<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">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.)<div class=""><br class=""></div><div class="">Richard<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On May 21, 2019, at 5:52 PM, Nicolas Frisby <<a href="mailto:nicolas.frisby@gmail.com" class="">nicolas.frisby@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="auto" class=""><div dir="auto" class="">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.</div><div dir="auto" class=""><br class=""></div><div dir="auto" class="">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.</div><div dir="auto" class=""><br class=""></div><div dir="auto" class="">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))`.</div><div dir="auto" class=""><br class=""></div><div dir="auto" class="">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.</div><div dir="auto" class=""><br class=""></div><div dir="auto" class="">Thanks. -Nick</div></div><br class=""><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, May 21, 2019, 00:29 Richard Eisenberg <<a href="mailto:rae@richarde.dev" target="_blank" rel="noreferrer" class="">rae@richarde.dev</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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.)<br class="">
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> On May 21, 2019, at 3:48 AM, Nicolas Frisby <<a href="mailto:nicolas.frisby@gmail.com" rel="noreferrer noreferrer" target="_blank" class="">nicolas.frisby@gmail.com</a>> wrote:<br class="">
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> 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.<br class="">
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If something like this ends up in GHC, Data.Type.Bool seems like the right place.<br class="">
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Richard</blockquote></div>
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