#4867: ghci displays negative floats incorrectly (was: Incorrect result from
trig
functions)
-------------------------------+--------------------------------------------
Reporter: gwright | Owner: gwright
Type: bug | Status: new
Priority: high | Milestone: 7.0.2
Component: GHCi | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: MacOS X | Blocking:
Architecture: x86_64 (amd64) | Failure: Incorrect result at runtime
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Comment(by gwright):
The source of this trouble seems to be in `formatRealFloat` in
`libraries/Base/GHC/Float.lhs`.
Here's the top of that function:
{{{
formatRealFloat :: (RealFloat a) => FFFormat -> Maybe Int -> a -> String
formatRealFloat fmt decs x
| isNaN x = "NaN"
| isInfinite x = if x < 0 then "-Infinity" else "Infinity"
| x < 0 || isNegativeZero x = '-' : doFmt fmt (floatToDigits (toInteger
base) (-x))
| otherwise = doFmt fmt (floatToDigits (toInteger base)
x)
where
base = 10
...
}}}
The clue is that the incorrectly displayed numbers always have the correct
sign. The guard `| x < 0 || isNegativeZero` is satisfied, so the minus
sign is prefixed to the formatted digits. In retrospect, if I had paid
more attention to the gdb traces I did earlier, I would have seen this.
Still don't know exactly what goes wrong yet.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4867#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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