expr

Expression evaluator for Go

Features

• Operators: + - * / % ! < <= > >= == === != !=== ?: ?? , [] () & | ^
• Types: String, Number, Boolean, and custom types
• Supports custom functions, types, associative arrays, and variables.
• Parenthesized expressions
• Javascript-like syntax with automatic type conversions
• Native uint64 and int64 types using the u64 and i64 suffix on number literals
• Stateless: No variable assignments and no statements.

Using

To start using expr, install Go and run go get:

$ go get github.com/tidwall/expr

Basic expressions

For example:

1 + 1
(10 * 5 <= 50) && (50 > 100 || 8 >= 7)
1e+10 > 0 ? "big" : "small"

In Go, you're code may look like the following.

res, _ := expr.Eval(`1 + 1`, nil)
fmt.Println(res)
res, _ := expr.Eval(`(10 * 5 <= 50) && (50 > 100 || 8 >= 7)`, nil)
fmt.Println(res)
res, _ := expr.Eval(`1e+10 > 0 ? "big" : "small"`, nil)
fmt.Println(res)

// Output: 
// 2
// true
// big

Advanced expressions

Using a custom evaluation extender we can extend the eval function to support arithmetic and comparisons on custom types, such as time.Time that is built into Go. We can also provide some extra user data that exposes extra variables to the evaluator.

Example expressions:

this.timestamp
this.timestamp - dur('1h')
now() + dur('24h')
this.timestamp < now() - dur('24h') ? "old" : "new"
((this.minX + this.maxX) / 2) + "," + ((this.minY + this.maxY) / 2)

In Go, you would provide a custom Extender to the Eval function.

package main

import (
	"fmt"
	"time"

	"github.com/tidwall/expr"
)

func main() {
	// Create a user data map that can be referenced by the Eval function.
	this := make(map[string]expr.Value)

	// Add a bounding box to the user dictionary.
	this["minX"] = expr.Number(112.8192)
	this["minY"] = expr.Number(33.4738)
	this["maxX"] = expr.Number(113.9146)
	this["maxY"] = expr.Number(34.3367)

	// Add a timestamp value to the user dictionary.
	ts, _ := time.Parse(time.RFC3339, "2022-03-31T09:00:00Z")
	this["timestamp"] = expr.Object(ts)

	// Set up an evaluation extender for referencing the user data, and
	// using functions and operators on custom types.
	ext := expr.NewExtender(
		func(info expr.RefInfo, ctx *expr.Context) (expr.Value, error) {
			if info.Chain {
				// The reference is part of a dot chain such as:
				//   this.minX
				if this, ok := ctx.UserData.(map[string]expr.Value); ok {
					return this[info.Ident], nil
				}
				return expr.Undefined, nil
			}
			switch info.Ident {
			case "now":
				// The `now()` function
				return expr.Function("now"), nil
			case "dur":
				// The `dur(str)` function
				return expr.Function("duration"), nil
			case "this":
				// The `this` UserData
				return expr.Object(ctx.UserData), nil
			}
			return expr.Undefined, nil
		},
		func(info expr.CallInfo, ctx *expr.Context) (expr.Value, error) {
			if info.Chain {
				// Only use globals in this example.
				// No chained function like `user.name()`.
				return expr.Undefined, nil
			}
			switch info.Ident {
			case "now":
				// Return the current date/time.
				return expr.Object(time.Now()), nil
			case "dur":
				// Parse the duration using the first argument.
				d, err := time.ParseDuration(info.Args.At(0).String())
				if err != nil {
					return expr.Undefined, err
				}
				// Valid time.Duration, return as an Int64 value
				return expr.Int64(int64(d)), nil
			default:
				return expr.Undefined, nil
			}
		},
		func(info expr.OpInfo, ctx *expr.Context) (expr.Value, error) {
			// Try to convert a and/or b to time.Time
			left, leftOK := info.Left.Value().(time.Time)
			right, rightOK := info.Right.Value().(time.Time)
			if leftOK && rightOK {
				// Both values are time.Time.
				// Perform comparison operation.
				switch info.Op {
				case expr.OpLt:
					return expr.Bool(left.Before(right)), nil
				}
			} else if leftOK || rightOK {
				// Either A or B are time.Time.
				// Perform arithmatic add/sub operation and return a
				// recalcuated time.Time value.
				var x time.Time
				var y int64
				if leftOK {
					x = left
					y = info.Right.Int64()
				} else {
					x = right
					y = info.Left.Int64()
				}
				switch info.Op {
				case expr.OpAdd:
					return expr.Object(x.Add(time.Duration(y))), nil
				case expr.OpSub:
					return expr.Object(x.Add(-time.Duration(y))), nil
				}
			}
			return expr.Undefined, nil
		},
	)

	// Set up a custom expr.context that holds user data and the extender.
	ctx := expr.Context{UserData: this, Extender: ext}

	var res expr.Value

	// Return the timestamp.
	res, _ = expr.Eval(`this.timestamp`, &ctx)
	fmt.Println(res)

	// Subtract an hour from the timestamp.
	res, _ = expr.Eval(`this.timestamp - dur('1h')`, &ctx)
	fmt.Println(res)

	// Add one day to the current time.
	res, _ = expr.Eval(`now() + dur('24h')`, &ctx)
	fmt.Println(res)

	// See if timestamp is older than a day
	res, _ = expr.Eval(`this.timestamp < now() - dur('24h') ? "old" : "new"`, &ctx)
	fmt.Println(res)

	// Get the center of the bounding box as a concatenated string.
	res, _ = expr.Eval(`((this.minX + this.maxX) / 2) + "," + ((this.minY + this.maxY) / 2)`, &ctx)
	fmt.Println(res)

	// Output:
	// 2022-03-31 09:00:00 +0000 UTC
	// 2022-03-31 08:00:00 +0000 UTC
	// 2022-04-02 06:00:40.834656 -0700 MST m=+86400.000714835
	// old
	// 113.36689999999999,33.905249999999995
}