[GHC] #15630: panic! Simplifier ticks exhausted

11 Sep 2018

      #15630: panic! Simplifier ticks exhausted
--------------------------------------+---------------------------------
           Reporter:  micahshahn      |             Owner:  (none)
               Type:  bug             |            Status:  new
           Priority:  normal          |         Milestone:  8.6.1
          Component:  Compiler        |           Version:  8.2.2
           Keywords:                  |  Operating System:  Windows
       Architecture:  x86_64 (amd64)  |   Type of failure:  None/Unknown
          Test Case:                  |        Blocked By:
           Blocking:                  |   Related Tickets:
Differential Rev(s):                  |         Wiki Page:
--------------------------------------+---------------------------------
 Compiling the following with -O2 causes a panic.

 {{{#!hs
 module GHCPanic where

 data IValue = IDefault
             | IInt Int
             | IBlob String

 (?) :: Applicative m => (IValue -> m a) -> IValue -> m (Maybe a)
 (?) _ IDefault = pure Nothing
 (?) p x = Just <$> p x

 getInt :: IValue -> Either () Int
 getInt (IInt i) = Right i
 getInt v = Left ()

 getString :: IValue -> Either () String
 getString (IBlob b) = Right $ b
 getString v = Left ()

 (<+>) :: Applicative m => (m (a -> b), [IValue]) -> (IValue -> m a) -> (m
 b, [IValue])
 (<+>) (f, (v:vs)) p = (f <*> (p v), vs)

 data TestStructure = TestStructure
     { _param1 :: Int
     , _param2 :: Maybe String
     , _param3 :: Maybe Int
     , _param4 :: Maybe String
     , _param5 :: Maybe Int
     , _param6 :: Maybe Int
     , _param7 :: Maybe String
     , _param8 :: Maybe String
     , _param9 :: Maybe Int
     , _param10 :: Maybe Int
     , _param11 :: Maybe String
     , _param12 :: Maybe String
     , _param13 :: Maybe Int
     , _param14 :: Maybe Int
     , _param15 :: Maybe String
     }

 getMenuItem :: [IValue] -> Either () TestStructure
 getMenuItem vs = fst $ (pure TestStructure, vs)
              <+> getInt
              <+> (getString ?)
              <+> (getInt ?)
              <+> (getString ?)
              <+> (getInt ?)
              <+> (getInt ?)
              <+> (getString ?)
              <+> (getString ?)
              <+> (getInt ?)
              <+> (getInt ?)
              <+> (getString ?)
              <+> (getString ?)
              <+> (getInt ?)
              <+> (getInt ?)
              <+> (getString ?)
 }}}

 {{{
 ghc.exe: panic! (the 'impossible' happened)
   (GHC version 8.2.2 for x86_64-unknown-mingw32):
         Simplifier ticks exhausted
   When trying UnfoldingDone $j_s1y9
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   To see detailed counts use -ddump-simpl-stats
   Total ticks: 71323
   Call stack:
       CallStack (from HasCallStack):
         prettyCurrentCallStack, called at
 compiler\utils\Outputable.hs:1133:58 in ghc:Outputable
         callStackDoc, called at compiler\utils\Outputable.hs:1137:37 in
 ghc:Outputable
         pprPanic, called at compiler\simplCore\SimplMonad.hs:199:31 in
 ghc:SimplMonad
 }}}

 This seems similar to #8319 which was marked as being fixed.

 It compiles (albeit very very slowly!) if I remove the last parameter and
 the last application of (<+>).

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15630
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler