The website asks what they do in Haskell. The answer is property modification and reading, as well as very powerful traversal constructs, use lenses (https://hackage.haskell.org/package/lens , tutorial at https://hackage.haskell.org/package/lens-tutorial-1.0.5/docs...).

In Lil[0], this is how ordinary assignment syntax works. Implicitly defining a dictionary stored in a variable named "cat" with a field "age":

    cat.age:3
    # {"age":3}

    l:1,(list 2,list cat),4
    # (1,(2,{"age":3}),4)

    l[1][1].age:9
    # (1,(2,{"age":9}),4)
    cat
    # {"age":3}

    l:l,5
    # (1,(2,{"age":9}),4,5)

It seems like this is proposing syntactic sugar to make mutating and non-mutating operations be on equal footing.

> The more interesting example is reassigning the deeply nested l to make the cat inside older, without mutating the original cat

Isn't that mutating l, though? If you're concerned about mutating cat, shouldn't you be concerned about mutating l?

Yeah this looks like lenses at first glance

q has the concept of amend, which is similar: https://code.kx.com/q4m3/6_Functions/#683-general-form-of-fu...

It's quite handy, though the syntax for it is rather clunky compared to the rest of the language in my opinion.