Expr is an engine that can evaluate expressions.

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:

# 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

Inspired by

• Symfony's The ExpressionLanguage component,
• Rob Pike's talk Lexical Scanning in Go.

• Works with any valid Go object (structs, maps, etc)
• Static and dynamic typing (example)

  code := "groups[0].Title + user.Age"
  p, err := expr.Parse(code, expr.Define("groups", []Group{}), expr.Define("user", User{}))
  // err: invalid operation: groups[0].Name + user.Age (mismatched types string and int)

• User-friendly error messages

  unclosed "("
  (boo + bar]
  ----------^
  

• Reasonable set of basic operators
• Fast (faster otto and goja, see bench)

go get -u github.com/antonmedv/expr

• See for developer documentation,
• See The Expression Syntax page to learn the syntax of the Expr expressions.

Executing arbitrary expressions.

env := map[string]interface{}{
    "foo": 1,
    "bar": struct{Value int}{1},
}

out, err := expr.Eval("foo + bar.Value", env)

Static type checker with struct as environment.

type env struct {
	Foo int
	Bar bar
}

type bar struct {
	Value int
}

p, err := expr.Parse("Foo + Bar.Value", expr.Env(env{}))

out, err := expr.Run(p, env{1, bar{2}})

Using env's methods as functions inside expressions.

type env struct {
	Name string
}

func (e env) Title() string {
	return strings.Title(e.Name)
}


p, err := expr.Parse("'Hello ' ~ Title()", expr.Env(env{}))

out, err := expr.Run(p, env{"world"})

Using embedded structs to construct env.

type env struct {
	helpers
	Name string
}

type helpers struct{}

func (h helpers) Title(s string) string {
	return strings.Title(s)
}


p, err := expr.Parse("'Hello ' ~ Title(Name)", expr.Env(env{}))

out, err := expr.Run(p, env{"world"})

MIT