[Haskell-cafe] better error messages for YAML

[Adam Bergmark]

For aeson there is a <?> combinator in an "internal" modue lfor adding
custom error messages (https://github.com/bos/aeson/issues/475). There's
definitely room for improving error messages though, I hope you'd consider
helping out with that in aeson rather than rolling your own :)

But what Michael says makes a lot of sense, going through aeson is not
optimal. Especially since yaml is a superset (in terms of constructs) of
json.

- Adam

On Thu, Nov 3, 2016 at 10:55 AM Michael Snoyman <michael at snoyman.com> wrote:

> The Text.Libyaml[1] module provides a streaming interface, which allows
> more flexibility and doesn't jump through the aeson Value type at all.
> Currently, it doesn't include location information, but I _think_ the
> underlying libyaml C library makes that information available. I'd
> certainly be open to a PR to add something like:
>
> decodeWithPosition :: ByteString -> Producer m (Event, Position)
>
> And that way you don't have to rewrite a parser or the FFI bindings.
>
> [1] https://www.stackage.org/haddock/lts-7.7/yaml-0.8.20/Text-Libyaml.html
>
> On Thu, Nov 3, 2016 at 11:47 AM, Tobias Dammers <tdammers at gmail.com>
> wrote:
>
> Hello cafe,
>
> I have a bit of a conundrum here.
>
> The situation is such that I'm using the YAML package to parse YAML
> configuration files; some of the values in the file format require
> further parsing (using my own ginger library), and all that works fine.
>
> One problem though; since this is aimed at end users, I want to preset
> error messages that pinpoint the exact error location in case the ginger
> parse fails. However, the ginger parser doesn't get to see the entire
> YAML source, because it runs on just an individual string value inside
> the YAML, and the YAML parser itself doesn't report source positions
> either. The unfortunate result is that while I get "technically correct"
> parser errors from Ginger, the source positions they report are relative
> to the string value, i.e., all errors are reported as being on line 1 of
> an unnamed source file.
>
> The relevant code is here:
>
>
> https://github.com/tdammers/sprinkles/blob/master/src/Web/Sprinkles/Replacement.hs#L40
>
> The only solution I can think of right now involves writing my own YAML
> parser, and probably also an Aeson drop-in replacement; I don't really
> want to do that though.
>
> Is there any other way to get what I want?
>
> --
> Tobias Dammers - tdammers at gmail.com
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