[GHC] #8854: Missing class method not caught at compile time, program freezes when missing method is called.

[GHC]

#8854: Missing class method not caught at compile time, program freezes when
missing method is called.
----------------------------------+----------------------------------
       Reporter:  RaminHAL9001    |             Owner:
           Type:  bug             |            Status:  new
       Priority:  normal          |         Milestone:
      Component:  Compiler        |           Version:  7.6.3
       Keywords:                  |  Operating System:  Linux
   Architecture:  x86_64 (amd64)  |   Type of failure:  Runtime crash
     Difficulty:  Unknown         |         Test Case:
     Blocked By:                  |          Blocking:
Related Tickets:  #7633           |
----------------------------------+----------------------------------
 I created a type class where two of four functions must be satisfied for a
 minimal complete definition. GHC did not catch an instance where the
 minimal complete definition was not satisfied, the program compiled
 without an error or warning.

 When running the compiled program it would freeze when calling the missing
 method. Also when running the program in GHCi and stepping up to the use
 of the missing method, GHCi would freeze before reaching the missing
 function. This made it very difficult to debug. It '''did not''' throw a
 "NoMethodError" exception, and there '''was no''' segment violation caught
 by the operating system, nor does the program terminate with the
 "<<loop>>" message output on the command line.

 Compiling with language extensions:
 {{{
 -XTemplateHaskell -XScopedTypeVariables -XRankNTypes
 -XMultiParamTypeClasses -XFunctionalDependencies -XFlexibleInstances
 -XFlexibleContexts -XDeriveDataTypeable
 }}}

 Using compiler flags:
 {{{
 -prof -fprof-auto -threaded -Wall -fno-warn-name-shadowing -fno-warn-
 unused-do-bind -fno-warn-auto-orphans
 }}}

 '''To Reproduce the Issue'''

 I tried to reproduce the issue with a miniature program that isolates the
 problem, but GHC successfully detects a problem and terminates with the
 "<<loop>>" message written on the command line. My program is much larger
 and simply froze. However the code I wrote looks similar to the following
 situation...

 First I tried this, which gave me no problems at all:


 {{{
 class A x where
     f1 :: x
     f1 = f2

     f2 :: x
     f2 = f1

 }}}

 We have two functions f1 and f2, if f1 is not defined it defaults to f2,
 if f2 is not defined it defaults to f1, so you must define either f1 or
 f2, or you could also define both.

 Then I add two more methods:


 {{{
 class A x where
     f1 :: x
     f1 = f2

     f2 :: x
     f2 = f1

     g1 :: x
     g1 = g2

     g2 :: x
     g2 = g1

 }}}

 Just like with f1 f2, the g1 g2 functions each use their counterpart to
 define their default method. But this caused a huge problem.

 The minimal complete definition should be (f1 | f2) & (g1 | g2), but the
 compiler does not report any warning when I have not satisfied the minimal
 complete definition. Maybe one of my compiler flags disabled a would-be
 warning, I don't know yet. This was a red-flag to me, but I went on to
 test my program.

 As it turns out, there was a place in my code where I forgot to write an
 instance for g1 or g2 but I had not realize it. I only found out when I
 ran my test program and it froze. No "unsatisfied method" exception, not
 segment violations, it just froze. Although the program was still running
 and consuming a small amount of CPU resources, and I could cancel with
 CTRL-C in the command line.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8854>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler