[GHC] #5688: instance Read Integer/Rational/Double readsPrec out of memory and crash due to exponential notation

[GHC]

#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 maeder):

 The precise number given by the input string needs to be converted to the
 nearest representable number (by the round-to-nearest rule of IEEE 754
 floating-point arithmetic).

 If we assume that `fromRational` does this correctly, it is enough to
 roughly estimate when the value will definitely be +/- Infinity/Zero (in
 order to avoid computing an overly large Rational value causing this
 ticket).

 Roughly estimating makes failing inconsistent (at least not precise).

 It remains to check, if `fromRational` does the right thing. I assume it
 replaces "%" by "/" and converts the two arguments from Integer to
 Double/Float first. (Where is the code for this?)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5688#comment:47>
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