Fast inverse square root in Haskell - Two ways

fsqrt.hs

import Data.Bits
import Data.Word
import Foreign ( Ptr, new, castPtr, Storable(peek, poke) )
import System.IO.Unsafe
import Unsafe.Coerce

qsqrt :: Float -> Float
qsqrt x = 1 / qRsqrt x

qsqrt' :: Float -> Float
qsqrt' x = 1 / qRsqrt' x

-- Quake 3 inverse square root, using C-style pointer casting
qRsqrt :: Float -> Float
qRsqrt number = unsafePerformIO $ do
    let x2 = number * 0.5
    y <- new number
    let i = castPtr y :: (Ptr Word32)
    magic <- peek i
    let magic' = 0x5f3759df - (magic `shiftR` 1)
    poke i magic'
    y' <- peek y
    return $ y' * (1.5 - (x2 * y' * y'))

-- Quake 3 inverse square root, using unsafe coerce
qRsqrt' :: Float -> Float
qRsqrt' number =
    let x2 = number * 0.5
        magic = unsafeCoerce number :: Word32
        magic' = 0x5f3759df - (magic `shiftR` 1)
        number' = unsafeCoerce magic' :: Float
    in (number' * (1.5 - (x2 * number' * number')))

-- Compare sqrt 1 and sqrt 2 for both methods
test () = map (\x -> (qsqrt x, qsqrt' x, qsqrt x == qsqrt' x)) (take 2 [1.0..])