#5872: bug in default implementation of popCount
------------------------------+---------------------------------------------
Reporter: lerkok | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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base 4.5.0.0
(http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/src
/Data-Bits.html#popCount) gives the default implementation of popCount of
the Bits class like this:
{{{
popCount :: a -> Int
popCount = go 0
where
go !c 0 = c
go c w = go (c+1) (w .&. w - 1)
}}}
Alas, .&. binds more tightly than -, thus the last expression parses as
`((w .&. w) - 1)` not as `(w .&. (w-1))`, as it was intended.
At the least, this causes `popCount :: Integer -> Int` to behave
erratically:
{{{
Prelude Data.Bits> map popCount [2::Integer .. 4]
[2, 3, 4]
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5872>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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