[Haskell-cafe] A faithful strictly-accumulating writer

[David Feuer]

No, that's not quite right either (too lazy this time). The idea is morally
right, but I guess it doesn't quite work out for strictness. Anyway, this
is all a side track.

On Fri, Aug 30, 2019, 12:17 PM David Feuer <david.feuer at gmail.com> wrote:

> Hmm... that implementation of `pass` isn't quite right for lazy WriterT. I
> guess it needs to be
>
> pass m = do
>       ~(a, f) <- m
>       edit f
>       pure a
>
> On Fri, Aug 30, 2019, 11:46 AM David Feuer <david.feuer at gmail.com> wrote:
>
>> 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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>>>
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