example_test.go

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package ast_test
     6  
     7  import (
     8  	"fmt"
     9  	"go/ast"
    10  	"go/format"
    11  	"go/parser"
    12  	"go/token"
    13  	"strings"
    14  )
    15  
    16  // This example demonstrates how to inspect the AST of a Go program.
    17  func ExampleInspect() {
    18  	// src is the input for which we want to inspect the AST.
    19  	src := `
    20  package p
    21  const c = 1.0
    22  var X = f(3.14)*2 + c
    23  `
    24  
    25  	// Create the AST by parsing src.
    26  	fset := token.NewFileSet() // positions are relative to fset
    27  	f, err := parser.ParseFile(fset, "src.go", src, 0)
    28  	if err != nil {
    29  		panic(err)
    30  	}
    31  
    32  	// Inspect the AST and print all identifiers and literals.
    33  	ast.Inspect(f, func(n ast.Node) bool {
    34  		var s string
    35  		switch x := n.(type) {
    36  		case *ast.BasicLit:
    37  			s = x.Value
    38  		case *ast.Ident:
    39  			s = x.Name
    40  		}
    41  		if s != "" {
    42  			fmt.Printf("%s:\t%s\n", fset.Position(n.Pos()), s)
    43  		}
    44  		return true
    45  	})
    46  
    47  	// Output:
    48  	// src.go:2:9:	p
    49  	// src.go:3:7:	c
    50  	// src.go:3:11:	1.0
    51  	// src.go:4:5:	X
    52  	// src.go:4:9:	f
    53  	// src.go:4:11:	3.14
    54  	// src.go:4:17:	2
    55  	// src.go:4:21:	c
    56  }
    57  
    58  // This example shows what an AST looks like when printed for debugging.
    59  func ExamplePrint() {
    60  	// src is the input for which we want to print the AST.
    61  	src := `
    62  package main
    63  func main() {
    64  	println("Hello, World!")
    65  }
    66  `
    67  
    68  	// Create the AST by parsing src.
    69  	fset := token.NewFileSet() // positions are relative to fset
    70  	f, err := parser.ParseFile(fset, "", src, 0)
    71  	if err != nil {
    72  		panic(err)
    73  	}
    74  
    75  	// Print the AST.
    76  	ast.Print(fset, f)
    77  
    78  	// Output:
    79  	//      0  *ast.File {
    80  	//      1  .  Package: 2:1
    81  	//      2  .  Name: *ast.Ident {
    82  	//      3  .  .  NamePos: 2:9
    83  	//      4  .  .  Name: "main"
    84  	//      5  .  }
    85  	//      6  .  Decls: []ast.Decl (len = 1) {
    86  	//      7  .  .  0: *ast.FuncDecl {
    87  	//      8  .  .  .  Name: *ast.Ident {
    88  	//      9  .  .  .  .  NamePos: 3:6
    89  	//     10  .  .  .  .  Name: "main"
    90  	//     11  .  .  .  .  Obj: *ast.Object {
    91  	//     12  .  .  .  .  .  Kind: func
    92  	//     13  .  .  .  .  .  Name: "main"
    93  	//     14  .  .  .  .  .  Decl: *(obj @ 7)
    94  	//     15  .  .  .  .  }
    95  	//     16  .  .  .  }
    96  	//     17  .  .  .  Type: *ast.FuncType {
    97  	//     18  .  .  .  .  Func: 3:1
    98  	//     19  .  .  .  .  Params: *ast.FieldList {
    99  	//     20  .  .  .  .  .  Opening: 3:10
   100  	//     21  .  .  .  .  .  Closing: 3:11
   101  	//     22  .  .  .  .  }
   102  	//     23  .  .  .  }
   103  	//     24  .  .  .  Body: *ast.BlockStmt {
   104  	//     25  .  .  .  .  Lbrace: 3:13
   105  	//     26  .  .  .  .  List: []ast.Stmt (len = 1) {
   106  	//     27  .  .  .  .  .  0: *ast.ExprStmt {
   107  	//     28  .  .  .  .  .  .  X: *ast.CallExpr {
   108  	//     29  .  .  .  .  .  .  .  Fun: *ast.Ident {
   109  	//     30  .  .  .  .  .  .  .  .  NamePos: 4:2
   110  	//     31  .  .  .  .  .  .  .  .  Name: "println"
   111  	//     32  .  .  .  .  .  .  .  }
   112  	//     33  .  .  .  .  .  .  .  Lparen: 4:9
   113  	//     34  .  .  .  .  .  .  .  Args: []ast.Expr (len = 1) {
   114  	//     35  .  .  .  .  .  .  .  .  0: *ast.BasicLit {
   115  	//     36  .  .  .  .  .  .  .  .  .  ValuePos: 4:10
   116  	//     37  .  .  .  .  .  .  .  .  .  Kind: STRING
   117  	//     38  .  .  .  .  .  .  .  .  .  Value: "\"Hello, World!\""
   118  	//     39  .  .  .  .  .  .  .  .  }
   119  	//     40  .  .  .  .  .  .  .  }
   120  	//     41  .  .  .  .  .  .  .  Ellipsis: -
   121  	//     42  .  .  .  .  .  .  .  Rparen: 4:25
   122  	//     43  .  .  .  .  .  .  }
   123  	//     44  .  .  .  .  .  }
   124  	//     45  .  .  .  .  }
   125  	//     46  .  .  .  .  Rbrace: 5:1
   126  	//     47  .  .  .  }
   127  	//     48  .  .  }
   128  	//     49  .  }
   129  	//     50  .  FileStart: 1:1
   130  	//     51  .  FileEnd: 5:3
   131  	//     52  .  Scope: *ast.Scope {
   132  	//     53  .  .  Objects: map[string]*ast.Object (len = 1) {
   133  	//     54  .  .  .  "main": *(obj @ 11)
   134  	//     55  .  .  }
   135  	//     56  .  }
   136  	//     57  .  Unresolved: []*ast.Ident (len = 1) {
   137  	//     58  .  .  0: *(obj @ 29)
   138  	//     59  .  }
   139  	//     60  .  GoVersion: ""
   140  	//     61  }
   141  }
   142  
   143  // This example illustrates how to remove a variable declaration
   144  // in a Go program while maintaining correct comment association
   145  // using an ast.CommentMap.
   146  func ExampleCommentMap() {
   147  	// src is the input for which we create the AST that we
   148  	// are going to manipulate.
   149  	src := `
   150  // This is the package comment.
   151  package main
   152  
   153  // This comment is associated with the hello constant.
   154  const hello = "Hello, World!" // line comment 1
   155  
   156  // This comment is associated with the foo variable.
   157  var foo = hello // line comment 2
   158  
   159  // This comment is associated with the main function.
   160  func main() {
   161  	fmt.Println(hello) // line comment 3
   162  }
   163  `
   164  
   165  	// Create the AST by parsing src.
   166  	fset := token.NewFileSet() // positions are relative to fset
   167  	f, err := parser.ParseFile(fset, "src.go", src, parser.ParseComments)
   168  	if err != nil {
   169  		panic(err)
   170  	}
   171  
   172  	// Create an ast.CommentMap from the ast.File's comments.
   173  	// This helps keeping the association between comments
   174  	// and AST nodes.
   175  	cmap := ast.NewCommentMap(fset, f, f.Comments)
   176  
   177  	// Remove the first variable declaration from the list of declarations.
   178  	for i, decl := range f.Decls {
   179  		if gen, ok := decl.(*ast.GenDecl); ok && gen.Tok == token.VAR {
   180  			copy(f.Decls[i:], f.Decls[i+1:])
   181  			f.Decls = f.Decls[:len(f.Decls)-1]
   182  			break
   183  		}
   184  	}
   185  
   186  	// Use the comment map to filter comments that don't belong anymore
   187  	// (the comments associated with the variable declaration), and create
   188  	// the new comments list.
   189  	f.Comments = cmap.Filter(f).Comments()
   190  
   191  	// Print the modified AST.
   192  	var buf strings.Builder
   193  	if err := format.Node(&buf, fset, f); err != nil {
   194  		panic(err)
   195  	}
   196  	fmt.Printf("%s", buf.String())
   197  
   198  	// Output:
   199  	// // This is the package comment.
   200  	// package main
   201  	//
   202  	// // This comment is associated with the hello constant.
   203  	// const hello = "Hello, World!" // line comment 1
   204  	//
   205  	// // This comment is associated with the main function.
   206  	// func main() {
   207  	// 	fmt.Println(hello) // line comment 3
   208  	// }
   209  }
   210
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