id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3563,A couple of additions to Data.Bits,porges,,"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)
}}}",proposal,closed,normal,Not GHC,libraries/base,6.11,wontfix,,merehap@… gabriel@…,Unknown/Multiple,Unknown/Multiple,None/Unknown,Unknown,,,,