Package printer


Package printer implements printing of AST nodes.




Package files

nodes.go printer.go

func FprintSource

func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error

Fprint "pretty-prints" an AST node to output. It calls Config.Fprint with default settings. Note that gofmt uses tabs for indentation but spaces for alignment; use format.Node (package go/format) for output that matches gofmt.


package main

import (

func parseFunc(filename, functionname string) (fun *ast.FuncDecl, fset *token.FileSet) {
	fset = token.NewFileSet()
	if file, err := parser.ParseFile(fset, filename, nil, 0); err == nil {
		for _, d := range file.Decls {
			if f, ok := d.(*ast.FuncDecl); ok && f.Name.Name == functionname {
				fun = f
	panic("function not found")

func main() {
	// Parse source file and extract the AST without comments for
	// this function, with position information referring to the
	// file set fset.
	funcAST, fset := parseFunc("example_test.go", "ExampleFprint")

	// Print the function body into buffer buf.
	// The file set is provided to the printer so that it knows
	// about the original source formatting and can add additional
	// line breaks where they were present in the source.
	var buf bytes.Buffer
	printer.Fprint(&buf, fset, funcAST.Body)

	// Remove braces {} enclosing the function body, unindent,
	// and trim leading and trailing white space.
	s := buf.String()
	s = s[1 : len(s)-1]
	s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))

	// Print the cleaned-up body text to stdout.


type CommentedNodeSource

A CommentedNode bundles an AST node and corresponding comments. It may be provided as argument to any of the Fprint functions.

type CommentedNode struct {
    Node     interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
    Comments []*ast.CommentGroup

type ConfigSource

A Config node controls the output of Fprint.

type Config struct {
    Mode     Mode // default: 0
    Tabwidth int  // default: 8
    Indent   int  // default: 0 (all code is indented at least by this much); added in Go 1.1

func (*Config) FprintSource

func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error

Fprint "pretty-prints" an AST node to output for a given configuration cfg. Position information is interpreted relative to the file set fset. The node type must be *ast.File, *CommentedNode, []ast.Decl, []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.

type ModeSource

A Mode value is a set of flags (or 0). They control printing.

type Mode uint
const (
    RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
    TabIndent                  // use tabs for indentation independent of UseSpaces
    UseSpaces                  // use spaces instead of tabs for alignment
    SourcePos                  // emit //line directives to preserve original source positions

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Licensed under the Creative Commons Attribution License 3.0.