The index of the first bit

I can see in http://hackage.haskell.org/package/base-4.7.0.0/docs/Data-Bits.html#v:bit how to convert from an Int, n, to a Bit that has the nth bit set, using

bit :: Int -> a

However, how can I do the inverse of this? (Assuming the input Bit just has 1 bit set in it?)