[GHC] #14052: Significant GHCi speed regression with :module and `let` in GHC 8.2.1

[GHC]

#14052: Significant GHCi speed regression with :module and `let` in GHC 8.2.1
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  GHCi              |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Note that in `GenExample`, the crucial bit is here:

 {{{#!hs
 genExample :: Int -> IO ()
 genExample i = putStr $ unlines
   [ ":m *Foo"
   , "example : \"expr" ++ show i ++ "\""
   , "let foo = it"
   , "\"marker\""
   , "let it = foo"
   ]
 }}}

 Both the `:m *Foo` part, as well as `let foo = it`/`let it = foo`, are
 crucial to triggering the slowdown. If one of them is commented out, then
 GHCi 8.2.1 achieves parity in speed with 8.0.2.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14052#comment:1>
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