[Haskell-cafe] Examples of Continuation monad that impossible to understand and maintain?

[Andreas Källberg]

You can even implement goto on top of Cont. See for example this: https://www.reddit.com/r/haskell/comments/1jk06q/goto_in_haskell/

> On 1 Sep 2021, at 15:55, Isaac Elliott <isaace71295 at gmail.com> wrote:
> 
> I don't have any examples. Given that Cont essentially implements unstructured control flow, I think that examples of `goto` misuse would apply by analogy.
> 
> On Wed, 1 Sep 2021, 4:31 pm YueCompl via Haskell-Cafe, <haskell-cafe at haskell.org <mailto:haskell-cafe at haskell.org>> wrote:
> I can understand the purpose if it is advising against overusing surface syntax in CPS, but here the approach is to hide continuation beneath the beloved do notation on the surface, and monad laws plus possibly further laws to be added, will make it safer to program programs by end programmers. 
> 
> I do realize CPS is powerful yet dangerous (unsafe), abuse of CPS could be easy and quite unintentional, but what about abuse of "Continuation monad"?
> 
>> On 2021-08-31, at 22:46, Jeff Clites via Haskell-Cafe <haskell-cafe at haskell.org <mailto:haskell-cafe at haskell.org>> wrote:
>> 
>> Based on the preceding paragraph, I think that by “abuse” it means overuse, as in using CPS when you could have used straightforward code. I can imagine someone doing at the source code level the sort of things that would be done by a CPS-based compiler (converting everything possible to CPS), and ending up with a mess.
>> 
>> For example, imagine you started with this code snippet:
>> 
>>   let x = f a
>>         y = g x
>>   in h x y
>> 
>> If you fully convert that to CPS you’d end up with 3 continuations (I think) and it would be much harder to understand. And adding an additional let binding later might involve a bunch of restructuring.
>> 
>> I assume it just means that sort of thing. When someone first learns about continuations and their generality, it can be tempting to go overboard.
>> 
>> Jeff
>> 
>> On Aug 31, 2021, at 3:03 AM, YueCompl via Haskell-Cafe <haskell-cafe at haskell.org <mailto:haskell-cafe at haskell.org>> wrote:
>> 
>>> Dear Cafe,
>>> 
>>> I'm wrapping up my CPS codebase to provide some monadic interface, it appears almost the Cont monad, so the following statement is a pretty valid caveat to me now:
>>> 
>>> > Abuse of the Continuation monad can produce code that is impossible to understand and maintain.
>>> 
>>> Which can be viewed in context of Hackage at: https://hackage.haskell.org/package/mtl/docs/Control-Monad-Cont.html#:~:text=Abuse%20of%20the%20Continuation%20monad%20can%20produce%20code%20that%20is%20impossible%20to%20understand%20and%20maintain <https://hackage.haskell.org/package/mtl/docs/Control-Monad-Cont.html#:~:text=Abuse%20of%20the%20Continuation%20monad%20can%20produce%20code%20that%20is%20impossible%20to%20understand%20and%20maintain> 
>>> 
>>> But I can't find concrete examples demonstrating the "impossible to understand and maintain" situation, in figuring out what pitfalls I'd rather to avoid.
>>> 
>>> Please share what you know about it, many appreciations!
>>> 
>>> Background of my CPS necessarity: 
>>> 
>>> Library code need to delegate STM transaction boundary delimitation to (scripting) application code, though `inlineSTM :: STM a -> m a` can be used to force some action to be within current tx, the usual `>>=` binding should honor whether a separate `atomically` tx should be issued for its rhs computation, as specified by the scripting context.
>>> 
>>> Thanks,
>>> Compl
>>> 
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