What is Julia’s composability useful for if it leaves me unable to trust my results?
But some of the examples in OP of non-composability are where edge cases (not all cases) will give you the wrong answers. In the worst case, in ways that aren't immediately obvious.
I think it's too optimistic to say that you just need to know that you should write some integration tests and then write some easy and obvious ("not especially hard") ones, and then you'll always catch these easily and early.
I don't actually even see any evidence here that these sorts of bugs are more common or more insidious than any other sort of bug, rather it looks to me that since dispatch-based composability is a bit of a new phenomenon, people haven't always been on the lookout for these bugs yet, or learned the patterns of where and what to test for to find them -- but once you've seen a few cases, patterns start appearing and it becomes more clear what you need to test, just like anything else.
The broader issue to me is that I think people often underestimate the degree to which Julia is a fundamentally different programming paradigm than any other language most people have used before (unless you've spent a lot of time in maybe CLOS or Dylan) -- there's not even an agreed-upon name for it, but I'd call it "dispatch-oriented programming". Often times people will come in to Julia expecting all their expertise and intuition from class-based OO or whatever other paradigm to just effortlessly transfer to Julia, and that's really not the case, which tends to lead to frustration.
