Avoid use of getters with symbols

[cortner]

I'm nervous about the use of getters using the symbols describing the fieldnames. To my mind this defeats the entire point of an abstract interface. I appreciate the elegance those with which one can access properties of a system. I therefore want to propose to replace calls such as

sys[inds, :position] 

with

sys[inds, position] 

This can be formally equivalent to position(sys[inds]) but internally something more efficient can be done. Because position has its own type this means it can be done in a type-stable way, where as using a Symbol one relies on constant-propagation. (I don't know how reliable this is?)

But the biggest advantage for me is that this way one can change the internals without changing the way we access.

A prototype of this is already in #101

[rkurchin]

I'm pretty sure I agree with you. Just to make sure I understand what you're getting at here, in your second proposed option, position is now simply the function position rather than the name of a field in the object? Definitely agree that we should avoid explicitly referencing those, for precisely the reason you point out.

[mfherbst]

Ok for me.

[cortner]

Interestingly, the discussion on the MD document went against what we agreed in this issue. Do you want to close it?