$ ./rpn.py <<< "3 4 + 5 * log2 print"
5.129283016944966

RPN is a proof-of-concept interpreter for entering Python commands in a simple reverse-Polish style. The interpreter has the following meta-commands to control the program's state:

• help: display this text
• stack: print the entire stack for debugging
• clear: delete the entire stack
• undo: restore the previous stack
• exec: run the following Python command in the current namespace

RPN supports the following operations on the stack:

• n collect: pop n elements of the stack and append them as a list
• dup: duplicate the top element on the stack
• n dups: duplicate the top n elements on the stack

Python built-in functions (and functions from math) are supported, and given a number of (popped) arguments from the stack equal to the number of arguments accepted by the function. Obviously this won't work as expected for some functions, so that's one limitation of this proof-of-concept.

Any other text is assumed to be an RPN expression. For example:

• 1 2 3: add the numbers 1 2 3 to the stack
• 1 2 +: add 3 to the stack
• 1 2 3 3 collect sum print: add 6 to the stack and print it

This last example is adding the numbers 1 2 3 3 to the stack, then collect pops the top value off the stack (3), which tells it to pop 3 more items of the stack, and put them back on the stack as a list. sum then pops this item and adds the sum (6). You could play with this by entering one value at a time at the REPL, followed by stack to see the current contents of the stack.

Note that python built-ins like sum and print are available, along with the built-in operators and functions from math, by default.