Re: [GHC] #14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies

[GHC]

#14046: “Illegal type synonym family application in instance” is too strict in the
presence of functional dependencies
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        Reporter:  lexi.lambda       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #3485             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AntC):

 > GHC rejects the following instance, complaining about the use of a type
 synonym family in the instance head ...

 ? This isn't a bug. GHC has never allowed type family applications in the
 instance head. (With FlexibleInstances, it does allow type synonyms.)
 #3485 is an enhancement request (and closed, for good reasons IMO).

 So the FunctionalDependency is a red herring.

 I agree with your suggestion on lens#754 the Template Haskell for lenses
 should be smarter.

 If there's an opportunity for an enhancement, I think there's a more
 general suggestion. Borrow the irrefutable patterns idea like:

 {{{

 instance HasValue (Rec a) ~(Fam a) where ...

 }}}

 And allow `~( ... )` to be any type or family application. Treat it as
 syntactic sugar for:

 {{{

 instance (b0 ~ Fam a) => HasValue (Rec a) b0 where ...      -- b0 is fresh

 }}}

 Also there's an opportunity for the compiler to see that instance (as
 compared with others):
  a. doesn't really overlap
  b. is consistent with FunDeps
  c. doesn't break the Coverage Condition

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14046#comment:4>
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