[GHC] #14582: Review and improve the Typeable API

[GHC]

#14582: Review and improve the Typeable API
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:  Typeable       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This ticket is to track improvements in the `Typeable` API

 There are a number of points I'm uncomfortable about

 * `Data.Typable` is presumably meant to be the public API, and is
   fairly small.  But `Data.Typeable.Internal` has a much larger
   API, which includes pattern synonyms and suchlike that appear
   to be for the benefit of clients, not just internal use.

 * The `Typeable` API has `type TypeRep = I.SomeTypeRep` which is
   different to the type-indexed `data TypeRep a` defined in
   `Data.Typeable.Internals`.  This is exteremly confusing.  Perhaps
   this is intended to be temporary, while we are moving over to the new
 type-indexed
   representation. But then what's the transition plan?

 * I cordially dislike this stuff about `IsApplication` in
   `Internals`
 ([https://git.haskell.org/ghc.git/commitdiff/1acb922bb1186662919c1dbc0af596584e5db3ac
 this commit]). It's hard for me to understand what's going on.  I
   believe that the two pattern synonyms `App` and `Con` are supposed
   to be exhaustive -- if so, let's just write a COMPLETE pragma.

 * The code has many uses of `unsafeCoerce`, whereas the drive of
   our "Typed reflection in Haskell" paper was to reduced the size
   of the trusted code base. I'd like to see a comment on each of
   those uses of `unsafeCoerce` to explain.

 * Do we really need `Con'` as well as `Con`?

 * The API could do with some careful documentation, in Haddock,
   but perhaps also an accomanying wiki page.  It's tricky stuff.

 Here's the [wiki:Typeable Typeable wiki page]

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14582>
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