How to read Yesod error messages effectively?

Question

(newbie alert)

Given the following error message, what's the quickest way to get to the source of the error:

08/Jul/2016:11:39:01 +0530 [Error#yesod-core] expected EPlain but got Nothing for: DerefBranch (DerefIdent (Ident "show")) (DerefString "abcdef") @(yesod_3MCr4WfhviiELXmo3fAaXL:Yesod.Core.Class.Yesod ./Yesod/Core/Class/Yesod.hs:625:5)
GET /
  Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
  Status: 500 Internal Server Error 0.054158s

Handler/Home.hs:38:11:
    No instance for (Text.Julius.ToJavascript String)
      arising from a use of ‘Text.Julius.toJavascript’
    In the second argument of ‘(GHC.Base..)’, namely
      ‘Text.Julius.toJavascript’
    In the second argument of ‘(GHC.Base..)’, namely
      ‘(Text.Julius.unJavascript GHC.Base.. Text.Julius.toJavascript)’
    In the expression:
      Text.Shakespeare.EPlain
      GHC.Base..
        (Text.Julius.unJavascript GHC.Base.. Text.Julius.toJavascript)
Build failure, pausing...

I'm working with a simple scaffolded site (no DB) and have purposely messed up with homepage.julius. In this particular instance I know exactly what the error is, but how can one know just by looking at the error message?

Answer 1

After perusing http://hackage.haskell.org/package/shakespeare-2.0.8/docs/Text-Julius.html, it looks like you are trying to embed a plain string in something that wants javascript.

That is usually prevented so that someone can't inject bad code into a page when you are trying to just display a string they've supplied. So just call rawJS on it and it should embed? Or perhaps you are using the wrong interpolation type for the variable you are supplying (@,^, or #). Yesod is particular about that.

This just just a guess as I don't use julius.