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

Nicolas Frisby nicolas.frisby at
Tue May 21 15:52:59 UTC 2019

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 <=?

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> 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>
> 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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