[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 simonpj):

 > But this relies on laziness, no?

 No, it doesn't rely on laziness. At no stage will GHC construct that
 Rational.  The desugarer will generate something like
 {{{
 Var 'fromRational' `App` (Var 'readRational' `App` Lit (LitString
 "1e100"))
 }}}
 That is, it'll generate Core that will, when compiled and run, compute the
 rational (at runtime).  But the compiler just manipulates this Core data
 structure, there is no bad Rational in it.  Does that help?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15646#comment:19>
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