Re: [MirageOS-devel] Using Result instead of Option in libraries

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

> On 14 Oct 2016, at 15:05, David Scott <scott.dj@xxxxxxxxx> wrote:
> 
> 
> On Fri, Oct 14, 2016 at 2:51 PM, Anil Madhavapeddy <anil@xxxxxxxxxx> wrote:
>> We have quite a few base libraries that use the pattern of
>> 
>> val foo_exn : ... -> 'a
>> @raises
>> 
>> val foo: ... -> 'a option
>> Gobbles the exception and returns Some/None
>> 
>> Should we take the Mirage3 opportunity to port libraries like Ipaddr to 
>> using the Result type instead, so it would be
>> 
>> val foo : ... -> ('a, [`Msg of string]) result
>> 
>> instead, using the Result type?  That would let libraries use combinators 
>> such as Rresult, and not gobble errors from parsing silently.  It would be 
>> an incompatible API bump so we would need to bump all consumers of, e.g. 
>> Ipaddr  http://docs.mirage.io/ipaddr/Ipaddr_unix.V4.html simultaneously.

> Personally I like the sound of this-- it's nice to use combinators to parse a 
> more complicated structure and be able to emit something more useful than 
> "None" as an error. Should we write a "recommended conventions" doc or wiki 
> page to include things like `foo_exn` and `foo` examples above?

Yeah, once we agree on the conventions :-)  Once we have everything using 
Result.t, we also need to find the right set of combinators.

- There is Rresult for basic Result.t handling: 
http://erratique.ch/software/rresult/doc/Rresult.html
- Lwt_result has a slightly different set of combinators 
https://github.com/ocsigen/lwt/blob/master/src/core/lwt_result.mli

So I guess we need to decide if we publish an Rresult_lwt.t which lifts up 
"('a,'b) result" into an Lwt.t with the same API as Rresult otherwise.

Anil

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