request: a Nat ordering constraint that is not an equality constraint

Richard Eisenberg rae at
Tue May 21 16:13:48 UTC 2019

You're right that, without special support, the IsTrue approach won't work with any deductions from Givens. But -- short of strapping on an SMT solver -- we're always going to fall short there, so we should analyze a particular on-the-ground use case before taking any drastic action. (It sounds like you agree with this.)


> On May 21, 2019, at 5:52 PM, Nicolas Frisby <nicolas.frisby at> wrote:
> Yes, it seems possible that a user space declaration of <= via IsTrue as in my first email could get much of the desired behavior. I plan on trying it with the work code base soon, maybe even today -- it'll probably do better than my current workaround.
> If, however, we want the Nat solver to do anything at all with a Given `IsTrue (n <=? m)`, then I think it will need changes. I don't know that machinery well, but it seems very likely it would ignore such Givens.
> For example, I would naively expect the Nat solver should discharge a Wanted `IsTrue (n <=? m)` from two Givens `(IsTrue (n <=? x),IsTrue (x <=? m))`.
> Simon's exploration of IsTrue/IsEqual might shed more light on what exactly the Nat solver should and should not do with such a Given. If it's in fact nothing at all, then yes, maybe a user space solution fully supplants the proposed Passive.<=. But I currently anticipate that it should do something with such Givens.
> Thanks. -Nick
> On Tue, May 21, 2019, 00:29 Richard Eisenberg <rae at <mailto:rae at>> wrote:
> This is an interesting proposal. When I started reading it, I wondered why anyone would want to avoid the current definition. But you motivate that part well. I would want a larger test of the IsTrue approach to make sure it does what you want before supporting this. But wait: couldn't you write your GHC.TypeLits.Passive today, in a library, with no ill effect? If so, there isn't a strict reason GHC needs to adopt this. (Of course, if the new definition proves useful, then it might make sense to do so in time.)
> > On May 21, 2019, at 3:48 AM, Nicolas Frisby <nicolas.frisby at <mailto:nicolas.frisby at>> wrote:
> > 
> > P.P.S. - Is there a standard place to find something like `IsTrue`? More generally: a test for type equality that does not drive unification? Thanks again.
> If something like this ends up in GHC, Data.Type.Bool seems like the right place.
> Richard

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