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 useful to be able to talk about them in the surface language
of Haskell, because otherwise it isn't clear how to talk about F a ~ G b
style constraints, which arise in practice when you work with type families.
-Edward
On Sun, Jan 11, 2015 at 6:04 PM, Dominic Steinitz
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: https://downloads.haskell.org/~ghc/latest/docs/users_guide.pdf section 7.11. Folk on #ghc referred me to http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/. I can find papers that refer to ~ in F_C (aka FC?) but as far as I can tell not in the Haskell language itself.
Many thanks
Dominic Steinitz dominic@steinitz.org http://idontgetoutmuch.wordpress.com
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