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

[Joey Adams]

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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