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

[Nicolas Frisby]

Yep, agreed. Totally practical.

FYI: my code base at work seems quite happy with the IsTrue approach; a
very happy simplification! Like so:

```
class IsTrue
  (msg :: ErrorMessage) (b :: Bool) where
  isTrue :: Proxy# msg -> b :~: 'True

instance IsTrue msg 'True where
  isTrue = \_ -> Refl

instance TypeError msg => IsTrue msg 'False
  isTrue = \_ -> error "impossible"

type (<=) n m = IsTrue (Msg n m) (n GHC.TypeLits.<=? m)

type Msg (n :: Nat) (m :: Nat) =
    'ShowType n
  ':<>:
    'Text " is not <= "
  ':<>:
    'ShowType m
```

(An `absurd` for TypeError might be nice?

Via isTrue, I can explicitly convert between the two <= constraints
wherever I need to. Which I could use to explicitly manage those
hypothetical hypotheticals we were discussing.

Just to spell it out: the SMT solver would indeed supercede TcTypeNats, but
TcTypeNats in turn already supercedes the IsTrue-based encoding. So I've
already lost some <= deductions. But my work code doesn't use any of them
in this middle ground -- they're all simple enough that IsTrue works as-is
or complicated enough that TcTypeNats didn't work already (I'm grumpily
writing and invoking Trusted Code Base "axia" in those cases -- I'm
avoiding plugin dependencies at work for now.)

On Tue, May 21, 2019, 09:14 Richard Eisenberg <rae at richarde.dev> wrote:

> 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.)
>
> Richard
>
> On May 21, 2019, at 5:52 PM, Nicolas Frisby <nicolas.frisby at gmail.com>
> 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 richarde.dev> 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 gmail.com>
>> 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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