[GHC] #10116: Closed type families: Warn if it doesn't handle all cases

[GHC]

#10116: Closed type families: Warn if it doesn't handle all cases
-------------------------------------+-------------------------------------
        Reporter:  andrewthad        |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by gkaracha):

 Replying to [comment:7 rwbarton]:
 > This might fall out of the implementation anyways, but a special case to
 be aware of is non-exhaustiveness in implicit kind variables, such the
 return kind of a polykinded type family, as arose in #11275:
 > {{{
 > type family Hm :: k where Hm = Int
 > }}}
 > Despite appearances this type family is not exhaustive: `Hm :: (* -> *)`
 will not reduce.
 Yes, this makes a lot of sense, it really is non-exhaustive. If you think
 of
 the full type, I think that `Hm` would look like the following:
 {{{
 type family Hm (k :: BOX) where
   Hm * = Int
 }}}
 Hence, we can compute the uncovered set to be:
 {{{
 U1 = { k |> not (k ~ *) }
 }}}
 I think this is the only *clean* way to treat this,
 pretty close to what already happens with GADTs: Have both kind and type
 arguments
 explicit (kinds first) and represent the non-matched cases with kind
 inequalities.

 I suspect that proper kind inequalities (See
 [http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f
 /axioms-extended.pdf])
 will be needed in
 order for the results of the check to be properly used by the type checker
 but I think that
 just for the warning it can be done in a simpler way. Can't be sure
 though!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10116#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler