#8823: showFloat for higher precision types produces strange results for some values --------------------------+------------------------------------------------ Reporter: axman6 | Owner: Type: bug | Status: new Priority: low | Milestone: Component: | Version: 7.8.1-rc1 Prelude | Operating System: Unknown/Multiple Keywords: | Type of failure: Incorrect result at runtime Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: | Unknown | Blocked By: | Related Tickets: | --------------------------+------------------------------------------------ I've written a library which is a quda-double type a la the QD C/C++ package, and showFloat does not behave correctly for numbers with such high precision. My type has ~212 bits of precision, and when using showFloat from Numeric, I get strange results for integer values: {{{ show (1 :: QDouble) = "0.00000000000000000000000000000000000000000000001e47" show (1.1 :: QDouble) = "1.1" show (1000 :: QDouble) = "0.00000000000000000000000000000000000000000000001e50" -- These seems to suggest it happens for any number with only a -- few high bits set to 1 in the result of decodeFloat show (1.125 :: QDouble) = "0.00000000000000000000000000000000000000000000001125e47" show (1.625 :: QDouble) = "0.00000000000000000000000000000000000000000000001625e47" }}} The problem seems to be related to the result of floatDigits, which starts causing problem when it's larger than 56 (floatDigits x, show x): {{{ (56,"1.0") (57,"01.0") (60,"001.0") (212,"0.00000000000000000000000000000000000000000000001e47") }}} My fix has been to use a modified version of showFloat from Numeric by changing the floatToDigits function to include a fix for times when large numbers of zeros are produced: {{{ fixup2 (xs,k) = let (zs,ys) = span (==0) xs in (ys,k-length zs) in fixup2 (map fromIntegral (reverse rds), k) }}} This fixes the symptom but not the issue itself (though it seems like a reasonable thing to have any result returned by floatToDigits. I have attached as minimal test case as I could come up with. Using floatToDigits from Numeric causes the strange behaviour, while floatToDigits' included in the test case does not. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8823 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler