A couple of additions to Data.Bits

Population count is an often-needed function when bitfiddling, so I think a version should be supplied. I also have included functions for converting to and from lists of Booleans:

-- population count
popCount :: (Bits a, Num t) => a -> t
popCount x = count' (bitSize x) x 0
  where
  count' 0 _ acc = acc
  count' n x acc = count' (n-1) (x `shiftR` 1) (acc + if x .&. 1 == 1 then 1 else 0)
                -- this weird if/else is to preserve the nice type signature :)

-- converts a list of bools to a number
fromBools :: (Bits a) => [Bool] -> a
fromBools = foldl' (\i b -> (i `shiftL` 1) .|. if b then 1 else 0) 0 -- likewise

-- converts a number to a list of bools
toBools :: (Bits a) => a -> [Bool]
toBools x = reverse (toBools' (bitSize x) x)
  where
  toBools' 0 _ = []
  toBools' n x = (x .&. 1 == 1) : toBools' (n-1) (x `shiftR` 1)

Trac metadata

Trac field	Value
Version	6.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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