[GHC] #13536: Program which terminates instantly in GHC 8.0.2 runs for minutes with 8.2.1

[GHC]

#13536: Program which terminates instantly in GHC 8.0.2 runs for minutes with 8.2.1
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      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 rwbarton):

 Here is a version without any of the random or quickcheck stuff. (I used
 the actual `i`, `b`, `v` values from the test and wrote down an arbitrary
 strict function `f`.)
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Main where

 import Control.Monad (ap, liftM, liftM2, liftM3, replicateM)
 import Data.Int (Int32)

 main :: IO ()
 main = do
   let f :: (Bool, Bool) -> (Bool, Bool) -> (Bool, Bool)
       f (True, False) (False, False) = (False, True)
       f _ _ = (True, False)
       ((i, b), v) =
 ((False,True),[(False,True),(False,False),(True,True),(True,False),(False,False),(False,True),(True,True),(True,True),(False,True),(True,False),(False,False),(True,True),(True,True),(False,False),(False,False),(False,True),(True,False),(True,False),(True,True),(True,True),(False,True),(True,False),(True,False),(True,True),(False,False),(True,True),(False,False),(True,False),(False,True),(True,True)])
   print $ foldlTest f (i, b) v

 type FoldlTest a = (a -> a -> a) -> a -> [a] -> Bool

 foldlTest :: FoldlTest (Bool, Bool)
 foldlTest f (i, b) v =
   foldl f (i, b) v == foldl (\x -> f (unmodel x)) (i, b) v

 class TestData a where
   type Model a
   unmodel :: Model a -> a

 instance TestData Bool where
   type Model Bool = Bool
   unmodel = id

 instance (Eq a, Eq b, TestData a, TestData b) => TestData (a,b) where
   type Model (a,b) = (Model a, Model b)
   unmodel (a,b) = (unmodel a, unmodel b)
 }}}

 Observations so far:

 * Making the match in `unmodel` lazy (`unmodel ~(a,b) = ...`) makes the
 program fast again.

 * Adding an explicit export list `module Main (main) where` also makes the
 program fast again.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13536#comment:6>
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