[GHC] #12882: Unexpected constraint when using ExistentialQuantification

[GHC]

#12882: Unexpected constraint when using ExistentialQuantification
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 But presumably the reason you want the type checker to know `t` is an
 instance of `Typeable` is in order to use functions which use the
 constraint, like `typeRep`. And the `Typeable t` dictionary is exactly
 what will be used at runtime to implement that function. So, there is no
 getting around the need for the dictionary, either passed into your
 function as a constraint `Typeable t =>` or unpacked from the data as in
 `tst1`.

 (I'm a bit confused how you think this could work at runtime. Maybe you
 are imagining that a function receives information about its type
 parameters at runtime and could use that data to look up the needed
 `Typeable` instance in some global table? Actually, there is no type data
 at runtime aside from what you explicitly introduce in the form of
 `Typeable` contexts nor any table of instances. And even `Typeable` is not
 that special, and could be reimplemented to some extent by a used-defined
 class.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12882#comment:5>
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