How to implement type-level list concatenation as a GHC primitive type family

[Hécate]

Hi GHC devs,

I was wondering recently, considering that type family evaluation is 
notoriously slow, how one would implement type-level list concatenation 
directly within GHC in a way that is much less expensive to run. So I am 
looking for pointers, as this part of GHC is fairly unknown to me.

Thinking about it, I'm actually unsure where the tyfam evaluation is 
happening.

Any advice would be appreciated.

Cheers,
Hécate

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