Language Basics

Starlark Language Basics

A quick tour of Starlark syntax and features.

Data Types

Primitives

# Numbers
count = 42
pi = 3.14159

# Strings
name = "Starlark"
multiline = """
This is a
multiline string
"""

# Booleans
enabled = True
disabled = False

# None
empty = None

Collections

# Lists (mutable until frozen)
fruits = ["apple", "banana", "cherry"]
fruits.append("date")

# Tuples (immutable)
point = (10, 20)

# Dictionaries
person = {
    "name": "Alice",
    "age": 30,
    "languages": ["Python", "Go"],
}

# Sets (via dict keys)
unique = {x: True for x in [1, 2, 2, 3]}.keys()  # [1, 2, 3]

Functions

Basic Functions

def add(x, y):
    return x + y

result = add(2, 3)  # 5

Named Arguments

def greet(name, greeting="Hello"):
    return "{}, {}!".format(greeting, name)

greet("World")              # Hello, World!
greet("World", "Hi")        # Hi, World!
greet(greeting="Hey", name="You")  # Hey, You!

Variadic Functions

# *args - positional arguments
def sum_all(*numbers):
    total = 0
    for n in numbers:
        total += n
    return total

sum_all(1, 2, 3, 4)  # 10

# **kwargs - keyword arguments
def configure(**options):
    return options

configure(debug=True, verbose=False)  # {"debug": True, "verbose": False}

Control Flow

Conditionals

def classify(age):
    if age < 13:
        return "child"
    elif age < 20:
        return "teenager"
    else:
        return "adult"

For Loops

# Iterate over list
for fruit in ["apple", "banana"]:
    print(fruit)

# Iterate with index
for i, fruit in enumerate(["apple", "banana"]):
    print(i, fruit)

# Iterate over dict
for key, value in {"a": 1, "b": 2}.items():
    print(key, value)

# Range-based loop
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

Note

Starlark has no while loops to prevent infinite execution. Use for with range() instead.

Comprehensions

# List comprehension
squares = [x * x for x in range(5)]  # [0, 1, 4, 9, 16]

# With condition
evens = [x for x in range(10) if x % 2 == 0]  # [0, 2, 4, 6, 8]

# Dict comprehension
word_lengths = {word: len(word) for word in ["hello", "world"]}
# {"hello": 5, "world": 5}

String Operations

name = "Starlark"

# Methods
name.upper()          # "STARLARK"
name.lower()          # "starlark"
name.startswith("Star")  # True
name.replace("ark", "fish")  # "Starlfish"

# Formatting
"Hello, {}!".format("World")  # "Hello, World!"
"x={}, y={}".format(10, 20)   # "x=10, y=20"

# Join and split
", ".join(["a", "b", "c"])  # "a, b, c"
"a,b,c".split(",")          # ["a", "b", "c"]

Loading Modules

# Load symbols from another file
load("//lib:utils.star", "helper", "CONSTANT")

# Load with alias
load("//lib:math.star", my_add = "add")

# Use loaded symbols
result = helper()
value = CONSTANT
sum = my_add(1, 2)

Note

Unlike Python’s import, Starlark’s load() executes at parse time and only imports specific symbols.

Structs

Starlark doesn’t have classes, but you can use structs for structured data:

# Create a struct constructor
def make_person(name, age):
    return struct(
        name = name,
        age = age,
        is_adult = age >= 18,
    )

alice = make_person("Alice", 25)
print(alice.name)      # "Alice"
print(alice.is_adult)  # True

Providers (Bazel)

In Bazel rules, providers are the standard way to pass information:

MyInfo = provider(fields = ["value", "files"])

def _my_rule_impl(ctx):
    return [MyInfo(
        value = "hello",
        files = ctx.files.srcs,
    )]

Best Practices

1. Prefer immutability - Don’t modify data after creation
2. Use descriptive names - Starlark code is often configuration
3. Keep functions pure - Same inputs, same outputs
4. Document with docstrings - Tools like skydoc extract them
5. Test your Starlark - Use skytest for unit tests

Try It Online

Practice Starlark in the browser:

• Starlark Playground