[GHC] #13011: Simplifier ticks exhausted: a 10-line case

[GHC]

#13011: Simplifier ticks exhausted: a 10-line case
-------------------------------------+-------------------------------------
           Reporter:  L.K.Rebellion  |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  Simplifier     |  Operating System:  Linux
  ticks exhausted                    |   Type of failure:  Compile-time
       Architecture:  x86            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #8319 #12776
Differential Rev(s):                 |         Wiki Page:
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 I was trying to define a function that could take itself as an argument.

 {{{#!hs
 newtype MobiusFn a = MobiusFn {
     func :: MobiusFn a -> a
 }

 spin :: MobiusFn a -> a
 spin mf = func mf mf
 }}}

 Use it to find suffixes of a `String`

 {{{#!hs
 suffixes :: String -> [String]
 suffixes = spin $ MobiusFn suffixesMF
     where suffixesMF _ [] = []
           suffixesMF mf s@(_:xs) = s : spin mf xs
 }}}

 Add `main` and compile
 {{{#!hs
 main = readLn >>= (print . suffixes)
 }}}

 The compiler panicked with
 {{{
 [1 of 1] Compiling Main             ( test.hs, test.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for i386-unknown-linux):
         Simplifier ticks exhausted
   When trying UnfoldingDone mf_s1UD
 }}}

 But if we ignore the empty string case, it works well with non-empty
 strings.

 {{{#!hs
 suffixes :: String -> [String]
 suffixes = spin $ MobiusFn suffixesMF
     where suffixesMF _ s@[_] = [s]
           suffixesMF mf s@(_:xs) = s : spin mf xs
 }}}

 Two versions tested.
 {{{
 GHC version 8.0.1 for i386-unknown-linux
 GHC version 7.10.3 for i386-unknown-linux
 }}}
 Thank you for reading.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13011>
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