[GHC] #8427: GHC accepts invalid program because of EPS poisoning

[GHC]

#8427: GHC accepts invalid program because of EPS poisoning
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        Reporter:  errge             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.10.1
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  GHC accepts       |       Difficulty:  Moderate (less
  invalid program                    |  than a day)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by simonpj):

 * priority:  lowest => normal


Comment:

 You are quite right. The current situation is manifestly a compromise.

 The "right" solution is
  * to create a fresh class-instance environment (call it `myInstEnv`) when
 starting to compile a module
  * whenever you need an interface file, go look it up; if it is not there,
 load it; and then (in both cases) add its class instances to `myInstEnv`
  * similarly add any orphan modules
  * to avoid repeatedly adding stuff to `myInstEnv` we'll need a set of
 module whose class instance are already in it.
  * discard `myInstEnv` when we are done with this module

 Similarly for rules and family instances.

 Once done we can delete some code in GHCi that does something very like
 this for the ghci command line.

 None of this is hard, just needs doing.  Volunteers?

 I'll make priority 'normal'.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8427#comment:2>
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