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.
Features
- 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)
Install
go get -u github.com/antonmedv/expr
Documentation
- See
for developer documentation,
- See The Expression Syntax page to learn the syntax of the Expr expressions.
Examples
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"})
License
MIT