README ¶
Expr
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)
- Strict mode with type checks
- 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 godoc.org/github.com/antonmedv/expr for developer documentation,
- See The Expression Syntax page to learn the syntax of the Expr expressions.
License
MIT
Documentation ¶
Overview ¶
Package expr is an engine that can evaluate expressions.
// Evaluate expression on data. result, err := expr.Eval("expression", data) // Or precompile expression to ast first. node, err := expr.Parse("expression") // And run later. result, err := expr.Run(node, data)
Passing in Variables ¶
You can pass variables into the expression, which can be of any valid Go type (including structs):
// Maps data := map[string]interface{}{ "Foo": ... "Bar": ... } // Structs data := Payload{ Foo: ... Bar: ... } // Pass object result, err := expr.Eval("Foo == Bar", data)
Expr uses reflection for accessing and iterating passed data. For example you can pass nested structures without any modification or preparation:
type Cookie struct { Key string Value string } type User struct { UserAgent string Cookies []Cookie } type Request struct { User *user } req := Request{&User{ Cookies: []Cookie{{"origin", "www"}}, UserAgent: "Firefox", }} ok, err := expr.Eval(`User.UserAgent matches "Firefox" and User.Cookies[0].Value == "www"`, req)
Passing in Functions ¶
You can also pass functions into the expression:
data := map[string]interface{}{ "Request": req, "Values": func(xs []Cookie) []string { vs := make([]string, 0) for _, x := range xs { vs = append(vs, x.Value) } return vs }, } ok, err := expr.Eval(`"www" in Values(Request.User.Cookies)`, data)
Parsing and caching ¶
If you planning to execute some expression lots times, it's good to parse it first and only one time:
// Parse expression to AST. ast, err := expr.Parse(expression) // Run given AST ok, err := expr.Run(ast, data)
Strict mode ¶
Expr package support strict parse mode in which some type checks performed during parsing. To parse expression in strict mode, define all of used variables:
expression := `Request.User.UserAgent matches "Firefox"` node, err := expr.Parse(expression, expr.Define("Request", request{}))
Parse function will check used variables, accessed filed, logical operators and some other type checks.
If you try to use some undeclared variables, or access unknown field, an error will be returned during paring:
expression := `Request.User.Cookies[0].Timestamp` node, err := expr.Parse(expression, expr.Define("Request", request{})) // err: Request.User.Cookies[0].Timestamp undefined (type expr_test.cookie has no field Timestamp)
Also it's possible to define all used variables and functions using expr.With and struct:
type payload struct { Request *Request Values func(xs []Cookie) []string } node, err := expr.Parse(expression, expr.With(payload{}))
Or with map:
data := map[string]interface{}{ "Request": req, "Values": func(xs []Cookie) []string {...}, } node, err := expr.Parse(expression, expr.With(data))
Printing ¶
Compiled ast can be compiled back to string expression using stringer fmt.Stringer interface:
node, err := expr.Parse(expression) code := fmt.Sprintf("%v", node)
Number type ¶
Inside Expr engine there is no distinguish between int, uint and float types (as in JavaScript). All numbers inside Expr engine represented as `float64`. You should remember about it if you use any of binary operators (`+`, `-`, `/`, `*`, etc). Otherwise type remain unchanged.
data := map[string]int{ "Foo": 1, "Bar": 2, } out, err := expr.Eval(`Foo`, data) // int out, err := expr.Eval(`Foo + Bar`, data) // float64
Index ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
Types ¶
type Node ¶
type Node interface { Type(table typesTable) (Type, error) Eval(env interface{}) (interface{}, error) }
Node represents items of abstract syntax tree.