#5688: instance Read Integer/Rational/Double readsPrec out of memory and crash
due
to exponential notation
-------------------------------+--------------------------------------------
Reporter: gracjan | Owner: igloo
Type: bug | Status: new
Priority: highest | Milestone: 7.4.2
Component: libraries/base | Version: 6.12.3
Resolution: | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Runtime crash | Difficulty: Unknown
Testcase: | Blockedby:
Blocking: | Related:
-------------------------------+--------------------------------------------
Comment(by igloo):
OK, in the `Read Double/Float` instances, we now have access to the
integer part, the fractional part, and the exponent. The remaining part of
the problem is how to convert these into a `Double/Float`.
One approach that doesn't work well is converting the `x.y` part and then
multiplying by `(10 ^ z)`. For example, given `4.2e-1` we would like to
get this `Double`:
{{{
Prelude> 0.42 :: Double
0.42
}}}
but we would actually get this `Double`:
{{{
Prelude> (4.2 :: Double) * (10 ** (-1))
0.42000000000000004
}}}
Something else that works with the current code, is:
{{{
Prelude> read (show (10 ^ 1000) ++ "e-1000") :: Double
1.0
}}}
despite this:
{{{
Prelude> read (show (10 ^ 1000)) :: Double
Infinity
}}}
and similarly
{{{
Prelude> read ("0." ++ replicate 1000 '0' ++ "1e1001") :: Double
1.0
}}}
despite:
{{{
Prelude> read ("0." ++ replicate 1000 '0' ++ "1") :: Double
0.0
}}}
So my thoughts are that the first example should be supported (i.e. give
us 0.42), but the second and third should break (I'm not sure what they
should return. Whichever of Infinity, -Infinity, NaN and 0.0 best fits the
implementation, I guess).
What do people think?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5688#comment:41>
GHC <http://www.haskell.org/ghc/>
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