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@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@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@gmail.com
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