[Haskell-cafe] A faithful strictly-accumulating writer

David Feuer david.feuer at gmail.com
Fri Aug 30 04:09:26 UTC 2019

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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