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# Operator Override
**Expr** supports operator overriding. For example, you may rewrite such expression:
```js
Now().Sub(CreatedAt)
```
To use `-` operator:
```js
Now() - CreatedAt
```
To override operator use [expr.Operator](https://pkg.go.dev/github.com/antonmedv/expr?tab=doc#Operator):
```go
package main
import (
"fmt"
"time"
"github.com/antonmedv/expr"
)
func main() {
code := `(Now() - CreatedAt).Hours() / 24 / 365`
// We can define options before compiling.
options := []expr.Option{
expr.Env(Env{}),
expr.Operator("-", "Sub"), // Override `-` with function `Sub`.
}
program, err := expr.Compile(code, options...)
if err != nil {
panic(err)
}
env := Env{
CreatedAt: time.Date(1987, time.November, 24, 20, 0, 0, 0, time.UTC),
}
output, err := expr.Run(program, env)
if err != nil {
panic(err)
}
fmt.Print(output)
}
type Env struct {
datetime
CreatedAt time.Time
}
// Functions may be defined on embedded structs as well.
type datetime struct{}
func (datetime) Now() time.Time { return time.Now() }
func (datetime) Sub(a, b time.Time) time.Duration { return a.Sub(b) }
```
**Expr** uses defined functions on `Env` for operator overloading. If types of operands match types of a function,
the operator will be replaced with the function call.
Complete example can be found here: [dates_test.go](examples/dates_test.go).
* [Contents](README.md)
* Next: [Visitor and Patch](Visitor-and-Patch.md)
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