+
+
+**Expr** package provides an engine that can compile and evaluate expressions.
+An expression is a one-liner that returns a value (mostly, but not limited to, booleans).
+It is designed for simplicity, speed and safety.
+
+The purpose of the package is to allow users to use expressions inside configuration for more complex logic.
+It is a perfect candidate for the foundation of a _business rule engine_.
+The idea is to let configure things in a dynamic way without recompile of a program:
+
+```coffeescript
+# Get the special price if
+user.Group in ["good_customers", "collaborator"]
+
+# Promote article to the homepage when
+len(article.Comments) > 100 and article.Category not in ["misc"]
+
+# Send an alert when
+product.Stock < 15
+```
+
+## Features
+
+* Seamless integration with Go (no need to redefine types)
+* Static typing ([example](https://godoc.org/github.com/antonmedv/expr#example-Env)).
+ ```go
+ out, err := expr.Compile(`name + age`)
+ // err: invalid operation + (mismatched types string and int)
+ // | name + age
+ // | .....^
+ ```
+* User-friendly error messages.
+* Reasonable set of basic operators.
+* Builtins `all`, `none`, `any`, `one`, `filter`, `map`.
+ ```coffeescript
+ all(Tweets, {.Size <= 280})
+ ```
+* Fast ([benchmarks](https://github.com/antonmedv/golang-expression-evaluation-comparison#readme)): uses bytecode virtual machine and optimizing compiler.
+
+## Install
+
+```
+go get github.com/antonmedv/expr
+```
+
+## Documentation
+
+* See [Getting Started](docs/Getting-Started.md) page for developer documentation.
+* See [Language Definition](docs/Language-Definition.md) page to learn the syntax.
+
+## Expr Code Editor
+
+
+
+
+
+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 →](https://antonmedv.github.io/expr/)
+
+## Examples
+
+[Play Online](https://play.golang.org/p/z7T8ytJ1T1d)
+
+```go
+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](https://play.golang.org/p/4S4brsIvU4i)
+
+```go
+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)
+}
+```
+
+## Contributing
+
+**Expr** consist of a few packages for parsing source code to AST, type checking AST, compiling to bytecode and VM for running bytecode program.
+
+Also expr provides powerful tool [exe](cmd/exe) for debugging. It has interactive terminal debugger for our bytecode virtual machine.
+
+
+ 
+
|
+
Aviasales are actively using Expr for different parts of the search engine. + |
+
+
Mystery Minds uses Expr to allow easy yet powerful customization of its matching algorithm. + |
+
