Re: Constrained Type Families?

I unfortunately don’t have the exact code I was working on in front of me, but I was playing around with the -XTypeInType extension to see if I could use it to implement some notion of sub-kinding to start doing “Data Types a la Carte” style things at the type level. Constrained type families seemed like the most natural way to “promote” those concepts, but maybe there’s a more obvious way I’m missing. I’d be happy to send you a follow up email tomorrow when I’m back at the office if you think this would be a good motivating use case for the feature. V/r, -Evan

On Mar 8, 2016, at 8:21 PM, Richard Eisenberg wrote:

On Mar 8, 2016, at 7:17 PM, Evan Austin mailto:e.c.austin@gmail.com> wrote:

...

The wiki page for Phase I of Dependent Haskell describes an approach to constrained type families: https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1#Typefamilyequa... https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Phase1#Typefamilyequa...

Did that land in GHC 8.0 and, if so, is the updated syntax documented somewhere?

No, it didn't make it. The motivating test case seemed contrived and so we punted on this one.

Do you have a use case that really needs this feature? That would help to motivate it for 8.2 or beyond.

Thanks! Richard