How to append an existing object in jsonnet?

Question

How to append to an existing list?

This is not valid:

local list = ['a', 'b', 'c'];

local list = list + ['e'];

Answer 1

What you experienced is due to locals being recursive in jsonnet. So in local list = list + ['e'] the list on the right hand side is the same list as on the left hand side, resulting in infinite recursion when you try to evaluate it.

So this will work as you would expect:

local list = ['a', 'b', 'c'];
local list2 = list + ['e'];

This time it correctly refers to the previously defined list.

If you wonder why it was designed this way, it is useful, because means you can write recursive functions:

local foo(x) = if x == 0 then [] else foo(x - 1) + [x];
foo(5)

Which is exactly the same as writing:

local foo = function(x) if x == 0 then [] else foo(x - 1) + [x];
foo(5)