[Haskell-cafe] servant streaming question

[Zoran Bošnjak]

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 at haskell.foundation> 
To: "Zoran Bošnjak" <zoran.bosnjak at via.si> 
Cc: "haskell-cafe" <haskell-cafe at 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 < [ mailto:zoran.bosnjak at via.si | zoran.bosnjak at 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-Shell.html ] 
[ https://hackage.haskell.org/package/turtle-1.6.1/docs/Turtle-Prelude.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 | https://hackage.haskell.org/package/servant-0.20/docs/Servant-Types-SourceT.html ] 

As a simple example: 

[ http://turtle.prelude.ls/ | 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 $ [ http://turtle.prelude.ls/ | Turtle.Prelude.ls ] "somePath" 

Appreciate any suggestion. 

regards, 
Zoran 
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