[GHC] #8447: A combination of type-level comparison and subtraction does not work for 0

[GHC]

#8447: A combination of type-level comparison and subtraction does not work for 0
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        Reporter:  nushio                     |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  low                        |        Milestone:
       Component:  Compiler (Type checker)    |          Version:  7.6.3
      Resolution:                             |         Keywords:
Operating System:  Linux                      |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Comment (by diatchki):

 Hello,

 the issue here is that type functions are kind of strict at the moment, so
 we evaluate both branched of the `If` are checked, and then we have some
 code that spots the one is impossible.

 GHC used to reason like this:
 0 - 5 ~ r --> 5 + r ~ 0 --> (5 ~ 0, r ~ 0) --> Impossible

 For the time being I've weakened things a bit, so this does not happen
 anymore.
 However, we should probably try to think of a more general solution
 because we really want GHC to be good at reasoning, rather than bad!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8447#comment:4>
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