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# Visitor and Patch
[ast](https://pkg.go.dev/github.com/antonmedv/expr/ast?tab=doc) package provides `ast.Visitor` interface and `ast.Walk` function.
You can use it for traveling ast tree of compiled program.
For example if you want to collect all variable names:
```go
package main
import (
"fmt"
"github.com/antonmedv/expr/ast"
"github.com/antonmedv/expr/parser"
)
type visitor struct {
identifiers []string
}
func (v *visitor) Enter(node *ast.Node) {}
func (v *visitor) Exit(node *ast.Node) {
if n, ok := (*node).(*ast.IdentifierNode); ok {
v.identifiers = append(v.identifiers, n.Value)
}
}
func main() {
tree, err := parser.Parse("foo + bar")
if err != nil {
panic(err)
}
visitor := &visitor{}
ast.Walk(&tree.Node, visitor)
fmt.Printf("%v", visitor.identifiers) // outputs [foo bar]
}
```
## Patch
Implemented visitor can be applied before compiling AST to bytecode in `expr.Compile` function.
```go
program, err := expr.Compile(code, expr.Patch(&visitor{}))
```
This can be useful for some edge cases, there you want to extend functionality of **Expr** language.
In next example we are going to replace expression `list[-1]` with `list[len(list)-1]`.
```go
package main
import (
"fmt"
"github.com/antonmedv/expr"
"github.com/antonmedv/expr/ast"
)
func main() {
env := map[string]interface{}{
"list": []int{1, 2, 3},
}
code := `list[-1]` // will output 3
program, err := expr.Compile(code, expr.Env(env), expr.Patch(&patcher{}))
if err != nil {
panic(err)
}
output, err := expr.Run(program, env)
if err != nil {
panic(err)
}
fmt.Print(output)
}
type patcher struct{}
func (p *patcher) Enter(_ *ast.Node) {}
func (p *patcher) Exit(node *ast.Node) {
n, ok := (*node).(*ast.IndexNode)
if !ok {
return
}
unary, ok := n.Index.(*ast.UnaryNode)
if !ok {
return
}
if unary.Operator == "-" {
ast.Patch(&n.Index, &ast.BinaryNode{
Operator: "-",
Left: &ast.BuiltinNode{Name: "len", Arguments: []ast.Node{n.Node}},
Right: unary.Node,
})
}
}
```
Type information is also available. Here is an example, there all `fmt.Stringer` interface automatically
converted to `string` type.
```go
package main
import (
"fmt"
"reflect"
"github.com/antonmedv/expr"
"github.com/antonmedv/expr/ast"
)
func main() {
code := `Price == "$100"`
program, err := expr.Compile(code, expr.Env(Env{}), expr.Patch(&stringerPatcher{}))
if err != nil {
panic(err)
}
env := Env{100_00}
output, err := expr.Run(program, env)
if err != nil {
panic(err)
}
fmt.Print(output)
}
type Env struct {
Price Price
}
type Price int
func (p Price) String() string {
return fmt.Sprintf("$%v", int(p)/100)
}
var stringer = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
type stringerPatcher struct{}
func (p *stringerPatcher) Enter(_ *ast.Node) {}
func (p *stringerPatcher) Exit(node *ast.Node) {
t := (*node).Type()
if t == nil {
return
}
if t.Implements(stringer) {
ast.Patch(node, &ast.MethodNode{
Node: *node,
Method: "String",
})
}
}
```
* [Contents](README.md)
* Next: [Optimizations](Optimizations.md)
|
