Constrained Type Families?

Evan Austin e.c.austin at
Wed Mar 9 01:54:05 UTC 2016

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.



> On Mar 8, 2016, at 8:21 PM, Richard Eisenberg <eir at> wrote:
> On Mar 8, 2016, at 7:17 PM, Evan Austin <e.c.austin at <mailto:e.c.austin at>> wrote:
>> The wiki page for Phase I of Dependent Haskell describes an approach to constrained type families:
>> <>
>> 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

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