I have managed to convert in one direction like this:

```

sourceToShell :: forall a. SourceIO a -> Shell a
sourceToShell (S.SourceT act1) = Shell act2 where
   act2 :: FoldShell a r -> IO r
   act2 (FoldShell step begin done) = act1 (go begin) where
   go x = \case
   S.Stop -> done x
   S.Error e -> fail e
   S.Skip cont -> go x cont
   S.Yield val cont -> step x val >>= flip go cont
   S.Effect eff -> eff >>= go x

```

But in the other way (that is: shellToSource) I could not find any clean solution. I've got information from turtle's author that it's not possible directly by simple manupulation of data structures. It should however be possible by forking a new thread, write elements from Shell to some shared buffer and let StepT read from this buffer. But I have decided to re-implement some small set of turtle's functions (like 'ls') to produce SourceIO, instead of Shell, so this conversion is not needed any more.

Zoran

From: "Bryan Richter" <bryan@haskell.foundation>
To: "Zoran Bošnjak" <zoran.bosnjak@via.si>
Cc: "haskell-cafe" <haskell-cafe@haskell.org>
Sent: Monday, July 10, 2023 9:19:44 AM
Subject: Re: [Haskell-cafe] servant streaming question

In short, yes I think it should be possible. But messing with Shell / Fold always feels like doing a brain teaser rather than programming. If you figure it out, I would like to hear about it. :P

On Fri, 30 Jun 2023 at 21:00, Zoran Bošnjak <zoran.bosnjak@via.si> wrote:

Hi all,
I would like to reuse some streaming functions from the turtle package in the context of the servant streaming handler.

Relevant references:
https://hackage.haskell.org/package/turtle-1.6.1/docs/Turtle-Shell.html
https://hackage.haskell.org/package/turtle-1.6.1/docs/Turtle-Prelude.html
https://hackage.haskell.org/package/servant-0.20/docs/Servant-Types-SourceT.html

As a simple example:

Turtle.Prelude.ls function represents stream of FilePaths (directory listing of some path)
ls :: FilePath -> Shell FilePath

Servant streaming is based around SourceT IO a, for example:
type ListFiles = "ls" :> StreamGet NewlineFraming PlainText (SourceT IO FilePath)

The problem is that the streaming (Shell a) is not exactly the same as servant's (SourceT IO a). But as far as I understand, they both represent "stream of values of type 'a'". My question is: Is a generic conversion function possible? Something like:

shellToSource :: forall a. Shell a -> SourceIO a
shellToSource = ??

... such that I could reuse the 'ls' and write a streaming servant handler like this:

listFilesHandler :: Handler (SourceIO FilePath)
listFilesHandler = pure $ shellToSource $ Turtle.Prelude.ls "somePath"

Appreciate any suggestion.

regards,
Zoran
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