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 (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 Typeablewikipage