[Haskell-beginners] How to call popCnt64#?

Hello Haskellers, Does anyone know how to call popCnt64# from the GHC.Prim module? This was my failed attempt: λ> popCnt64# 1 <interactive>:14:11: Couldn't match kind ‘*’ with ‘#’ When matching types a0 :: * Word# :: # Expected type: Integer -> Word# Actual type: Integer -> a0 In the first argument of ‘popCnt64#’, namely ‘1’ In the expression: popCnt64# 1 In an equation for ‘it’: it = popCnt64# 1 -John