[GHC] #8168: ghc "Simplifier ticks exhausted" "When trying UnfoldingDone"

[GHC]

#8168: ghc "Simplifier ticks exhausted" "When trying UnfoldingDone"
-------------------------------------+-------------------------------------
        Reporter:  sp55aa            |            Owner:
            Type:  bug               |           Status:  new
        Priority:  high              |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:  Simplifier,
Operating System:  Windows           |  UnfoldingDone, simpl-tick-factor
 Type of failure:  GHC rejects       |     Architecture:  x86
  valid program                      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+-------------------------------------

Old description:

> ghc fail to compile this file:
>
> ---- begin Y.hs ----
> newtype Rec a = Rec { unRec :: Rec a -> a }
>
> y :: (a -> a) -> a
> y = \f -> (\x -> f (unRec x x)) (Rec (\x -> f (unRec x x)))
>
> foo :: (a -> a) -> a -> a
> foo f n = f n
>
> main = print $ y foo 4
> ---- end Y.hs ----
>
> C:\Users\user\Desktop>ghc Y.lhs
> [1 of 1] Compiling Main             ( Y.lhs, Y.o )
> ghc: panic! (the 'impossible' happened)
>   (GHC version 7.6.3 for i386-unknown-mingw32):
>         Simplifier ticks exhausted
>     When trying UnfoldingDone a_sqe{v} [lid]
>     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: 5601
>
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

New description:

 ghc fail to compile this file:
 {{{
 ---- begin Y.hs ----
 newtype Rec a = Rec { unRec :: Rec a -> a }

 y :: (a -> a) -> a
 y = \f -> (\x -> f (unRec x x)) (Rec (\x -> f (unRec x x)))

 foo :: (a -> a) -> a -> a
 foo f n = f n

 main = print $ y foo 4
 ---- end Y.hs ----
 }}}
 Try it:
 {{{
 C:\Users\user\Desktop>ghc Y.lhs
 [1 of 1] Compiling Main             ( Y.lhs, Y.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.3 for i386-unknown-mingw32):
         Simplifier ticks exhausted
     When trying UnfoldingDone a_sqe{v} [lid]
     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: 5601
 }}}

--

Comment (by simonpj):

 Yes see http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs.html.
 At least the simplifier doesn't go into an infinite loop any more.  Better
 solutions welcome.

 Simon

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8168#comment:1>
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