[GHC] #15993: Bitwise-oriented semigroup and monoid newtype wrappers for Data.Bits.Bits instances

4 Dec 2018

      #15993: Bitwise-oriented semigroup and monoid newtype wrappers for Data.Bits.Bits
instances
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           Reporter:  koz_           |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.3
          Component:                 |           Version:  8.6.2
  libraries/base                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I've found myself needing these often, and given the existence of similar
 newtypes for directing `(<>)` and `mempty` for a range of other types,
 these are conspicuous by their absence.  Additionally, while `oneBits`
 isn't technically necessary, it's a lot more concise than `complement
 zeroBits`, and I found myself needing it often.

 To that end, I've sketched up this implementation. Technically speaking,
 GND isn't needed, but it means I don't have to repeat myself a lot. These
 could be made even more concise with `coerce`, but I decided not to do
 that, since that would require `TypeApplications`. I've limited derivation
 to methods that are specifically about bitwise operations, rather than
 stuff like `Num`.

 {{{#!hs
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}

 module Data.Bits.Extra where

 import           Data.Bits                      ( Bits(..)
                                                 , FiniteBits(..)
                                                 )

 -- | Monoid under bitwise AND.
 newtype Conj a = Conj { getConj :: a }
   deriving (Eq, Bounded, Enum, Bits, FiniteBits)

 instance (Bits a) => Semigroup (Conj a) where
   (Conj x) <> (Conj y) = Conj (x .&. y)

 instance (Bits a) => Monoid (Conj a) where
   mempty = Conj . complement $ zeroBits

 -- | Monoid under bitwise OR.
 newtype Disj a = Disj { getDisj :: a }
   deriving (Eq, Bounded, Enum, Bits, FiniteBits)

 instance (Bits a) => Semigroup (Disj a) where
   (Disj x) <> (Disj y) = Disj (x .|. y)

 instance (Bits a) => Monoid (Disj a) where
   mempty = Disj zeroBits

 -- | Semigroup under bitwise XOR.
 newtype Xor a = Xor { getXor :: a }
   deriving (Eq, Bounded, Enum, Bits, FiniteBits)

 instance (Bits a) => Semigroup (Xor a) where
   (Xor x) <> (Xor y) = Xor (x `xor` y)

 -- | Semigroup under bitwise \'equality\'; defined as '1' if the
 corresponding
 -- bits match, '0' otherwise.
 newtype Iff a = Iff { getIff :: a }
   deriving (Eq, Bounded, Enum, Bits, FiniteBits)

 instance (Bits a) => Semigroup (Iff a) where
   (Iff x) <> (Iff y) = Iff . complement $ (x `xor` y)

 -- not strictly necessary, but would be a big help
 -- probably should be INLINE
 oneBits :: (Bits a) => a
 oneBits = complement zeroBits
 }}}

 Potentially this could include more, such as instances of `Ord` based on
 lexicographic ordering on the bits, rather than defaulting to the
 underlying one (such as the one for `Int`, which I believe is numeric).
 However, that's a separate issue. I also don't know if these belong in
 `Data.Bits` or `Data.Semigroup` (or `Data.Monoid` I guess).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15993
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