[DNF2]

> Julia is fastest with immutable structures--why provide a built-in syntax for complex assignment to mutable types, but then relegate lenses to a library that only FP aficionados will use?

This is not really accurate. Performance in Julia is heavily organized around mutability, in particular for arrays. The main reason Julia does not fully embrace immutability for everything is, simply, performance.

[electroly]

There is some discussion about this from smarter people than me over here (same thread): https://news.ycombinator.com/item?id=45769149

> Counterintuitively Julia recommends the use of immutable data types for performance reasons...

Those folks are likely more able to respond to your counterclaim than me.