Proposal: add replay function to Control.Monad.Cont.Class

[Alexandre Esteves]

Agreed that 'label' is a better name than goto/replay.
It didn't even hit me until I saw setJump that `m (m a)` doesn't allow
representing recursive functions with arguments, so `a -> m(a -> m b, a)`
does seem much more expressive to me
Starting with my original examples, introducing the recursive binding when
there are arguments would look like

`(numberPromptStep, (x,y)) <- setJump (x0, y0)`
`fix $ \numberPromptStep x y -> do`
`numberPromptStep x y = do`

It is slightly annoying that the later 2 can simply add an argument while
setJump requires using the uncurried version but I don't see a way around
that since >>= itself works that way
`m (m a)` is essentially the 0-tuple version of `a -> m(a -> m b, a)` which
makes me wonder if it's even worth having. Sure, it's convenient not to
have these unit/() around, but maybe the same argument (heh) could be made
for 2-argument and 3-argument versions.

do
  numberPromptStep <- setJump
  (..)
  numberPromptStep
do
  (numberPromptStep, x, y) <- setJump2 x0 y0
  (..)
  numberPromptStep x' y'

Given that, I think `a -> m (a -> m b, a)` is the important one be it
called label or setJump and maybe there can be a specialized 0-tuple
version (e.g. label_ / setJump_) offering the `m (m a)` special case. I
don't feel strongly about it though, since unlike `for_` it wouldn't
actually relax constraints, only have a simpler signature.



On Sun, Mar 7, 2021 at 7:33 PM David Feuer <david.feuer at gmail.com> wrote:

> Why not both?
>
> On Sun, Mar 7, 2021, 1:36 PM Zemyla <zemyla at gmail.com> wrote:
>
>> I kind of don't like it, because the continuation doesn't return anything
>> but itself. I'd prefer something that works more like the setjmp function
>> in C, taking a value and returning the value plus a function that lets it
>> return the new value:
>>
>> setJump :: MonadCont m => a -> m (a -> m b, a)
>> setJump a = callCC $ \k -> let
>>   go b = k (go, b)
>>   in pure (go, a)
>>
>> On Sun, Mar 7, 2021, 11:36 Carter Schonwald <carter.schonwald at gmail.com>
>> wrote:
>>
>>> Yeah
>>>
>>> On Sun, Mar 7, 2021 at 12:16 PM Edward Kmett <ekmett at gmail.com> wrote:
>>>
>>>> I rather like label. +1 from me.
>>>>
>>>> On Sun, Mar 7, 2021 at 1:25 AM Tom Ellis <
>>>> tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> wrote:
>>>>
>>>>> On Sun, Mar 07, 2021 at 03:25:16AM +0000, Alexandre Esteves wrote:
>>>>> > replay :: MonadCont m => m (m a)
>>>>> > replay = callCC $ pure . fix
>>>>> >
>>>>> > Using this in a do-notation block allows one to bind a name to the
>>>>> > sub-block that starts immediately after.
>>>>> > I reached for continuations to try to get this behavior for use with
>>>>> > recursive flows without disrupting the reading of the main forward
>>>>> flow and
>>>>> > found how to do it in https://jsdw.me/posts/haskell-cont-monad/
>>>>> under the
>>>>> > name "goto".
>>>>> >
>>>>> > While "goto" is as familiar as a name can be, I feel "replay" more
>>>>> > accurately conveys, that you can only 'go back', by stating what
>>>>> actually
>>>>> > is happening - that a sub-block we're currently evaluating is
>>>>> replayed from
>>>>> > its beginning.
>>>>>
>>>>> Looks like a "label" more than a "goto" to me.  Would "label" be a
>>>>> good name?
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