Problems with type inference, `HasAwsCredentials env String` and `Member (Reader env) effs`

UsingHasConfig.hs

#!/usr/bin/env stack
-- stack --resolver lts-12.16 script --package freer-simple --package lens
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
module Test where

import Control.Lens (makeLensesWith, camelCaseFields)
import Control.Lens ((^.))
import Control.Monad.Freer (Eff, Member, LastMember)
import qualified Control.Monad.Freer as Freer
import Control.Monad.Freer.Reader (Reader)
import qualified Control.Monad.Freer.Reader as Reader

data Config = Config
  { configAwsCredentials :: String
  , configOrganization :: String
  } deriving (Show)

makeLensesWith camelCaseFields ''Config

test :: (HasAwsCredentials env String, Member (Reader env) effs, LastMember IO effs) => Eff effs String
test = do
  config <- Reader.ask
  pure $ config ^. awsCredentials

main :: IO ()
main = do
  let
    config = Config
      { configAwsCredentials = "Test Creds"
      , configOrganization   = "Codetalk"
      }
  creds <- Freer.runM . Reader.runReader config $ test
  putStrLn creds
  
  
{-
λ ~/Desktop ./UsingHasConfig.hs

/Users/tehnix/Desktop/UsingHasConfig.hs:27:13: error:
    • Overlapping instances for Member (Reader s0) effs
        arising from a use of ‘Reader.ask’
      Matching givens (or their superclasses):
        Member (Reader env) effs
          bound by the type signature for:
                     test :: forall env (effs :: [* -> *]).
                             (HasAwsCredentials env String, Member (Reader env) effs,
                              LastMember IO effs) =>
                             Eff effs String
          at /Users/tehnix/Desktop/UsingHasConfig.hs:25:1-103
      Matching instances:
        instance (Data.OpenUnion.Internal.FindElem t r,
                  Data.OpenUnion.Internal.IfNotFound t r r) =>
                 Member t r
          -- Defined in ‘Data.OpenUnion.Internal’
      (The choice depends on the instantiation of ‘s0, effs’)
    • In a stmt of a 'do' block: config <- Reader.ask
      In the expression:
        do config <- Reader.ask
           pure $ config ^. awsCredentials
      In an equation for ‘test’:
          test
            = do config <- Reader.ask
                 pure $ config ^. awsCredentials
   |
27 |   config <- Reader.ask
   |             ^^^^^^^^^^

/Users/tehnix/Desktop/UsingHasConfig.hs:28:20: error:
    • Could not deduce (HasAwsCredentials s0 String)
        arising from a use of ‘awsCredentials’
      from the context: (HasAwsCredentials env String,
                         Member (Reader env) effs, LastMember IO effs)
        bound by the type signature for:
                   test :: forall env (effs :: [* -> *]).
                           (HasAwsCredentials env String, Member (Reader env) effs,
                            LastMember IO effs) =>
                           Eff effs String
        at /Users/tehnix/Desktop/UsingHasConfig.hs:25:1-103
      The type variable ‘s0’ is ambiguous
      Relevant bindings include
        config :: s0
          (bound at /Users/tehnix/Desktop/UsingHasConfig.hs:27:3)
      These potential instance exist:
        instance HasAwsCredentials Config String
          -- Defined at /Users/tehnix/Desktop/UsingHasConfig.hs:23:1
    • In the second argument of ‘(^.)’, namely ‘awsCredentials’
      In the second argument of ‘($)’, namely ‘config ^. awsCredentials’
      In a stmt of a 'do' block: pure $ config ^. awsCredentials
   |
28 |   pure $ config ^. awsCredentials
   |                    ^^^^^^^^^^^^^^

/Users/tehnix/Desktop/UsingHasConfig.hs:37:51: error:
    • Ambiguous type variable ‘env0’ arising from a use of ‘test’
      prevents the constraint ‘(HasAwsCredentials
                                  env0 String)’ from being solved.
      Probable fix: use a type annotation to specify what ‘env0’ should be.
      These potential instance exist:
        instance HasAwsCredentials Config String
          -- Defined at /Users/tehnix/Desktop/UsingHasConfig.hs:23:1
    • In the second argument of ‘($)’, namely ‘test’
      In a stmt of a 'do' block:
        creds <- Freer.runM . Reader.runReader config $ test
      In the expression:
        do let config = ...
           creds <- Freer.runM . Reader.runReader config $ test
           putStrLn creds
   |
37 |   creds <- Freer.runM . Reader.runReader config $ test
   |                                                   ^^^^
-}

For posterity's sake, I'll include the fix here: We needed to enable three more language extensions,

...
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE MonoLocalBinds #-}
{-# LANGUAGE TypeApplications #-}
...

And test becomes,

test :: forall env effs. (HasAwsCredentials env String, Member (Reader env) effs, LastMember IO effs) => Eff effs String
test = do
  config <- Reader.ask @env
  pure $ config ^. awsCredentials

where you can notice the explicit forall and the Reader.ask @env type application. Finally we also added type annotations in main, with creds <- Freer.runM . Reader.runReader config $ test @Config.