@attractivechaos I don't know how to reply to your last reply, so I'll do it here. 1:length(A) is bad because it's using a standard construction for intervals of numbers, but using it for indices. We don't want to get rid of it because 1:5 or 0:0.2:1 is something that is very common and necessary, but I don't see how to tell one that they should instead use eachindex(A) except through proper docs. 1:length(A) is so common in MATLAB though that I am sure people will carry it over, and I'll PR to their library to fix it. I'm not sure how to fix a knowledge issue like that.
You're not understanding generic types and its relation to (1). There's only one way to access an array: getindex. That's the function that's called with A[i]. However, you can use an immutable to put a thin (zero-cost) wrapper over an array, and define dispatches to getindex to do whatever you need it to do. So it's both implicit syntactically because the user just does A[i], but it's explicit because the user has to choose a different type. getindex is then usually inlined and then compiled according to the type, making it a thin abstraction over the implementation.
There are iterators which don't have a size or length. You can write generic algorithms which require an AbstractArray which HasLength and query at compile time for things like that and throw appropriate errors (those are called traits).
There is still a lot of development to do here, but the basics like this are pretty much solved except when new users treat Julia like MATLAB, but I'm not sure how anyone could control for that.