Blackboard's set function seems to set variable to type string

[sven-hoek]

What I do

After creating a tree, I set blackboard entries from data read in from a JSON string, something like this

{
  "goal_position": 0.0,
  "goal_tolerance": 0.01,
  "serial_port": "/dev/ttyUSB0",
}

I currently restrict to primitive values, setting the parameters with something like this:

const auto parameters = nlohmann::json::parse(json_string);
for (const auto& [key, value] : parameters.items()) {
    if (value.is_string()) {
        // Necessary to avoid having quotes in the string-value
        blackboard->set(key, value.get<std::string>());
    } else if (value.is_primitive()) {
        blackboard->set(key, value.dump());
    } else {
        // discard
    }
}

In my tree, I want to set values according to those set blackboard entries, like so:

<Script code="goal_range_min := goal_position - goal_tolerance" />

What I expect

I expect the script node to succeed, resulting in a blackboard entry goal_range_min, which contains the difference of goal_position and goal_tolerance. In this example, it would be -0.01.

What I get

Error in script [goal_range_min := goal_position - goal_tolerance]

---

I get a bit more info when I do the calculation in two steps, i.e.

<Script code="goal_range_min := goal_position" />
<Script code="goal_range_min -= goal_tolerance" />

which will result in the following error:

Error in script [goal_range_min -= goal_tolerance]
Operator not supported for strings

---

Or, when I add a Script node at the beginning of the tree to set/overwrite an entry, i.e.

<Script code="goal_tolerance := 0.03" />

I get

Error in script [goal_tolerance := 0.03]
Error assigning a value to entry [goal_tolerance] with type [std::string]. 
The right operand has type [double] and can't be converted to [std::string]

However, when I delete the goal_tolerance entry from the JSON and only set it through the Script Node, while still setting the goal_position from the JSON, it seems to work.

Thoughts

This hints me that the blackboard entries have the type information to be a string. However, I would expect them to be without type information, according to this line and following: https://github.com/BehaviorTree/BehaviorTree.CPP/blob/3c62813b84fa9b26d8cb88dc38da17deb852f6a7/include/behaviortree_cpp/blackboard.h#L220

So, are mathematical operations between two AnyTypeAlloweds not possible? I assume if both are of unknown type, it is unknown what the operation is supposed to be (addition? string concatenation?).

But is there any solution? I could infer the data type from the JSON entry and use the right template instantiation of Blackboard::set<T>. Any other suggestion?