Kuroko - A bytecode-compiled scripting language

Kuroko is a bytecode-interpreted, dynamic, strongly-typed language with syntax similar to Python.

The bytecode VM / compiler is substantially based on Robert Nystrom's Crafting Interpreters.

At the moment, the intent for this project is to add a proper scripting language to Bim, to which both configuration scripts and syntax highlighting will be ported.

Kuroko, as its name should imply, will also be made available in ToaruOS as a general-purpose user language, and some utilities may end up being written in it.

Features

Kuroko inherits some core features by virtue of following Crafting Interpreters, including its basic type system, classes/methods/functions, and the design of its compiler and bytecode VM.

On top of this, Kuroko has:

• Python-style indentation-based block syntax.
• A syntax-highlighted repl, using ToaruOS's rline line editing library.
• Collection types including lists and dicts, with [] indexing syntax and inline declarations.
• List comprehensions ([foo(x) for x in [1,2,3,4]] and similar expressions).
• Iterator for loops (for i in l: ...).
• Exception handling with try/except/raise.
• A module import system for including additional code at runtime.

Examples

NOTE: Due to limitations with Github's markdown renderer, these snippets will be highlighted as Python code.

Hello World

Kuroko inherits a print statement from its Lox roots, which is similar to the one in Python 2:

print "Hello, world!"
# → Hello, world!

Multiple expressions can be supplied to print and will be concatenated with spaces:

print "Hello", 42, "!"
# → Hello 42 !

Basic Types

Kuroko's basic types are integers (which use the platform long type), double-precision floats, booleans (True and False), and None.

print 1 + 2 + 3
# → 6

When integer values are used in arithmetic operations, such as division, the result will be an integer as well:

print 1 / 2
# → 0

To get floating-point results, one of the arguments should be explicitly typed or converted:

print 1 / 2.0
# → 0.5

Implicit type conversion occurs late in evaluation, so be careful of integer overflows:

# Probably not what you want:
print 1000000000 * 1000000000 * 1000000000 * 3.0
# → -2.07927e+19
# Try something like this instead:
print 1000000000.0 * 1000000000 * 1000000000 * 3.0
# → 3e+27

Objects

Objects are values which live on the heap. Basic objects include strings, functions, classes, and instances.

Objects are passed by reference, though strings are immutable so this property is only relevant for other object types.

Strings

Strings can be concatenated, and other values can be appended to them.

print "Hello, " + 42 + "!"
# → Hello, 42!

Functions

Function syntax is essentially the same as in Python:

def greet(name):
    print "Hello, " + name + "!"
greet("user")
# → Hello, user!

Default arguments can be specified as follows:

def greet(name=None):
    if not name:
        print "Hello, world!"
    else:
        print "Hello, " + name + "!"
greet()
gree("user")
# → Hello, world!
#   Hello, user!

If a default argument value is not provided, the expression assigned to it will be evaluated as if it were at the top of the body of the function, like in Ruby (and not like in Python).

Blocks, including function def blocks and control flow structures like if and for, must be indented with spaces to a level greater than the enclosing block.

You may indent blocks to whatever level you desire, so long as ordering remains consistent, though the recommendtation indentation size is 4 spaces.

Tabs are not valid as indentation and will be ignored. It is recommended that you use an editor which provides a clear visual distinction between tabs and spaces, such as Bim.

if False:
	print "Oh no, that was a tab."
# → Oh no, that was a tab.

Blocks can also accept a single inline statement:

if True: print "The first rule of Tautology Club is the first rule of Tautology Club."
# → The first rule of Tautology Club is the first rule of Tautology Club.

Variables

In a departure from Python, Kuroko has explicit variable declaration and traditional scoping rules. Variables are declared with the let keyword and take the value None if not defined at declaration time:

let foo
print foo
# → None
foo = 1
print foo
# → 1

Closures

Functions are first-class values and may be returned from functions and stored in variables, producing closures.

When a function references local values from an outter scope, such as in the example below, the referenced variables will be captured.

def foo():
    let i = 1 # Local to this call to foo()
    def bar():
        print i # Reference to outer variable
        i = i + 1
    return bar # Produces a closure
let a = foo() # Each copy of `bar` gets its own `i`
let b = foo()
let c = foo()
a() # So these all print "1" as the first call,
b() # but each one also increments its own copy of i
c()
a() # So further calls will reference that copy
a()
a()
# → 1
#   1
#   1
#   2
#   3
#   4

Basic Objects and Classes

Objects and classes in Kuroko work a lot like Python or similar languages in that they have an arbitrary and mutable set of fields, which may be methods or other values.

To create a basic object without methods, the object class is provided:

let o = object()
o.foo = "bar"
print o.foo
# → bar

To supply methods, define a class:

class Foo():
    def printFoo():
        print self.foo
let o = Foo()
o.foo = "bar"
o.printFoo()
# → bar

The self keyword is implicit in all methods and does not need to be supplied in the argument list. You may optionally include it in the method declaration anyway:

class Foo():
    def printFoo(self):
        print self.foo
let o = Foo()
o.foo = "bar"
o.printFoo()
# → bar

When a class is instantiated, if it has an __init__ method it will be called automatically. __init__ may take arguments as well.

class Foo():
    def __init__(bar):
        self.foo = bar
    def printFoo(self):
        print self.foo
let o = Foo("bar")
o.printFoo()
# → bar

Some other special method names include __get__, __set__, and __str__, which will be explained later.

Note: Unlike in Python, most types are not actually instances of classes, though many of the same operations still apply to them.

Inheritence

Classes may inherit from a single super class:

class Foo():
    def __init__():
        self.type = "foo"
    def printType():
        print self.type

class Bar(Foo):
    def __init__():
        self.type = "bar"

let bar = Bar()
bar.printType()
# → bar

Methods can refer to the super class with the super keyword:

class Foo():
    def __init__():
        self.type = "foo"
    def printType():
        print self.type

class Bar(Foo):
    def __init__():
        self.type = "bar"
    def printType():
        super().printType()
        print "Also, I enjoy long walks on the beach."

let bar = Bar()
bar.printType()
# → bar
#   Also, I enjoy long walks on the beach.

You can determine at runtime if an object is an instance of a class, either directly or through its inheretince chain, with the isinstance builtin function:

class Foo:
class Bar:
class Baz(Bar):
let b = Baz()
print isinstance(b,Baz), isinstance(b,Bar), isinstance(b,Foo), isinstance(b,object)
# → True, True, False, True

All classes eventually inherit from the base class object, which provides default implementations of some special instance methods.

Collections

Kuroko has built-in classes for flexible arrays (list) and hashmaps (dict):

let l = list()
l.append(1)
l.append(2)
l.append("three")
l.append(False)
print l
# → [1, 2, three, False]
l[1] = 5
print l
# → [1, 5, three, False]
let d = dict()
d["foo"] = "bar"
d[1] = 2
print d
# → {1: 2, foo: bar}

These built-in collections can also be initialized as expressions, which act as syntactic sugar for the listOf and dictOf built-in functions:

let l = [1,2,"three",False] # or listOf(1,2,"three",False)
print l
# → [1, 2, three, False]
let d = {"foo": "bar", 1: 2} # or dictOf("foo","bar",1,2)
print d
# → {1: 2, foo: bar}

Lists can also be generated dynamically:

let fives = [x * 5 for x in [1,2,3,4,5]]
print fives
# → [5, 10, 15, 20, 25]

Exceptions

Kuroko provides a mechanism for handling errors at runtime. If an error is not caught, the interpreter will end and print a traceback.

def foo(bar):
    print "I expect an argument! " + bar
foo() # I didn't provide one!
# → Traceback, most recent first, 1 call frame:
#     File "<stdin>", line 1, in <module>
#   ArgumentError: foo() takes exactly 1 argument (0 given)

To catch exceptions, use try/except:

def foo(bar):
    print "I expect an argument! " + bar
try:
    foo() # I didn't provide one!
except:
    print "oh no!"
# → oh no!

Runtime exceptions are passed to the except block as a special variable exception. As of this writing, runtime exceptions from the VM are strings.

def foo(bar):
    print "I expect an argument! " + bar
try:
    foo() # I didn't provide one!
except:
    print "oh no, there was an exception: " + exception.arg
# → oh no, there was an exception: foo() takes exactly 1 argument (0 given)

Exceptions can also be generated from code:

def login(password):
    if password != "supersecret":
        raise "Wrong password, try again!"
    print "[Hacker voice] I'm in."
login("foo")
# → Traceback, most recent first, 2 call frames:
#     File "<stdin>", line 5, in <module>
#     File "<stdin>", line 3, in login
#   Wrong password, try again!

The except block is optional, and an exception may be caught and ignored.

def login(password):
    if password != "supersecret":
        raise "Wrong password, try again!"
    print "[Hacker voice] I'm in."
try:
    login("foo")
# (no output)

Modules

Modules allow scripts to call other scripts.

# modules/demomodule.krk
let module = object()
module.foo = "bar"
return module

# demo.krk
import demomodule
print demomodule.foo
# → bar

When modules are imported, they run in a function local context and variables they declare do not live in the global namespace.

To put variables into the global namespace, use the export keyword:

# modules/demomodule.krk
let module = object()
foo = "bar"
export foo
return module

# demo.krk
import demomodule
print foo
# → bar

Loops

Kuroku supports C-style for loops, while loops, and Python-style iterator for loops.

for i = 1, i < 5, i = i + 1:
    print i
# → 1
#   2
#   3
#   4

let i = 36
while i > 1:
    i = i / 2
    print i
# → 18
#   9
#   4
#   2
#   1

let l = [1,2,3]
for i in l:
    print i
# → 1
#   2
#   3

Iterators

The special method __iter__ should return an iterator. An iterator should be a function which increments an internal state and returns the next value. If there are no values remaining, return the iterator itself.

An example of an iterator is the range built-in class, which is defined like this:

class range:
    "Helpful iterable."
    def __init__(self, min, max):
        self.min = min
        self.max = max
    def __iter__(self):
        let i=self.min
        let l=self.max
        def _():
            if i>=l:
                return _
            let o=i
            i++
            return o
        return _

Objects which have an __iter__ method can then be used with the for ... in ... syntax:

for i in range(1,5):
    print i
# → 1
#   2
#   3
#   4

Indexing

Objects with the methods __get__ and __set__ can be used with square brackets []:

class Foo:
    def __get__(ind):
        print "You asked for ind=" + ind
        return ind * 5
    def __set__(ind, val):
        print "You asked to set ind=" + ind + " to " + val
let f = Foo()
print f[4]
f[7] = "bar"
# → You asked for ind=4
#   20
#   You asked to set ind=7 to bar

String Slicing

Substrings can be extracted from strings via slicing:

print "Hello world!"[3:8]
# → lo wo
print "Hello world!"[:-1]
# → Hello world
print "Hello world!"[-1:]
# → !

NOTE: Slicing is not yet provided for other types, and step values are currently not supported.

String Conversion

If an object implements the __str__ method, it will be called to produce string values when concatenating or printing.

class Foo:
    def __str__():
        return "(I am a Foo!)"
let f = Foo()
print f
# → (I am a Foo!)

File I/O

The module fileio provides an interface for opening, reading, and writing files, including stdin/stdout/stderr.

To open and read the contents of a file:

import fileio
let f = fileio.open("README.md","r")
print f.read()
f.close()

To write to stdout (notably, without automatic line feeds):

import fileio
fileio.stdout.write("hello, world")

To read lines from stdin:

import fileio

while True:
    fileio.stdout.write("Say something: ")
    fileio.stdout.flush()

    let data = fileio.stdin.readline()
    if data[-1] == '\n':
        data = data[:-1]
    if data == "exit":
        break
    print "You said '" + data + "'!"

Decorators

Decorators allow functions and methods to be wrapped.

def decorator(func):
    print "I take the function to be decorated as an argument:", func
    def wrapper():
        print "And I am the wrapper."
        func()
        print "Returned from wrapped function."
    return wrapper

@decorator
def wrappedFunction():
    print "Hello, world"

wrappedFunction()
# → I take a function to be decorated as an argument: <function wrappedFunction>
#   And I am the wrapper.
#   Hello, world
#   Returned from wrapped function

The resulting function will have the same signature as the original function, so wrappers may take arguments to pass to the wrapped function, or may take their own arguments (or both).

Method wrappers work similarly, though be sure to explicitly provide a name (other than self) for the object instance:

def methodDecorator(method):
    def methodWrapper(instance, anExtraArgument):
        method(instance)
        print "I also required this extra argument:", anExtraArgument
    return methodWrapper

class Foo():
    @methodDecorator
    def theMethod():
        print "I am a method, so I can obviously access", self
        print "And I also didn't take any arguments, but my wrapper did:"

let f = Foo()
f.theMethod("the newly required argument")
# → I am a method, so I can obviously access <instance of Foo at ...>
#   And I also didn't take any arguments, but my wrapper did:
#   I also required this extra argument: the newly required argument

Decorators are expressions, just like in Python, so to make a decorator with arguments create a function that takes those arguments and returns a decorator:

def requirePassword(password):
    print "I am creating a decorator."
    def decorator(func):
        print "I am wrapping", func, "and attaching",password
        def wrapper(secretPassword):
            if secretPassword != password:
                print "You didn't say the magic word."
                return
            func()
        return wrapper
    return decorator

@requirePassword("hunter2")
def superSecretFunction():
    print "Welcome!"

superSecretFunction("a wrong password")
print "Let's try again."
superSecretFunction("hunter2")
# → I am wrapping <function superSecretFunction> and attaching hunter2
#   You didn't say the magic word.
#   Let's try again.
#   Welcome!

Keyword Arguments

Arguments may be passed to a function by specifying their name instead of using their positional location.

def aFunction(a,b,c):
    print a,b,c

aFunction(1,2,3)
aFunction(1,c=3,b=2)
aFunction(b=2,c=3,a=1)
# → 1 2 3
#   1 2 3
#   1 2 3

This will be slower in execution than a normal function call, as the interpreter will need to figure out where to place arguments in the requested function by examining it at runtime, but it allows for functions to take many default arguments without forcing the caller to specify the values for everything leading up to one they want to specifically set.

def aFunction(with=None,lots=None,of=None,default=None,args=None):
    print with,lots,of,default,args

aFunction(of="hello!")
# → None None hello! None None

*args and **kwargs

When used as parameters in a function signature, * and ** before an identifier indicate that the function will accept arbitrary additional positional arguments and keyword arguments respectively. These options are typically applied to variables named args and kwargs, and they must appear last (and in this order) in the function signature if used.

The variable marked with * will be provided as an ordered list, and ** will be an unordered dict of keyword names to values.

def takesArgs(*args):
    print args
takesArgs(1,2,3)
# → [1, 2, 3]
def takesKwargs(**kwargs):
    print kwargs
takesKwargs(a=1,b=2,c=3)
# → {'a': 1, 'b': 2, 'c': 3}
def takesEither(*args,**kwargs):
    print args, kwargs
takesEither(1,2,a=3,b=4)
# → [1, 2] {'a': 3, 'b': 4}
def takesARequiredAndMore(a,*args):
    print a, args
takesARequiredAndMore(1,2,3,4)
# → 1 [2, 3, 4]

Argument Expansion

When used in a function argument list, * and ** before a list and dict expression respectively, will expand those values into the argument list.

let l = [1,2,3]
def foo(a,b,c):
    print a,b,c
foo(*l)
# → 1 2 3
let d = {"foo": "a", "bar": 1}
def func(foo,bar):
    print foo, bar
func(**d)
# → a 1

If an expanded list provides too many, or too few, arguments, an ArgumentError will be raised.

If an expanded dict provides parameters which are not requested, an ArgumentError will be raised.

If an expanded dict provides an argument which has already been defined, either as a positional argument or through a named parameter, an error will be raised.

Note: Argument expansion is not currently supported for functions provided by C modules.

About the REPL

Kuroko's repl provides an interactive environment for executing code and seeing results.

When entering code at the repl, lines ending with colons (:) are treated specially - the repl will continue to accept input and automatically insert indentation on a new line. Please note that the repl's understanding of colons is naive: Whitespace or comments after a colon which would normally be accepted by Kuroko's parser will not be understood by the repl - if you want to place a comment after the start of a block statement, be sure that it ends in a colon so you can continue to enter statements.

Pressing backspace when the cursor is preceded by whitespace will delete up to the last column divisible by 4, which should generally delete one level of indentation automatically.

The tab key will also produce spaces when the cursor is at the beginning of the line or preceded entirely with white space.

The repl will display indentation level indicators in preceeding whitespace as a helpful guide.

When a blank line or a line consisting entirely of whitespace is entered, the repl will process the full input.

Code executed in the repl runs in a global scope and reused variable names will overwrite previous definitions, allowing function and class names to be reused.

The repl will display the last value popped from the stack before returning. Note that unlike with the print statement, objects printed in this way from the repl will not be converted to strings, so they may display differently.

What's different from Python?

You may be looking at the code examples and thinking Kuroko looks a lot more like Python than "syntax similar to Python" suggests. Still, there are some differences, and they come in two forms: Intentional differences and unintentional differences.

Unintentional differences likely represent incomplete features. Intentional differences are design decisions specifically meant to differentiate Kuroko from Python and usually are an attempt to improve upon or "fix" perceived mistakes.

Two notable intentional differences thus far are:

• Kuroko's variable scoping requires explicit declarations. This was done because Python's function-level scoping, and particularly how it interacts with globals, is often a thorn in the side of beginner and seasoned programmers alike. It's not so much seen as a mistake as it is something we don't wish to replicate.
• Default arguments to functions are evaluated at call time, not at definition time. How many times have you accidentally assigned an empty list as a default argument, only to be burned by its mutated descendent appearing in further calls? Kuroko doesn't do that - it works more like Ruby.