[GHC] #6018: Injective type families

[GHC]

#6018: Injective type families
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              Reporter:  lunaris     |            Owner:  jstolarek
                  Type:  feature     |           Status:  new
  request                            |        Milestone:  7.10.1
              Priority:  normal      |          Version:  7.4.1
             Component:  Compiler    |         Keywords:  TypeFamilies,
            Resolution:              |  Injective
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #4259
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 Replying to [comment:58 jstolarek]:
 > I thought about it and I believe this should be possible. But we could
 only infer normal injectivity. I believe an algorithm for inferring
 injectivity in some of the parameters (ie. things like `result a -> b c`
 or `result -> b c` for a 3-arity type family) would be exponential in the
 number of arguments.

 I don't know anything about this, but what's wrong with exponential time
 here? This inference only happens once per closed type family, right? How
 many variables are there likely to be?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6018#comment:62>
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