[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):

 > Why not do the parsing in compile time generate Core for this instead:

 Yes, I agree.  That's what I meant in comment:14 with `fl_before` etc, but
 I expressed it badly.  Let me try to do better.  Accounting for monoidal's
 excellent point we could have
 {{{
 data FractionalLit
   = FL { fl_text :: SourceText     -- How the value was written in the
 source
        , fl_neg :: Bool            -- See Note [Negative zero]
        , fl_mantissa, fl_exp :: Integer
                                    -- Denotes <mantissa>E<exp>
        }
 }}}
 So `1.077E400` would be represented with `fl_mantissa = 1077` and `fl_exp
 = 397`.

 Then we desugar to
 {{{
 Var 'makeRational'  `App` (Lit 1077) `App` (Lit 397)
 }}}
 where `makeRational` is a new library function defined thus
 {{{
 makeRational :: Integer -> Integer -> Integer
 makeRational i e = (i % 1) * (10 ^^ e)
 }}}
 Probably we want it to inline, but that's a separate matter.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:24>
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