Control.Lensで指定したビットフィールドを取り出す演算子(.#.)をつくったところ
"bfget"の型がどうなっているのかわからなくなってしまいました。
お知恵をお借りしたく... (ghc version 7.6.3, lens-4.3.3)
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{-# LANGUAGE TemplateHaskell #-}
module A where

import Control.Lens (ALens',makeLenses,cloneLens,lens,to,(.~),(%~),(&),(^.))
import Data.Word (Word32,Word64)
import Data.Bits (Bits,FiniteBits,complement,shiftL,shiftR,rotateL,(.|.),(.&.))
import Test.HUnit (Test,test,runTestTT,(~=?))

data Regs = Regs { _r0 :: Word32, _r1 :: Word64 } deriving (Show, Eq)
$(makeLenses ''Regs)

(.#.) :: (Functor f, FiniteBits a, Bounded a, Integral a) =>
ALens' Regs a -> (Int, Int) -> ((a -> f a) -> Regs -> f Regs)
(.#.) r (lsb, nbits) = let
-- ??? bfget :: Integral a => Regs -> a
bfget rs = rs ^. (cloneLens r) . to (flip shiftR lsb . (.&. complement mask))
bfset :: Integral a => Regs -> a -> Regs
bfset rs x = rs & (cloneLens r) %~ \ w -> (mask .&. w) .|. (complement mask .&. shiftL (fromIntegral x) lsb)
mask :: (Bits a, Bounded a) => a
mask = flip rotateL lsb $ shiftL maxBound nbits
in lens bfget bfset