[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:  (none)
            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 osa1):

 The discussion may be confusing but the proposed change is actually quite
 simple. Just refactor one type and follow the type errors. In more
 details:

 - Update the `FractionalLit` type as shown in comment:14. There are two
   variants, pick one. I'd pick the one with
 `fl_before`/`fl_after`/`fl_exp`.

 - `readRational` (the slow function that causes this ticket) is used by
   DynFlags and CmmLex too, so to keep things simpler let's keep the
 original
   `readRational` and add a new variant that returns `FractionalLit`
 (instead of
   `Rational`). This is the fast variant.

 - Replace `readFractionalLit` in Lexer.x with the new fast variant of
   `readRational`.

 Sounds like a good first ticket to me.

 Simon,

 > No, it won'r blow up when desugaring. Just desugar the constant to
 > fromRational (readRational "1E1000"). Nothing blows up there!

 But this relies on laziness, no? I thought you said this is not a good
 idea in
 comment:14.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:18>
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