Re: [Haskell-cafe] Wrapping external process communication in a nice API?

On Mon, May 31, 2021 at 17:31 Mario Lang wrote:

Tristan Cacqueray writes:

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On Sat, May 29, 2021 at 16:59 Mario Lang wrote:

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Hi.

For my chessIO package, I wrote a module to communicate with UCI (Universal Chess Interface) speaking chess engines.

This sort of thing is pretty new to me. I settled to use attoparsec for parsing and TChan to feed incoming data to the library users. This is the first thing that I found which looked like it could work...

I would be thrilled if someone with more experience in this area would give my approach a review. Let me know if something is glaringly unidiomatic, I am sure you will find things.

https://github.com/mlang/chessIO/blob/master/src/Game/Chess/UCI.hs

It seems like it would be safer to use withCreateProcess instead of createProcess to ensure the process is not left unattended.

Maybe I am unimaginative, but it seems to me I can not use withCreateProcess as I need to read/write the Handles outside of the initialisation phase. It seems to me withCreateProcess would kill my external process once my initialisation function is done. Or do I misunderstand bracket somehow? AIUI, withCreateProcess always calls cleanupProcess once the action is done, right?

I guess you would have to replace `start'` with something like: ```haskell withEngine :: Time unit -> ... -> (Engine -> IO ()) -> IO () ``` And let your user provide the `Engine -> IO ()` callback. Alternatively, perhaps the `managed` library can be used to get something closer to your existing `start'`, e.g.: http://hackage.haskell.org/package/managed-1.0.8/docs/Control-Monad-Managed....

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This is for me to learn, and to avoid putting weird stuff on hackage... I actually prefer private mail, but a GitHub issue or even a PR are also highly welcome.

I had the same need for a matrix bot where the events come from a ssh process stream. There is `streaming-commons` and `conduit-extra`, but I am not familiar enough with conduit to make it work.

In the end I used `turtle` to callback with each line output. For what it worths, here is my current implementation:

https://github.com/softwarefactory-project/gerritbot-matrix/blob/71644a21a9a...

Thanks. The output of my subprocess is also line-based, so I use ByteString.hGetLine and a call into attoparsec to make sense of the output I receive. Works so far.

You're welcome. Perhaps such line-based process wrapper could be a useful addition to the hackage registry? Note that I also tried to convert the Turtle.inproc Shell to a `pipes` or `streaming`, but without success. -Tristan