Using ImplicitParams to guide type inference

implicit-inference.hs

-- asks is frequently too ambiguous
impAsks
  :: forall a b es
   . (?impReader :: Proxy a)
  => Reader a :> es
  => (a -> b)
  -> Eff es a
impAsks = asks

withImpReader
  :: forall a k
   . (?impReader :: Proxy a => k)
  -> k
withImpReader k = let ?impReader = Proxy @a in k

-- This example contains usage of 'asks' that would
-- normally be ambiguous but are not due to the implicit param
example :: Eff '[Reader Int, Reader String] Int
example =
  withImpReader @Int $ do
    x <- impAsks (+1)
    withImpReader @String $ do
      s <- impAsks (++ "hello world")
      pure (x + length s)