#3676: realToFrac doesn't sanely convert between floating types
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Reporter: draconx | Owner: simonmar
Type: bug | Status: assigned
Priority: normal | Milestone: _|_
Component: libraries (other) | Version: 6.12.1
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: x86_64 (amd64) | Failure: None/Unknown
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Changes (by simonmar):
* milestone: 6.12.2 => _|_
Comment:
I rolled back the patch to libraries/base:
{{{
Tue Feb 23 10:16:03 GMT 2010 Simon Marlow <marlo...@gmail.com>
* UNDO: Handle NaN, -Infinity and Infinity in the toRational for
Float/Double (#3676)
}}}
in retrospect it wasn't a good idea to shoehorn these strange values into
Rational.
The position now is that `toRational (1/0 :: Double)` is undefined. The
Haskell standard does not define the result (it should), and GHC gives
unpredictable results. Hence, `realToFrac` cannot convert accurately
between floating-point types.
Someone should:
* propose a proer API for conversion between floating-point types, e.g.
`FloatConvert` above, but make a full proposal.
* decide whether `toRational (1/0::Double)` should be undefined or an
exception. If it is an exception, then we cannot optimise `realToFrac`
to a direct conversion, e.g. `floatToDouble#`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3676#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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