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# Optimizations
Expr has a bunch of optimization which will produce more optimal program during compile step.
## In array
```js
value in ['foo', 'bar', 'baz']
```
If expr finds an `in` or `not in` expression with an array, it will be transformed into:
```js
value in {"foo": true, "bar": true, "baz": true}
```
## Constant folding
Arithmetic expressions with constants is computed on compile step and replaced with result.
```js
-(2-5)**3-2/(+4-3)+-2
```
Will be compiled to just single number:
```js
23
```
So in expressions it's safe to use some arithmetics for better readability:
```js
percentage > 0.3 * 100
```
As it will be simpified to:
```js
percentage > 30
```
## In range
```js
user.Age in 18..32
```
Will be replaced with binary operator:
```js
18 <= user.Age && user.Age <= 32
```
`not in` operator will also work.
## Const range
```js
1..10_000
```
Ranges computed on compile stage, repleced with preallocated slices.
## Const expr
If some function marked as constant expression with `expr.ConstExpr`. It will be replaced with result
of call, if all arguments are constants.
```go
expr.ConstExpt("fib")
```
```js
fib(42)
```
Will be replaced with result of `fib(42)` on compile step. No need to calculate it during runtime.
[ConstExpr Example](https://pkg.go.dev/github.com/antonmedv/expr?tab=doc#ConstExpr)
## Reuse VM
It is possible to reuse a virtual machine between re-runs on the program.
This adds a small increase in performance (from 4% to 40% depending on a program).
```go
package main
import (
"fmt"
"github.com/antonmedv/expr"
"github.com/antonmedv/expr/vm"
)
func main() {
env := map[string]interface{}{
"foo": 1,
"bar": 2,
}
program, err := expr.Compile("foo + bar", expr.Env(env))
if err != nil {
panic(err)
}
// Reuse this vm instance between runs
v := vm.VM{}
out, err := v.Run(program, env)
if err != nil {
panic(err)
}
fmt.Print(out)
}
```
* [Contents](README.md)
|
