[GHC] #11240: Simplifier ticks exhausted on Y combinator

17 Dec 2015

      #11240: Simplifier ticks exhausted on Y combinator
-------------------------------------+-------------------------------------
           Reporter:  sweirich       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The code from this stackoverflow question:
 https://programmers.stackexchange.com/questions/215712/type-checking-and-
 recursive-types-writing-the-y-combinator-in-haskell-ocaml

 i.e.
 {{{#!hs
 newtype Mu a = Roll { unroll :: Mu a -> a }

 fix :: (a -> a) -> a
 fix = \f -> (\x -> f (unroll x x)) $ Roll (\x -> f (unroll x x))
 }}}

 produces the following output when compiled with
 GHC 7.10.2 or 7.10.3:

 {{{
 sweirich$ ghc --make Mu.hs
 [1 of 1] Compiling Mu               ( Mu.hs, Mu.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-apple-darwin):
         Simplifier ticks exhausted
   When trying UnfoldingDone a_sml
   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: 4962

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 The code compiles with newtype replaces by data.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11240
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