Reproduction of a bad type error message

[chrisdone-artificial]

@austin-artificial found this good repro example of a bad type error message.

main = do
  env0 <- Environment.getEnvironment
  let env1 = [("X", "Y")]

  env <- List.concat [env0, env1]

  Main.runServer env

runServer = \env ->
  Process.runProcess $ Process.setEnv env $ Process.proc "server" []

❯ ./result/bin/hell examples/33-type-error.hell
hell: Unification error: Couldn't match type

  Text

against type

  (((,) Text) Text)

arising from examples/33-type-error.hell:3:15
CallStack (from HasCallStack):
  error, called at src/Hell.hs:174:33 in main:Main

The error is reported as if the let is in the wrong, when really the [env0, env1] is where the conflicting types are happening.

There's prior art for good error messages: HIW 2014: Lennart Augustsson: Better type-error messages - YouTube

But there might be more light-weight approaches, such as listing all unification errors, and not just stopping at the first.