Unpacking coercions

[Ryan Scott]

In case this wasn't clear, the context of this discussion in this GHC
proposal [1], where David is trying to work around the fact that data types
with existential Coercible constraints do not support unpacking. (By
"unpacking", I mean putting an {-# UNPACK #-} pragma in front of a field of
that type does what you would expect.)

An example program which demonstrates the performance issue can be found
here [2]. That comment concerns unboxed equality vs. boxed equality, but
the same unpacking problems that affect boxed equality also affect
Coercible.

Ryan S.
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[1] https://github.com/ghc-proposals/ghc-proposals/pull/116
[2]
https://github.com/ghc-proposals/ghc-proposals/pull/116#issuecomment-382346662
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