Real-Complex type conversion within Exp / Acc

Question

I need to convert a real numerical value within the Exp or Acc type of the Accelerate library to its corresponding Complex value, e.g.:

toComplex :: Exp Double -> Exp (Complex Double)
toComplex' :: Acc (Vector Double) -> Acc (Vector (Complex Double))

I tried to pattern match like

toComplex :: Exp Double -> Exp (Complex Double)
toComplex a = (\(r, i) -> constant $ r :+ i) $ (a, constant 0)

and suspect that lift or unlift as in https://www.acceleratehs.org/examples/mandelbrot.html might somehow solve this problem but can't figure out correct application of those.

For my attempt ghc complains that

• Couldn't match type ‘Exp Double’ with ‘Double’
      Expected type: Exp (Complex Double)
        Actual type: Exp (Complex (Exp Double))