Compile Me!

ChocoPy is a programming language designed for classroom use in undergraduate compilers courses. ChocoPy is a restricted subset of Python 3, which can easily be compiled to a target such as RISC-V. The language is fully specified using formal grammar, typing rules, and operational semantics. ChocoPy is used to teach CS 164 at UC Berkeley. ChocoPy has been designed by Rohan Padhye and Koushik Sen, with substantial contributions from Paul Hilfinger.

New! A short paper on ChocoPy will appear at SPLASH-E'19. Check out the preprint.

At a glance, ChocoPy is:

Bonus: Due to static type safety and ahead-of-time compilation, most student implementations outperform the reference Python implementation on non-trivial benchmarks.

Try ChocoPy

# Search in a list
def contains(items:[int], x:int) -> bool:
    i:int = 0
    while i < len(items):
        if items[i] == x:
            return True
        i = i + 1
    return False

if contains([4, 8, 15, 16, 23], 15):
  print("Item found!")    # Prints this
  print("Item not found.")

Teaching Resources

The following resources are available for conducting a compilers course with ChocoPy:

These resources can be made available to instructors upon request.

Language Features

Here are some code examples demonstrating various features of ChocoPy. Edit and compile the programs below to observe the generated RISC-V assembly, which can itself be executed online. A detailed description of the ChocoPy language is available in the official language reference manual.

Static Type Checking

# A broken program
def is_even(x:int) -> bool:
    if x % 2 == 1:
        return 0      # FIXME
        return True

print(is_even("3"))   # FIXME

Nested functions with global/nonlocal variables

# Compute x**y
def exp(x: int, y: int) -> int:
    a: int = 0
    global invocations  # Count calls to this function

    def f(i: int) -> int:
        nonlocal a
        def geta() -> int:
            return a
        if i <= 0:
            return geta()
            a = a * x
            return f(i-1)
    a = 1
    invocations = invocations + 1
    return f(y)

invocations:int = 0
print(exp(2, 10))
print(exp(3, 3))

Lists, classes, and dynamic dispatch

# A resizable list of integers
class Vector(object):
    # Attributes
    items: [int] = None
    size: int = 0

    # Constructor
    def __init__(self:"Vector"):
        self.items = [0]

    # Returns current capacity
    def capacity(self:"Vector") -> int:
        return len(self.items)

    # Increases capacity of vector by one element
    def increase_capacity(self:"Vector") -> int:
        self.items = self.items + [0]
        return self.capacity()

    # Appends one item to end of vector
    def append(self:"Vector", item: int):
        if self.size == self.capacity():

        self.items[self.size] = item
        self.size = self.size + 1

# A faster (but more memory-consuming) implementation of vector
class DoublingVector(Vector):
    doubling_limit:int = 16

    # Overriding to do fewer resizes
    def increase_capacity(self:"DoublingVector") -> int:
        if (self.capacity() <= self.doubling_limit // 2):
            self.items = self.items + self.items
            # If doubling limit has been reached, fall back to
            # standard capacity increases
            self.items = self.items + [0]
        return self.capacity()
vec:Vector = None
num:int = 0

# Create a vector and populate it with The Numbers
vec = DoublingVector()
for num in [4, 8, 15, 16, 23, 42]:

ChocoPy does not support modules and imports, higher order functions, native dictionaries, and exceptions.

Contact for Instructors

Want to use ChocoPy to run your own compilers course? Send an email to