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

31 May 2021

      On Mon, May 31, 2021 at 19:03 Mario Lang wrote:
...
Tristan Cacqueray  writes:
...
On Mon, May 31, 2021 at 17:31 Mario Lang wrote:
...
Tristan Cacqueray  writes:
...
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.
Ahh, of course, push the problem up the caller chain :-)
And I guess if that caller needs a sort of handle, it would do it with a
channel to communicate with its action?
I know I was asking for idiomatic code, and I am thankful for
the pointer.  Exception safety is clearly a good goal. But, I wonder, is
limiting the API in such a way worth the trouble?
It looks like this change would trigger a complete rewrite of
the current module and all its clients.
I'd like to be confident this is the right thing to do before doing that.
Beware I'm in the same boat and I can't tell if this is the right thing
to do :-)

I like the with... idiom because it is straighforward and it should not
limit the API, users just need to provide a callback instead of binding
to get the resource.

Though it is more tedious to use from a REPL, where you need to prefix
each input with `withEngine ... $ \engine -> ...`
...
...
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....
Ahh, thanks again.  I'll have to digest all of this.
Cheers,
-Tristan