Stacker is a command-line calculator that uses reverse Polish notation (RPN) for mathematical calculations and features an RPN-based scripting language. - remokasu/stacker
So it looks like a totally different data flow style, and (I think) geared toward writing then running programs, whereas Stacker is more for interactive stack-oriented calculator tasks.
I'd say an important part of this calculator's interaction model is doing something, getting a result, then doing something else to that result. That's not too bad in the regular Python interpreter either.
For example, in Python:
>>> 5
5
>>> 4 + _
9
>>> 2 * _
18
In Stacker:
>>> 5
[5]
>>> 4 +
[9]
>>> 2 *
[18]
Does Hy have something like the Python interpreter's _?