Factorio Forums

Consider the following GUI element tags:
a = {"one"}
b = {"one", "two"},
c = {nil, "two"},
d = {nil, "two", "three"}

a and b are both sequences, but c and d are not.
the game will leave the sequences alone, but it will stringify the keys of the non-sequences.

Here's a command that adds a button to the player's screen with these tags, and prints out the tags the element has.
This is the output for the above command:
{a = {"one"}, b = {"one", "two"}, c = {["2"] = "two"}, d = {["2"] = "two", ["3"] = "three"}}
Notice how the keys for c and d are strings instead of numbers.

I'm not sure if this is intended behavior, but it's not documented on latest and left me really confused for about an hour. :P

As side information, when discussed on the Discord, it based on PropertyTrees being [compatible with/existing as] json. json has the two types of containers: arrays and objects. Arrays only have numbered indices (from 1-N, a Lua sequence) and objects only have strings keys. Anything not a sequence can only be stored in JSON in an object, which causes the surprise when numeric keys are stringified.

Thanks for the report however for now; this is simply how tags behave.