[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):

 The literal `1e100` means `fromRational (100...000 :: Rational)`, as
 specified in Haskell report. Constructing this number takes time and space
 proportional to the number of zeroes. It's not surprising it crashes badly
 when the exponent has 10 digits or more. On the other hand, the type of
 `10^1000000000` can be found quickly because the expression is not
 evaluated.

 The big integer is already created during parsing (as can be seen by
 compiling `main = print 1e1000` with `-ddump-parsed-ast`). With
 `-XNumDecimals`, we need to process the literal before we can tell its
 type: `1.234e3` is a valid `Integer` but `1.234e2` is not. I don't see any
 easy way to fix this and preserve backwards compatibility. Perhaps we
 could show a parse error when attempting to create an integer with an
 unrealistic exponent.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:6>
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