scrolls-py 0.4.0

A customizable interpreted microlanguage designed to run untrusted code.

Scrolls

Scrolls is a small interpreter originally designed to allow users of my discord bots to make custom commands. It prioritizes control over the interpreter to help prevent abuse, while still allowing tight integration with python code.

Links

• Documentation: https://a-bison.github.io/scrolls-py-docs/
• Source Code: https://github.com/a-bison/scrolls-py
• PyPI: https://pypi.org/project/scrolls-py/

Why?

The two other candidates for user scripts were python and Lua. Python code is a nightmare to sandbox properly, and the available Lua interpreters for python didn't give me the kind of control I wanted over the interpreter. In addition, Lua was a bit much for simple custom commands. So, I made my own interpreter.

There is a scripting language available for Rust called Rhai with a similar concept.

Also, I just kinda wanted to try making an interpreted language...

Goals

• Allow the developer to prevent abuse.
• Integrate tightly with the parent python application.
• Keep the syntax as simple as possible.

Getting Started

Check out the Links section above.

Installing

Scrolls may be installed through pip:

Linux

python3 -m pip install scrolls-py

Windows

py -3 -m pip install scrolls-py

Command Line Usage

The module comes with a built-in interpreter that may be invoked by executing the module. (Note that the linux version will be used from now on.)

python3 -m scrolls FILE

Try running some of the examples:

python3 -m scrolls ./examples/arithmetic.scrl

If no file is specified, the interpreter will run in interactive mode:

python3 -m scrolls

Example Code

Here are some sample programs showcasing the basic syntax of Scrolls. See the examples directory for more.

Fizzbuzz

!def(divisible? a b) {
    return $(eq? 0 $(% $a $b))
}

!for(n in $^(rangev 1 101)) {
    !if($(divisible? $n 15)) {
        print "FizzBuzz"
    } !elif($(divisible? $n 3)) {
        print "Fizz"
    } !elif($(divisible? $n 5)) {
        print "Buzz"
    } !else {
        print $n
    }
}

Nth Fibonacci Number

print "This will calculate fib N where fib 0 = 0, fib 1 = 1"
print "Enter N."
input n

set output_msg "Fibonacci number" $n "is"

!if($(< $n 2)) {
    print $^output_msg $n
    stop
}

set fib_prev 0
set fib 1
set i 2

!while($(<= $i $n)) {
    set tmp $fib_prev
    set fib_prev $fib
    set fib $(+ $tmp $fib)
    set i $(+ $i 1)
}

print $^output_msg $fib

Syntax Quirks

# In scrolls everything is a string
"print" "Everything is a string:"
!"for"("operator" "in" "+" "-" "*" "/") {
  "print" $($"operator" "5" "8")
}

String Escapes

print "\"test escapes\"\n\ttabbed\n\tlines\nunicode is supported: \u0398hello\u0398\n"

Programmatic Usage

Scrolls may be embedded into any python program:

import scrolls

# Create an interpreter. Note that an interpreter created this 
# way will not actually do anything. It's the responsibility of 
# the user to configure with the desired language features.
interpreter = scrolls.Interpreter()

# Configure the interpreter with the base language.
# scrolls.base_config is provided to make this common task
# a bit easier.
scrolls.base_config.configure(interpreter)

# Configure with stdio commands like input, and print
interpreter.command_handlers.add(scrolls.StdIoCommandHandler())

# Run your script.
script = """
print "Please enter your name:"
input name
!repeat(4) {
    print "Hello," $(cat $name "!")
}
"""
interpreter.run(script)

Known Issues

Interactive Control Calls

Multiple control calls without an explicit command separator will break parsing in interactive mode. For example, the following wrongly identifies else as a command call:

>>> !if($false) { print "from if" } !else() { print "from else" }
error:
0 !if($false) { print "from if" } !else() { print "from else" }
                                   ^
line 0: Command_call 'else' not found.

A workaround for this is to enter them one line at a time:

>>> !if($false) { print "from if" }
>>> !else() { print "from else" }
from else

Or use a semicolon:

>>> !if($false) { print "from if" }; !else() { print "from else" }
from else

This bug does not affect the parsing of whole scripts.

Acknowledgements

• hikari-lightbulb by tandemdude, which inspired the CallBase extension (see here).