[GHC] #11120: Missing type representations

[GHC]

#11120: Missing type representations
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.11
           Keywords:                 |  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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 When I say

 {{{
 {-# LANGUAGE DataKinds #-}

 module Bug where

 import Data.Typeable

 foo = typeRep (Proxy :: Proxy '[])
 }}}

 I get

 {{{
 GHC error in desugarer lookup in Bug:
   Can't find interface-file declaration for variable tc'[]
     Probable cause: bug in .hi-boot file, or inconsistent .hi file
     Use -ddump-if-trace to get an idea of which file caused the error
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.11.20151120 for x86_64-apple-darwin):
         initDs IOEnv failure
 }}}

 And I think there may be more trouble. Below are notes I have written to
 ghc-devs:

 ------------------------------

 I'm a bit confused by the new handling of `Typeable`.

 1. You say (in `Note [Grand plan for Typeable]`) that there is trouble
 making the `TyCon`/`Module` information for the types in `GHC.Types`. But
 what precisely goes wrong? I agree that it seems a bit fishy, but I don't
 actually see the spot where trouble lurks. Did you try this?

 2. Even more bizarre would be putting `TyCon`/`Module` info for `GHC.Prim`
 stuff (I'm thinking about the super-magical `TYPE` from my branch) right
 in `GHC.Prim`. But still I can't quite articulate what goes wrong. There
 is no Prim.hi file that would be wonky. And, provided that `GHC.Types`
 itself doesn't try to solve a `Typeable` constraint, no one would ever
 notice the weird dependency. I recognize that this means we'd have to
 build the info somewhere manually in GHC, but I don't think that would be
 too hard -- and I think easier than the current story around name-mangling
 just so that you can write the typereps by hand in
 `Data.Typeable.Internal`. There's also not very many lifted tycons in
 `GHC.Prim`. I count `TYPE` and `RealWorld`, and that's it.

     For what it's worth, a weird dependency from `GHC.Prim` to `GHC.Types`
 actually works in practice. I put `Levity` in `GHC.Types` but `TYPE ::
 Levity -> TYPE 'Lifted` in `GHC.Prim`. No one complained.

 3. Let's assume that we really can't clean up this mess. It still seems
 that several `TyCon`s are missing from `Data.Typeable.Internal`. Like
 promoted nil and cons, and `Nat`, and `Symbol`. At the least, we should
 put a loud comment in the export list of `GHC.Types` saying that
 everything defined there must be accompanied by a definition in
 `Data.Typeable.Internal`.

 4. `Data.Typeable.Internal` uses `mkGhcTypesTyCon`, which refers to
 `GHC.Types`. But this function is used also for things from `GHC.Prim`,
 like `(->)`. Solving `Typeable (->)` works fine. But I'm sure there's
 trouble lurking here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11120>
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