[was ghc-devs] Reasoning backwards with type families

David Feuer david.feuer at gmail.com
Thu Dec 14 02:06:34 UTC 2017

I still haven't really digested what you've written, but I wish to pick a
nit (below)

On Nov 20, 2017 3:44 AM, "Anthony Clayden" <anthony_clayden at clear.net.nz>

> On Thu Nov 16 01:31:55 UTC 2017, David Feuer wrote:


> For (&&), the obvious things you'd want are ...
> There's nothing inherently impossible about this sort of

No-ish but. It relies on knowing kind `Bool` is closed.
And GHC doesn't pay attention to that.
So you need to bring type family `Not`
into the reasoning; hence a SMT solver.

I don't think this is entirely correct. The fact that Bool is closed does
not seem relevant.  The key fact, I believe, is that (&&) is closed. Asking
GHC to reason like this about open type families smells much harder, but
maybe my sense of smell is off.
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