[GHC] #652: Have a single Data.Typeable hash table in GHCi

#652: Have a single Data.Typeable hash table in GHCi
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    Reporter:  simonmar        |        Owner:                  
        Type:  task            |       Status:  new             
    Priority:  normal          |    Milestone:  6.6             
   Component:  Runtime System  |      Version:  6.4.1           
    Severity:  normal          |     Keywords:                  
          Os:  Unknown         |   Difficulty:  Moderate (1 day)
Architecture:  Unknown         |  
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The hash table used by Data.Typeable is declared as a top-level IORef.
 This means in GHCi, there will be two Data.Typeable hash tables: one in
 the dynamically-loaded base package, and one in the statically-linked GHCi
 binary.  Therefore Dyanmics created in one world are incompatible with the
 other world.

 We have a hack to make sure the two hash tables don't use the same
 uniques, so at least a {{{TypeRep}}} created in one world will ''never''
 compare equal to a {{{TypeRep}}} from the other world (before this hack we
 could different TypeReps bogusly claiming to be equal).  However, we would
 like them to compare equal when they are equal.

 This implies that the packages in use in both world must be compatible
 (preferably identical).  This is because when a TypeRep compares equal, we
 must be sure that the value has the representation we expect.  Fortunately
 this is the case in a stage2 GHC.

 We should store a ptr (StablePtr?) to the hash table in an RTS global, so
 that there is only one per runtime instance.

-- 
Ticket URL: <http://cvs.haskell.org/trac/ghc/ticket/652>
GHC <http://www.haskell.org/ghc/>
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