[GHC] #8767: Add rules involving `coerce` to the libraries

[GHC]

#8767: Add rules involving `coerce` to the libraries
-------------------------------------+-------------------------------------
              Reporter:  nomeata     |            Owner:
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:              |          Version:  7.9
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:  8718
       Type of failure:              |  Related Tickets:  #2110
  None/Unknown                       |
             Test Case:              |
  tests/simplCore/should_run/T2110.hs|
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by dmcclean):

 I'm out of my depth here, not even sure this thread is about what I'm
 running in to.

 I want to coerce from `f (Quantity d v)` to `f v` where `f` is a functor,
 so that I can add up all the values without their wrappers in the way and
 then wrap the answer.

 This works a charm if I specialize `f` to `[]`, but doesn't compile
 ("because 'f (Quantity d v)' and 'f v' are different types") in the
 polymorphic version. (Full function below.)

 {{{
 sum :: forall f d v.(Num v, Foldable f, Functor f) => f (Quantity d v) ->
 Quantity d v
 sum = coerce . Data.Foldable.sum . (\x -> x :: f v) . coerce

 sum :: forall d v.(Num v) => [Quantity d v] -> Quantity d v
 sum = coerce . F.sum . (\x -> x :: [v]) . coerce
 }}}

 Is this thread about making a workaround for that?

 Is the reason that it's complicated because some functors have nominal
 roles? (Maybe a SortedList would break if you changed out `Ord` instances
 under it?) Or is there more to it than that?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8767#comment:17>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler