[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 monoidal):

 > 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.

 It's not even GCD: just computing the nominator of 1e1000000000 in the
 binary representation requires at least `log2(10**1000000000)` bits of
 memory, which is over 396GB.

 If we'd like to fix this, I see the following options:
 a. Move computation of n,d from typechecking to desugaring. This should
 allow `:t 1e1000000000`, but `let x _ = 1e1000000000` will still crash.
 b. Add a method to `Fractional`, say `fromMantissaExp :: Integer ->
 Integer -> Integer -> a` that will be called instead of `fromRational`,
 with a default implementation that calls `fromRational`. This will allow
 `let x _ = 1e1000000000` to work, we could make it return Infinity for
 `Double`. This will likely have performance implications (I'm not sure if
 positive or negative).
 c. Change the representation of `Ratio` to have an extra constructor that
 represents numbers of the form `N / D * 10**E`.

 All those solutions need some extra special cases for `-XNumDecimals`.
 It's doable, I'm not convinced it's worth the maintenance cost but I won't
 protest if it's done.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:10>
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