[GHC] #15646: ghci takes super long time to find the type of large fractional number

[GHC]

#15646: ghci takes super long time to find the type of large fractional number
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        Reporter:  Johannkokos       |                Owner:
                                     |  JulianLeviston
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  GHCi              |              Version:  8.4.3
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Wait, are you saying that typing 1.7e1234111111111111111111111 to take
 minutes and use up more than 20G of memory (residence, not allocation!)
 not surprising?

 I agree: that's absurd!

 HOwever, unlike Python, we can't just compute a suitable `Float`, because
 literals are overloaded.  The
 [https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-240003.2
 Report] does say that a floating point literal like `1e100` means
 `fromRational (n % d)` where "the integers n and d are chosen so that n/d
 =  f".

 I suspect that in computing `17 % 10000000000000000000000` we try to find
 the GCD of the two before we even start with `fromRational`, and you can
 see this isn't going to end well.

 What to do?  Probably we need a special case for `fromRational :: Rational
 -> Float` (and similarly `Double`); and maybe even a special literal
 representation inside GHC for `Rational`s  of form `N / 10000000000000`
 for some number of zeros.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:8>
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