Expr is a Go-centric expression language designed to deliver dynamic configurations with unparalleled accuracy, safety, and speed.

// Allow only admins and moderators to moderate comments.
user.Group in ["admin", "moderator"] || user.Id == comment.UserId

// Ensure all tweets are less than 240 characters.
all(Tweets, .Size <= 240)

Expr is a safe, fast, and intuitive expression evaluator optimized for the Go language. Here are its standout features:

• Memory-Safe: Expr is designed with a focus on safety, ensuring that programs do not access unrelated memory or introduce memory vulnerabilities.
• Side-Effect-Free: Expressions evaluated in Expr only compute outputs from their inputs, ensuring no side-effects that can change state or produce unintended results.
• Always Terminating: Expr is designed to prevent infinite loops, ensuring that every program will conclude in a reasonable amount of time.

• Seamless with Go: Integrate Expr into your Go projects without the need to redefine types.

• Ensures type correctness and prevents runtime type errors.

  out, err := expr.Compile(`name + age`)
  // err: invalid operation + (mismatched types string and int)
  // | name + age
  // | .....^

• Provides user-friendly error messages to assist with debugging and development.

• Rich Operators: Offers a reasonable set of basic operators for a variety of applications.
• Built-in Functions: Functions like all, none, any, one, filter, and map are provided out-of-the-box.

• Optimized for Speed: Expr stands out in its performance, utilizing an optimizing compiler and a bytecode virtual machine. Check out these benchmarks for more details.

go get github.com/antonmedv/expr

• See Getting Started page for developer documentation.
• See Language Definition page to learn the syntax.

Also, I have an embeddable code editor written in JavaScript which allows editing expressions with syntax highlighting and autocomplete based on your types declaration.

Learn more →

Play Online

package main

import (
	"fmt"
	"github.com/antonmedv/expr"
)

func main() {
	env := map[string]interface{}{
		"greet":   "Hello, %v!",
		"names":   []string{"world", "you"},
		"sprintf": fmt.Sprintf,
	}

	code := `sprintf(greet, names[0])`

	program, err := expr.Compile(code, expr.Env(env))
	if err != nil {
		panic(err)
	}

	output, err := expr.Run(program, env)
	if err != nil {
		panic(err)
	}

	fmt.Println(output)
}

Play Online

package main

import (
	"fmt"
	"github.com/antonmedv/expr"
)

type Tweet struct {
	Len int
}

type Env struct {
	Tweets []Tweet
}

func main() {
	code := `all(Tweets, {.Len <= 240})`

	program, err := expr.Compile(code, expr.Env(Env{}))
	if err != nil {
		panic(err)
	}

	env := Env{
		Tweets: []Tweet{{42}, {98}, {69}},
	}
	output, err := expr.Run(program, env)
	if err != nil {
		panic(err)
	}

	fmt.Println(output)
}

• Aviasales uses Expr as a business rule engine for our flight search engine.
• Wish.com uses Expr for decision-making rule engine in the Wish Assistant.
• Argo uses Expr in Argo Rollouts and Argo Workflows for Kubernetes.
• Crowdsec uses Expr in a security automation tool.
• FACEIT uses Expr to allow customization of its eSports matchmaking algorithm.
• qiniu uses Expr in trade systems.
• Junglee Games uses Expr for an in house marketing retention tool Project Audience.
• OpenTelemetry uses Expr in the OpenTelemetry Collector.
• Philips Labs uses Expr in Tabia, a tool for collecting insights on the characteristics of our code bases.
• CoreDNS uses Expr in CoreDNS, a DNS server.
• Chaos Mesh uses Expr in Chaos Mesh, a cloud-native Chaos Engineering platform.
• Milvus uses Expr in Milvus, an open-source vector database.
• Visually.io uses Expr as a business rule engine for our personalization targeting algorithm.
• Akvorado uses Expr to classify exporters and interfaces in network flows.

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