Starlark

Starlark is a lightweight, high-level programming language designed for embedded use in applications. It uses a subset of the Python syntax. By default, the code is deterministic and hermetic.^[1]

Starlark was released in 2015 as part of Bazel under the name Skylark^[3]. This first implementation was written in Java. In 2018, the language was renamed Starlark.^[4]

In 2017, a new implementation of Starlark in Go was announced.^[5]

In 2021, Meta announced an implementation of Starlark written in Rust,^[6] to be used for the Buck build system.^[7][8]

In addition to the Bazel^[9] and Buck build systems, Starlark is used by dozens of projects,^[10][11] including Isopod^[12], skycfg^[13], Uber's Starlark Worker^[14], and Tilt.^[15]

On GitHub, Starlark is among the top 50 languages based on the developer activity.^[16][17]

Starlark syntax is a strict subset of Python syntax.^[1] Similar to Python syntax, Starlark relies on indentation to delimit blocks, using the off-side rule.

Starlark's statements include:^[18]

• The = statement to assign a value to a variable
• The augmented assignment statements to modify a variable
• The if statement to execute conditionally a block of code (with else or elif)
• The for statement to iterate over an iterable object
• The def statement to define a function
• The break statement to exit a loop
• The continue statement to skip the rest of the current iteration and continues with the next
• The pass statement, serving as a NOP, syntactically needed to create an empty code block
• The return statement to return a value from a function.
• The load statement, which replaces Python import, to import a value from another module.^[19] Unlike Python, the order of load statements does not affect the semantics of the code.^[20]

Unlike Python, Starlark statements don't include: while, try, raise, class, with, del, assert, yield, import, match and case.^[21]

To ensure thread safety and support parallel computing, Starlark has a feature called freezing. At the end of the evaluation of a module, all values become immutable. This means that the values that can be accessed from multiple threads can no longer be modified, which removes the risk of race conditions.^[3][22]