What can cause jsaddle Results decode failed exception when using fun?

[Wizek]

I've been trying to use the fun and function functions from here: https://hackage.haskell.org/package/jsaddle-0.9.4.0/docs/Language-Javascript-JSaddle-Object.html#v:fun

And I've ran into exceptions like this:

jsaddle3-exe: jsaddle Results decode failed : "{\"tag\":\"Callback\",\"contents\":[0,{},-2,101,102,[103]]}"
CallStack (from HasCallStack):
  error, called at src/Language/Javascript/JSaddle/WebSockets.hs:85:40 in jsaddle-warp-0.9.5.0-EK8DFrM2ZlV7tdElT9ks2n:Language.Javascript.JSaddle.WebSockets

Here is a reproducible minimal example from the docs: Wizek/jsaddle3@967b32~2...967b32

Aside; I've also tried creating a minimal repro case with nix like this:

#!/usr/bin/env nix-shell
#! nix-shell -p "haskell.packages.ghc822.ghcWithPackages (p: with p; with haskell.lib; [(doJailbreak (dontCheck jsaddle-warp))])"
#! nix-shell -i "runghc"

{-# language OverloadedStrings #-}

import            Control.Monad.IO.Class
import            Language.Javascript.JSaddle.Warp
import            Language.Javascript.JSaddle
import            Language.Javascript.JSaddle.Debug
import            Control.Concurrent
import            Control.Monad
import            Data.Text
import            Control.Concurrent.MVar

main = run 3197 $ do
  result <- liftIO newEmptyMVar
  deRefVal $ call (eval "(function(f) {f('Hello');})") global [fun $ \ _ _ [arg1] -> do
    valToText arg1 >>= (liftIO . putMVar result)
    ]
  liftIO $ takeMVar result

But I've run into the following likely unrelated issue:

Configuring jsaddle-warp-0.9.5.0...
CallStack (from HasCallStack):
  die', called at libraries/Cabal/Cabal/Distribution/Simple/Configure.hs:948:20 in Cabal-2.0.1.0:Distribution.Simple.Configure
  configureFinalizedPackage, called at libraries/Cabal/Cabal/Distribution/Simple/Configure.hs:470:12 in Cabal-2.0.1.0:Distribution.Simple.Configure
  configure, called at libraries/Cabal/Cabal/Distribution/Simple.hs:570:20 in Cabal-2.0.1.0:Distribution.Simple
  confHook, called at libraries/Cabal/Cabal/Distribution/Simple/UserHooks.hs:67:5 in Cabal-2.0.1.0:Distribution.Simple.UserHooks
  configureAction, called at libraries/Cabal/Cabal/Distribution/Simple.hs:174:19 in Cabal-2.0.1.0:Distribution.Simple
  defaultMainHelper, called at libraries/Cabal/Cabal/Distribution/Simple.hs:119:27 in Cabal-2.0.1.0:Distribution.Simple
  defaultMain, called at Setup.lhs:5:10 in main:Main
Setup: Encountered missing dependencies:
websockets >=0.9.5.0 && <0.11
builder for ‘/nix/store/76y9xrhvb0hmq2s46qzsvqv1131z13yh-jsaddle-warp-0.9.5.0.drv’ failed with exit code 1
killing process 105483

A bit of background:

• Compiling with GHC
• I'm trying to explore these functions to overcome the issue stated in Q: Are synchronous, UI-blocking XHR requests meant to be issued by JSaddle? #51.

[Wizek]

I've updated the repro case a bit to get jsaddle too from the same GH commit; new commits will
be visible in this view: Wizek/jsaddle3@967b32~2...master

[Wizek]

If anyone is reading this later, in brief testing sessions fb9ba5a seems to have fixed the issue.