Expr is a Go-centric expression language designed to deliver dynamic configurations with unparalleled accuracy, safety, and speed. Expr combines simple syntax with powerful features for ease of use:

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

// Determine whether the request is in the permitted time window.
request.Time - resource.Age < duration("24h")

// Ensure all tweets are less than 240 characters.
all(tweets, len(.Content) <= 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/expr-lang/expr

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

Play Online

package main

import (
	"fmt"
	"github.com/expr-lang/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/expr-lang/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)
}

• Google uses Expr as one of its expression languages on the Google Cloud Platform.
• Uber uses Expr to allow customization of its Uber Eats marketplace.
• GoDaddy employs Expr for the customization of its GoDaddy Pro product.
• ByteDance incorporates Expr into its internal business rule engine.
• Aviasales utilizes Expr as a business rule engine for its flight search engine.
• Wish.com employs Expr in its decision-making rule engine for the Wish Assistant.
• Argo integrates Expr into Argo Rollouts and Argo Workflows for Kubernetes.
• OpenTelemetry integrates Expr into the OpenTelemetry Collector.
• Philips Labs employs Expr in Tabia, a tool designed to collect insights on their code bases.
• CrowdSec incorporates Expr into its security automation tool.
• CoreDNS uses Expr in CoreDNS, which is a DNS server.
• qiniu implements Expr in its trade systems.
• Junglee Games uses Expr for its in-house marketing retention tool, Project Audience.
• Faceit uses Expr to enhance customization of its eSports matchmaking algorithm.
• Chaos Mesh incorporates Expr into Chaos Mesh, a cloud-native Chaos Engineering platform.
• Visually.io employs Expr as a business rule engine for its personalization targeting algorithm.
• Akvorado utilizes Expr to classify exporters and interfaces in network flows.
• keda.sh uses Expr to allow customization of its Kubernetes-based event-driven autoscaling.
• Span Digital uses Expr in it's Knowledge Management products.
• Xiaohongshu combining yaml with Expr for dynamically policies delivery.

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