#4383: Uncanonical display of Double
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Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 6.12.3 | Keywords: Double, show
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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Comment(by daniel.is.fischer):
For starters, a better approximation to logBase 10 2 fixes this,
{{{
if b == 2 && base == 10 then
-- logBase 10 2 is slightly bigger than 3/10 so
-- the following will err on the low side. Ignoring
-- the fraction will make it err even more.
-- Haskell promises that p-1 <= logBase b f < p.
-- (p - 1 + e0) * 3 `div` 10
(p - 1 + e0) * 21306 `quot` 70777
*FloatShow> let x :: Double; x = 0.5 ^ 1030
*FloatShow> floatToDigits 10 x
([8,6,9,1,6,9,4,7,5,9,7,9,4],-310)
*FloatShow> showFloat x ""
"8.691694759794e-311"
}}}
but I'd rather check the code carefully to see whether something else
might need fixing (and maybe an earlier convergent of logBase 10 2 will
suffice).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4383#comment:2>
GHC <http://www.haskell.org/ghc/>
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