I suggest there is an easy translation of the one_of() ... to mathematical quantifiers and a reading order from left to right. https://en.wikipedia.org/wiki/Quantifier_(logic)   E.g. in the example one_of(1, 2) == one_of(1, 1, 2) your first listed answer should be correct (So the expression should be true).   -----Ursprüngliche Nachricht----- Von:Marcin Jaczewski via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Mi 27.09.2023 11:26 Betreff:Re: [std-proposals] Manifold comparison operator An:std-proposals_at_[hidden]; CC:Marcin Jaczewski <marcinjaczewski86_at_[hidden]>; wt., 26 wrz 2023 o 17:09 Lénárd Szolnoki via Std-Proposals <std-proposals_at_[hidden]> napisał(a): > > On Tue, 2023-09-26 at 12:29 +0200, Fabio Alemagna via Std-Proposals > wrote: > > Il giorno mar 26 set 2023 alle ore 11:13 Bjorn Reese > > <breese_at_[hidden]> ha scritto: > > > > > > On 9/24/23 16:05, Fabio Alemagna via Std-Proposals wrote: > > > > > > > It seems to be working fine, but comments and corrections are > > > > greatly welcome. > > > > > > Here is a simplification that uses std::apply: > > > > > >    https://godbolt.org/z/9bn4jn5v7 > > > > That looks indeed neater at some levels, but you did away with the > > type check, which renders the construct not able to deal with a > > heterogeneous set of value types any longer. > > > > However, meanwhile I came up with a revised version that uses > > policies > > and free functions rather than subclasses, to achieve the same syntax > > but with a leaner and more composable implementation. > > > > https://godbolt.org/z/4Wv7rq4se > > I don't know how to interpret `all_of(a, b) < any_of(c, d)` at a > glance. > > Is this "for every x in {a, b} exists y in {c, d} so that x < y", or is > it "there exists a y in {c, d} so that for every x in {a, b} x < y". > > I think these two are probably equivalent if a,b,c,d have a total > order, otherwise I don't think they necessarily are. > > For the partial order where a < c and b < d are true and other > comparisons are false the first statement is true and the second > statement is false. > > all_of(a,b) == any_of(a,b) is a similar example. > Better question is what is result of: ``` one_of(1, 2) == one_of(1, 1, 2) ``` I would expect that is should be symmetrical but on grouping of what is consider "one" then it can give `false` or `true`: ``` 1 == one_of(1, 1, 2) // false 2 == one_of(1, 1, 2) // true // -> true ``` vs ``` one_of(1, 2) == 1 //true one_of(1, 2) == 1 //true one_of(1, 2) == 2 //true // -> false ``` I think in this case the correct answer should be always `false`. We should consider this as a matrix of comparison results, and each `one_of`, `any_of`, `all_of`, `none_of` define how we group results. And some groupings have bigger precedence, like for `a * b + c * d` we first calculate both `*` then `+` in case of `a * b * c * d` order for natural numbers does not matter. I think this priority should be: ``` all_of none_of any_of one_of ``` If both sides are the same operator then we treat all as one group. e.g. for ``` //`any_of(a,b,c) op all_of(d,e,f)` same as `all_of(a,b,c) op any_of(d,e,f)` ((a op d) && (a op e) && (a op f)) || ((b op d) && (b op e) && (b op f)) || ((c op d) && (c op e) && (c op f)) //`any_of((a,b,c) op any_of(d,e,f)` (a op d) || (a op e) || (a op f) || (b op d) || (b op e) || (b op f) || (c op d) || (c op e) || (c op f) ``` BTW, aside of better `if` this function interact greatly with `std::find` like: ``` std::find(b, e, any_of(1, 2)); ``` Or with itself: ``` foo > any_of(one_of(1, 2), one_of(4, 5)) //-> foo is in [1,2) or [4,5) range ``` > Cheers, > Lénárd > -- > Std-Proposals mailing list > Std-Proposals_at_[hidden] > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals