[Haskell-cafe] A faithful strictly-accumulating writer

David Feuer david.feuer at gmail.com
Fri Aug 30 15:46:04 UTC 2019

I would definitely like to see `listen` split out, since it adds a sort of
introspection that can't be good for CPS-based implementations.

`pass` is a rather surprising piece of the API that definitely seems to
want its own subclass. Another way to express its idea is

    edit :: (w -> w) -> WriterT w m ()

This is equally powerful:

    edit f = pass $ pure ((), f)
    pass m = do
      (a, f) <- m
      edit f
      pure a

The essence of edit/pass is that it adds the ability to make arbitrary
modifications to the "log". That excludes instances for which the log
itself is not accessible at all.

instance MonadWriter String IO where
  tell = hPutStr stderr

On Fri, Aug 30, 2019, 12:49 AM Zemyla <zemyla at gmail.com> wrote:

> I wish we could separate listen and pass from the Writer monad, and
> eventually deprecate them. Doing so would solve so many problems.
> On Thu, Aug 29, 2019, 23:10 David Feuer <david.feuer at gmail.com> wrote:
>> On Thu, Aug 29, 2019, 11:42 PM Li-yao Xia <lysxia at gmail.com> wrote:
>>> This looks like an interesting problem, but I'm a little confused about
>>> the objective. In what sense is it "faithful"?
>>>  > to prevent the computation from gaining unauthorized access to the
>>> state.
>>> Does that property have a formal definition? Are we looking for a
>>> one-to-one correspondence between a "better" WriterT and the naive
>>> WriterT?
>> Exactly. They should be isomorphic as sets and "morally" isomorphic as
>> monads. There will of course be distinctions in strictness
>>> What about (w -> s -> s) instead of ((w -> w) -> (s -> s))? It seems
>>> that `pass` needs the (w -> w), but if we leave `pass` aside, does that
>>> still look about right?
>> Yes. That's the type I started with, and then I got to `pass` and
>> realized it just wouldn't do the trick.
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