[Haskell-cafe] can the default instances in GHC be recompiled with overlap?

[Jason Dusek]

  Per the GHC manual section on instance overlap:

    If an instance declaration is compiled without
    -XOverlappingInstances, then that instance can never be
    overlapped. This could perhaps be inconvenient.

  It is inconvenient for certain things. If I want to declare a
  special `instance Show [MyType] where...` I am out of luck.
  However, the solution need not involve a change to the way
  instances are handled -- if all the `GHC.*` instances were
  compiled with overlapping instances, I'd be able to declare my
  instance.

  Is there a substantial barrier, either theoretical or
  practical, to recompiling GHC this way?

--
_jsn


 |...GHC manual section...|
  http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-overlap