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go/ast: added example illustrating CommentMap use.

R=bradfitz
CC=golang-dev
https://golang.org/cl/43930043
This commit is contained in:
Robert Griesemer 2013-12-18 10:10:40 -08:00
parent 9d1832f227
commit 108d35bd8e

View File

@ -5,8 +5,10 @@
package ast_test
import (
"bytes"
"fmt"
"go/ast"
"go/format"
"go/parser"
"go/token"
)
@ -134,3 +136,75 @@ func main() {
// 57 . }
// 58 }
}
// This example illustrates how to remove a variable declaration
// in a Go program while maintaining correct comment association
// using an ast.CommentMap.
func ExampleCommentMap() {
// src is the input for which we create the AST that we
// are going to manipulate.
src := `
// This is the package comment.
package main
// This comment is associated with the hello constant.
const hello = "Hello, World!" // line comment 1
// This comment is associated with the foo variable.
var foo = hello // line comment 2
// This comment is associated with the main function.
func main() {
fmt.Println(hello) // line comment 3
}
`
// Create the AST by parsing src.
fset := token.NewFileSet() // positions are relative to fset
f, err := parser.ParseFile(fset, "src.go", src, parser.ParseComments)
if err != nil {
panic(err)
}
// Create an ast.CommentMap from the ast.File's comments.
// This helps keeping the association between comments
// and AST nodes.
cmap := ast.NewCommentMap(fset, f, f.Comments)
// Remove the first variable declaration from the list of declarations.
f.Decls = removeFirstVarDecl(f.Decls)
// Use the comment map to filter comments that don't belong anymore
// (the comments associated with the variable declaration), and create
// the new comments list.
f.Comments = cmap.Filter(f).Comments()
// Print the modified AST.
var buf bytes.Buffer
if err := format.Node(&buf, fset, f); err != nil {
panic(err)
}
fmt.Printf("%s", buf.Bytes())
// output:
// // This is the package comment.
// package main
//
// // This comment is associated with the hello constant.
// const hello = "Hello, World!" // line comment 1
//
// // This comment is associated with the main function.
// func main() {
// fmt.Println(hello) // line comment 3
// }
}
func removeFirstVarDecl(list []ast.Decl) []ast.Decl {
for i, decl := range list {
if gen, ok := decl.(*ast.GenDecl); ok && gen.Tok == token.VAR {
copy(list[i:], list[i+1:])
return list[:len(list)-1]
}
}
panic("variable declaration not found")
}