YAML Config error messages are pretty bad

[MaxGabriel]

The error messages you get from a project using the YAML config parser, like a scaffolded yesod project, are pretty bad. There isn't much indication of what caused the problem at all. For example, today I had some config like this:

slack:
  api-token: "_env:SLACK_API_TOKEN:1234"

And I removed 1234 because I didn't want to send Slack messages from my localhost after I was done testing. But that caused this error:

mercury-web-backend: Could not convert to expected type: expected Text, encountered Null
CallStack (from HasCallStack):
  error, called at src/Data/Yaml/Config.hs:201:20 in yaml-0.11.0.0-KWY10UhaZfM1oXgmBmK7aY:Data.Yaml.Config

Which doesn't give any indication of what it was trying to parse. I actually thought it was a different field for 10 minutes or so.

Edit: sorry, posted before I was finished writing.

[snoyberg]

I won't claim to understand how things work in aeson, but the change I introduced in 4210fa0 seems to produce better error messages. Can you give it a shot before I release?

[MaxGabriel]

Hm, I'm still getting bad error messages. I did this:

1. Scaffolded a yesod-postgres site

2. Added this to extra-deps, with allow-newer: true in my global config

extra-deps:
- git: https://github.com/snoyberg/yaml
  commit: 4210fa09e7b043ff18f71e32e74055fcbb418be1
  subdirs:
  - yaml
- libyaml-0.1.1.0@sha256:b3fcd8c44622c75e054c2267f3fec39a58a311748000310cbc8257a4683d3f02

3. Set copyright in settings.yml to copyright: "_env:COPYRIGHT:"

4. Ran stack run and got this:

/private/tmp/new-yaml5/src/Settings/StaticFiles.hs:1:1: error:
    Exception when trying to run compile-time code:
      expected Text, encountered Null
CallStack (from HasCallStack):
  error, called at src/Settings.hs:132:20 in new-yaml5-0.0.0-9rQ0xSiEFL2IyOnxx8O9Pf:Settings
    Code: staticFiles (appStaticDir compileTimeAppSettings)
  |
1 | {-# LANGUAGE NoImplicitPrelude #-}
  | ^

Completed 5 action(s).

--  While building package new-yaml5-0.0.0 using:
      /Users/maximiliantagher/.stack/setup-exe-cache/x86_64-osx/Cabal-simple_mPHDZzAJ_2.2.0.1_ghc-8.4.4 --builddir=.stack-work/dist/x86_64-osx/Cabal-2.2.0.1 build lib:new-yaml5 exe:new-yaml5 --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

[snoyberg]

Can you try to repro it against just the yaml library? It may be that this is something in Yesod.

[majkrzak]

Same problems occurs with non JSON-ed yaml parsing, so Data.Yaml.ParserreadYamlFile for example.