"Rob Stewart" wrote:
if (more_than_half_of(a) >= more_than_half_of(b)) ... doesn't make sense.
It does to me.
Suppose a is the set of female students and b the male students. Further suppose that >= compares grades. Then the expression asks whether more than half of the girls got grades at least as high as more than half of the boys.
You are right. Seems my deductive thinking isn't as strong as I always thought. ________________________________________________________
if ( is_true_that(all_of(a), a_binary_functor, one_of(b)) ) ...
If you allow that, then symmetry cannot be guarranteed. There's already a question as to whether the operators must be symmetrical. I've posited that asymmetry will lead to confusion, but many comparisons sound asymmetrical. Consider these two expressions:
none_of(a) == any_of(b) // 1
any_of(a) == none_of(b) // 2
1. Sounds like it should be true if none of the values in a equal any value in b.
2. Sounds like it should be true if any of the values in a equals none of the values in b.
No, the (2) should be true if any value from 'a' cannot be equaled to some single value from 'b'. This would make it symetrical to (1). Say: a = green, blue, blue b = blue, red none_of(a) == any_of(b) is false because there is blue (even 2 of them) from 'a' that match something in 'b' any_of(a) == none_of(b) is false because there are two different cases when blue from 'a' is in 'b'. Not because 'b' lacks 2 blues. IOW any_of() should care about cardinality. For the any_of(s1) == one_of(s2) one_of(s2) == any_of(s1) I think the "one_of" should read as "exactly_one_and_only_one_of" and "any_of" as "any_individual_item_of" This should restore the symetry. This is my understanding how these qualifiers should be implemented and thought about. The other interpretation didn't occured to me but perhaps I am still missing the 'aha!' moment. I didn't study the implementation deep enough (I would need to step through) but it should be symetrical.
Perhaps we should consider a functional style:
if (compare(all_of(a), one_of(b), _1 >= _2))
That would be if (compare(_1 >= _2, all_of(a), one_of(b))) woudn't it? ________________________________________________________
if (evaluate(all_of(a).where(filter1) , _1 >= _2 , one_of(b).where(filter2))) ...
That may be true, but I'm concerned about duplicating functionality and, thus, limiting flexibility. I guess we just need to be sure that this library interoperates easily with filters created using the facilities of the other libraries.
IMO syntax should decide what to use. If filter iterator is as easy to use as hypothetical embedded filter than I am all for for it. Expression that could be written on single line it is much prefered to alternatives: vector<int> a; set<int> b; if ( is_true_that(any_of(a), _1 * 2 >= _2, all_of(b)).where(_1 >= 0 && _2 > 10) ) { is still somehow readable (as long as the filter is simple). /Pavel