Re: [MirageOS-devel] result-y errors and MirageOS 3

[Thomas Gazagnaire <thomas@xxxxxxxxxxxxxx>]

> I will start merging all of this pretty soon, so expect some breakage if you 
> use the dev version of mirage.

All the 30 pull request (!!) have been merged. Please open an issue if you see 
any strange errors related to error types.

We decided to use the following error scheme after few months of going back and 
forth:

- Every mirage-type signature (e.g. BLOCK, FLOW, etc) now defines an `error` 
type (and sometime `write_error` too) which can either be abstract or which can 
be exposed as a private polymorphic variant, like `type error = private [> 
`Closed]`
- Every mirage-type signature now exposes a `pp_error` (and sometime 
`pp_write_error`) which can print the errors. 
- Some helper printers have been temporary defined in `Mirage_pp` (from the 
`mirage-runtime` module) which will disappear completely before 3.0 (see 
bellow).
- Module implementing these signature can use richer error types extending the 
base errors (see mirage-channel[0]). It is also perfectly fine to export these 
enriched error types in the mli.

The helper printers will move to individual related packages, like 
`Mirage_block.pp_error` (this specific function has already been moved, during 
the the signature split that I started to implement in the same patchset, to 
see how it works in practice; short answer: it works well). 

Individual packages (with pretty-printer and sometimes much more[1]) now exists 
for:
  - DEVICE (Mirage_device[2] in mirage-device)
  - CONSOLE (Mirage_console[3] in mirage-console)
  - TIME (Mirage_time[4] in mirage-time)
  - PCLOCK and MCLOCK (Mirage_clock[5] in mirage-clock)
  - RANDOM (Mirage_random[6] in mirage-random)
  - BLOCK (Mirage_block[7] and mirage-block)
  - FLOW (Mirage_flow[8] in mirage-flow)

And same thing for their respective lwt implementation (V1_LWT.CONSOLE is an 
alias to Mirage_console_lwt.S in mirage-console-lwt, etc.)

Thanks for everyone involved, especially Thomas Leonard, Mindy Preston and 
Hannes Mehnert for the groundwork and Jeremy Yallop and Leo White for the 
patience to explain the arcane of private polymorphic variants!

Best,
Thomas

[0]: https://github.com/mirage/mirage-channel/blob/master/src/channel.ml#L35
[1]: https://mirage.github.io/mirage-flow/
[2]: https://mirage.github.io/mirage-device/
[3]: https://mirage.github.io/mirage-console/
[4]: https://mirage.github.io/mirage-time/
[5]: https://mirage.github.io/mirage-clock/
[6]: https://mirage.github.io/mirage-random/
[7]: https://mirage.github.io/mirage-block/
[8[: https://mirage.github.io/mirage-flow/



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