[Haskell-cafe] Simple way to do something like ArrowChoice.right on a Conduit? (version 1.0.0)

[Michael Snoyman]

Wow, I hadn't realized that someone had implemented resumable sinks... and
now resumable conduits too! Very interesting.

I'm not sure if I entirely understand your use case, but in general it
should be possible to have multiple Conduits running one after the other.
Here's an example of restarting an accumulator after every multiple of 5:

https://www.fpcomplete.com/user/snoyberg/random-code-snippets/multiple-conduits

Michael


On Mon, Mar 4, 2013 at 6:55 PM, Joey Adams <joeyadams3.14159 at gmail.com>wrote:

> On Sun, Mar 3, 2013 at 10:24 PM, Joey Adams <joeyadams3.14159 at gmail.com>wrote:
>
>> ...
>>
>> Here's a possible API for a resumable Conduit:
>>
>>     newtype ResumableConduit i m o = -- hidden --
>>
>>     newResumableConduit :: Monad m => Conduit i m o -> ResumableConduit i
>> m o
>>
>>     -- | Feed the 'Source' through the conduit, and send any output from
>> the
>>     -- conduit to the 'Sink'.  When the 'Sink' returns, close the
>> 'Source', but
>>     -- leave the 'ResumableConduit' open so more data can be passed
>> through it.
>>     runResumableConduit
>>         :: Monad m
>>         => ResumableConduit i m o
>>         -> Source m i
>>         -> Sink o m r
>>         -> m (ResumableConduit i m o, r)
>>     ...
>>
>
>  While trying to implement this, I found a more elegant interface for
> resuming the ResumableConduit:
>
>     -- | Fuse a 'ResumableConduit' to a 'Sink'.  When the 'Sink' returns,
>     -- it returns the 'ResumableConduit' so the caller can reuse it.
>     (=$++) :: Monad m
>
>            => ResumableConduit i m o
>            -> Sink o m r
>            -> Sink i m (ResumableConduit i m o, r)
>
> This takes advantage of Sink's return value to forward the
> ResumableConduit.  I don't think a ($=++) can be implemented.
>
> Advantages:
>
>  * (=$++) is easier to implement than 'runResumableConduit' since it only
> has to fuse two pipes together instead of three.
>
>  * Pretty syntax: (resumable', a) <- source $$ resumable =$++ sink
>
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