Heterogeneous equality into base?

[Ryan Scott]

Sorry for only just discovering this thread now. A lot of this discussion
is in fact moot, since (:~~:) already is in base! Specifically, it's
landing in Data.Type.Equality [1] in the next version of base (bundled with
GHC 8.2). Moreover, it's constructor is named HRefl, so your wish has been
granted ;)

As for why it's being introduced in base, it ended up being useful for the
new Type-indexed Typeable that's also landing in GHC 8.2. In particular,
the eqTypeRep function [2] must return heterogeneous equality (:~~:), not
homogeneous equality (:~:), since it's possible that you'll compare two
TypeReps with different kinds.

Ryan S.
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[1]
http://git.haskell.org/ghc.git/blob/99adcc8804e91161b35ff1d0e5f718d18fcaa66a:/libraries/base/Data/Type/Equality.hs#l37
[2]
http://git.haskell.org/ghc.git/blob/99adcc8804e91161b35ff1d0e5f718d18fcaa66a:/libraries/base/Data/Typeable/Internal.hs#l311
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