[GHC] #15497: Coercion Quantification

[GHC]

#15497: Coercion Quantification
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        Reporter:  ningning          |                Owner:  (none)
            Type:  task              |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      Resolution:  fixed             |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5054
       Wiki Page:                    |
  https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase2|
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Comment (by simonpj):

 One other thing.  As I said in our call today, it's striking how narrow
 are the use-cases for a coercion-quantified type:

 * We can't ''declare'' a function with type `f :: (t1 ~# t2) => blah`,
 because currently `(t1 ~# t2)` is not a constraint (see Trac #15648) and
 is never implicitly instantiated.

 * We can't ''infer'' a function with type `f :: (t1 ~# t2) => blah`.  And
 if we did, it wouldn't be instantiated implicitly.

 And so ''a fortiori'' we can't have coercion-quantified variants of such
 types.

 '''So the ''only'' functions that can have these coercion-quantified types
 are the workers of data constructors'''.   Am I right?  If so, can we say
 so explicitly on the wiki page?

 That seems like a lot of work for a narrow use-case!  And yet, and yet...
 it's a very important case.

 Note also that we cannot write an exactly-equivalent type signature with
 GADT-syntax, because we don't have source-syntax of `t1 ~# t2`, let alone
 for `forall (co:t1~#t2). blah`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15497#comment:10>
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