Gosu (programming language)

Gosu
Developer	Guidewire and open source contributors
Stable release	1.18.6  / 28 June 2025 (28 June 2025)
Typing discipline	static
Platform	Execute on the Java Virtual Machine, statically and dynamically compiles to bytecode
OS	any supporting JVM
License	Apache License 2.0
Filename extensions	.gs, .gsp, .gst, .gsx
Website	gosu-lang.github.io
Influenced by
Java, C#
Influenced
Kotlin

Gosu is a statically typed general-purpose programming language that runs on the Java Virtual Machine. Its influences include Java, C#, and ECMAScript. Development of Gosu began in 2002 internally for Guidewire Software, and the language saw its first community release in 2010 under the Apache 2 license.

Gosu can serve as a scripting language, having free-form Program types (.gsp files) for scripting as well as statically verified Template files (.gst files). Gosu can optionally execute these and all other types directly from source without precompilation, which also distinguishes it from other static languages.

History

Gosu is often described as a Java variant that introduces practical improvements while maintaining compatibility and interoperability with Java. The language originated in 2002 at Guidewire Software, where it was initially known as GScript, a scripting language designed for use within Guidewire applications. As GScript evolved from a simple scripting tool into a general-purpose programming language, it was renamed Gosu by its lead developer, Scott McKinney.

Originally created as a “glue language” to allow Guidewire customers to define business rules, Gosu quickly found broader use within the company’s products, supporting areas such as user interface bindings, templating, and document management. Over time, Gosu became the primary implementation language across Guidewire’s application suite, surpassing Java in overall usage.

Guidewire released Gosu 0.7 beta to the public in November 2010, followed by 0.8 beta in December 2010 and 0.8.6 beta in mid-2011. These releases introduced several enhancements, including new typeloaders that allowed Gosu to dynamically represent external data formats such as JSON and XML as native Gosu types.

Guidewire continues to use and support Gosu extensively within its InsuranceSuite applications. While active development of new Gosu language features has been paused, the company continues to expand InsuranceSuite through RESTful APIs and integration frameworks accessible from both Gosu and Java.

Philosophy

Gosu language creator and development lead, Scott McKinney, emphasizes pragmatism, found in readability and discoverability, as the overriding principle that guides the language's design. For instance, Gosu's rich static type system is a necessary ingredient toward best of breed tooling via static program analysis, rich parser feedback, code completion, deterministic refactoring, usage analysis, navigation, and the like.

Syntax and semantics

Gosu follows a syntax resembling a combination of other languages. For instance, declarations follow more along the lines of Pascal with name-first grammar. Gosu classes can have functions, fields, properties, and inner classes as members. Nominal inheritance and composition via delegation are built into the type system as well as structural typing similar to the Go programming language.

Gosu supports several file types:

• Class (.gs files)
• Program (.gsp files)
• Enhancement (*.gsx files)
• Template (*.gst files)

In addition to standard class types Gosu supports enums, interfaces, structures, and annotations.

Program files facilitate Gosu as a scripting language. For example, Gosu's Hello, World! is a simple one-line program:

print("Hello, World!")

Gosu classes are also executable a la Java:

class Main {
  static function main(args: String[]) {
    print("Hello, World!")
  }
}

Data types

A unique feature of Gosu is its Open Type System, which allows the language to be easily extended to provide compile-time checking and IDE awareness of information that is typically checked only at runtime in most other languages. Enhancements let you add additional functions and properties to other types, including built-in Java types such as String, List, etc. This example demonstrates adding a print() function to java.lang.String.

enhancement MyStringEnhancement : String {
  function print() {
    print(this)
  }
}

Now you can tell a String to print itself:

"Echo".print()

The combination of closures and enhancements provide a powerful way of coding with Collections. The overhead of Java streams is unnecessary with Gosu:

var list = {1, 2, 3}
var result = list.where(\ elem -> elem >= 2)
print(result)

Uses

This general-purpose programming language is used primarily in Guidewire Software's commercial products.

Further reading

• McKinney, Scott (2010-07-26). "Introduction to Gosu" (PDF). JVM Language Summit 2010. Oracle.

Event

• Gross, Carson (2011-07-18). "Language Features As A Library: Using Gosu's Open Type System With External DSLs" (PDF). JVM Language Summit 2011. Oracle. Video

External links

• Official website
• "Corporate website". Guidewire Software.
• gosu-lang on GitHub