[GHC] #9974: Allow more general structural recursion without UndecidableInstances

[GHC]

#9974: Allow more general structural recursion without UndecidableInstances
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        Reporter:  dfeuer            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Changes (by goldfire):

 * cc: eir@… (added)


Comment:

 Yes, I definitely think GHC's termination checker could be beefed up. I
 believe one challenge here is that repeated variables are allowed on the
 left-hand sides, which causes havoc with a straightforward check for
 structural recursion (I believe). But I only barely know what I'm talking
 about here, so it may be much simpler than I think.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9974#comment:1>
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