They were introduced as part of the System Fc rewrite.
The Fc approach has the benefit of unifying a lot of the work on GADTs,
functional dependencies, type and data families, etc. all behind the scenes.
Every once in a while, (~) constraints can leak into the surface language
and it can be
Hi,
I am trying to find more background on these. They don’t exist in the Haskell
2010 Language Report, they didn’t exist in ghc 6.8.2 but make an appearance in
7.0.1. The documentation in the manual is rather sparse and doesn’t contain a
reference:
