap _|_'s around.
However, a Strict State or Writer is not actually strict in the state or log. This is admittedly a somewhat common misconception.
My concern is that such a combinator would exacerbate this confusion and that it doesn't generalize. e.g. This trick doesn't generalize to Strict.Writer, where you can `tell'`, but then you lose than on the next bind after you lazily smash it into something that is probably just mempty.
I understand your concern regarding the inability to generalize to Writer. That being said, does it do any harm to include a truly strict combinator in Strict.State? I for one end up rewriting modify' fairly often (or when I don't, I generally only realize after the first stack overflow). As I mentioned earlier, I think just the presence of "modify'" in the documentation would lead many new Haskellers to catch their mistake before even facing the dreaded "*** Exception: stack overflow". Regardless, it seems there is a general consensus around improving the treatment of laziness in the documentation. Unfortunately, not having mastered this myself, I'm not sure I'd be a terribly good candidate to write this language. Any volunteers? Cheers, - Ben