[GHC] #13006: Possible program should type check but does not using Implicit Parameters and Vectors

[GHC]

#13006: Possible program should type check but does not using Implicit Parameters
and Vectors
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        Reporter:  clinton           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2-rc2
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 It depends what you mean by "derive".

 Injectivity gives rise to so-called "Derived equalities" which guide
 unification (often called "improvement").  They do not provide
 ''evidence'' in the sense of System FC.  So, with your type family
 {{{
 f :: F a -> F a
 f x = x
 }}}
 would not be reported as an ambiguous type; without the injectivity
 constraint it would.

 I wish there was a comprehensive writeup about this... it's too much folk
 lore.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13006#comment:7>
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