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// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// The printer package implements printing of AST nodes.
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const debug = false // enable for debugging
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ignore = whiteSpace(0)
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blank = whiteSpace(' ')
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vtab = whiteSpace('\v')
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newline = whiteSpace('\n')
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formfeed = whiteSpace('\f')
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indent = whiteSpace('>')
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unindent = whiteSpace('<')
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esc2 = '\xfe' // an escape byte that cannot occur in regular UTF-8
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_ = 1 / (esc2 - tabwriter.Escape) // cause compiler error if esc2 == tabwriter.Escape
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esc = []byte{tabwriter.Escape}
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htabs = []byte("\t\t\t\t\t\t\t\t")
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newlines = []byte("\n\n\n\n\n\n\n\n") // more than the max determined by nlines
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formfeeds = []byte("\f\f\f\f\f\f\f\f") // more than the max determined by nlines
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var noPos token.Position // use noPos when a position is needed but not known
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var infinity = 1 << 30
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// Use ignoreMultiLine if the multiLine information is not important.
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var ignoreMultiLine = new(bool)
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// A pmode value represents the current printer mode.
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inLiteral pmode = 1 << iota
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// Configuration (does not change after initialization)
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nesting int // nesting level (0: top-level (package scope), >0: functions/decls.)
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written int // number of bytes written
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indent int // current indentation
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mode pmode // current printer mode
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lastTok token.Token // the last token printed (token.ILLEGAL if it's whitespace)
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// Buffered whitespace
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// The (possibly estimated) position in the generated output;
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// in AST space (i.e., pos is set whenever a token position is
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// known accurately, and updated dependending on what has been
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// The value of pos immediately after the last item has been
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// written using writeItem.
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lastTaggedLine int // last line for which a line tag was written
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// The list of all source comments, in order of appearance.
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comments []*ast.CommentGroup // may be nil
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cindex int // current comment index
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useNodeComments bool // if not set, ignore lead and line comments of nodes
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func (p *printer) init(output io.Writer, cfg *Config, fset *token.FileSet) {
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p.errors = make(chan os.Error)
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p.buffer = make([]whiteSpace, 0, 16) // whitespace sequences are short
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func (p *printer) internalError(msg ...interface{}) {
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fmt.Print(p.pos.String() + ": ")
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// nlines returns the adjusted number of linebreaks given the desired number
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// of breaks n such that min <= result <= max where max depends on the current
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func (p *printer) nlines(n, min int) int {
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max := 3 // max. number of newlines at the top level (p.nesting == 0)
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max = 2 // max. number of newlines everywhere else
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// write0 writes raw (uninterpreted) data to p.output and handles errors.
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// write0 does not indent after newlines, and does not HTML-escape or update p.pos.
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func (p *printer) write0(data []byte) {
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n, err := p.output.Write(data)
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// write interprets data and writes it to p.output. It inserts indentation
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// after a line break unless in a tabwriter escape sequence.
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// It updates p.pos as a side-effect.
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func (p *printer) write(data []byte) {
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for i, b := range data {
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// write segment ending in b
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p.write0(data[i0 : i+1])
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p.pos.Offset += i + 1 - i0
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if p.mode&inLiteral == 0 {
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// use "hard" htabs - indentation columns
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// must not be discarded by the tabwriter
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for ; j > len(htabs); j -= len(htabs) {
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p.pos.Offset += p.indent
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p.pos.Column += p.indent
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// next segment start
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case tabwriter.Escape:
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// ignore escape chars introduced by printer - they are
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// invisible and must not affect p.pos (was issue #1089)
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// write remaining segment
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func (p *printer) writeNewlines(n int, useFF bool) {
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p.write(formfeeds[0:n])
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p.write(newlines[0:n])
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// writeItem writes data at position pos. data is the text corresponding to
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// a single lexical token, but may also be comment text. pos is the actual
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// (or at least very accurately estimated) position of the data in the original
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// source text. writeItem updates p.last to the position immediately following
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func (p *printer) writeItem(pos token.Position, data []byte) {
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// continue with previous position if we don't have a valid pos
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if p.last.IsValid() && p.last.Filename != pos.Filename {
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// the file has changed - reset state
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// (used when printing merged ASTs of different files
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// e.g., the result of ast.MergePackageFiles)
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p.buffer = p.buffer[0:0]
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// do not update p.pos - use write0
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_, filename := filepath.Split(pos.Filename)
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p.write0([]byte(fmt.Sprintf("[%s:%d:%d]", filename, pos.Line, pos.Column)))
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// writeCommentPrefix writes the whitespace before a comment.
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// If there is any pending whitespace, it consumes as much of
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// it as is likely to help position the comment nicely.
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// pos is the comment position, next the position of the item
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// after all pending comments, prev is the previous comment in
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// a group of comments (or nil), and isKeyword indicates if the
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// next item is a keyword.
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func (p *printer) writeCommentPrefix(pos, next token.Position, prev *ast.Comment, isKeyword bool) {
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// the comment is the first item to be printed - don't write any whitespace
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if pos.IsValid() && pos.Filename != p.last.Filename {
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// comment in a different file - separate with newlines (writeNewlines will limit the number)
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p.writeNewlines(10, true)
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if pos.Line == p.last.Line && (prev == nil || prev.Text[1] != '/') {
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// comment on the same line as last item:
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// separate with at least one separator
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// first comment of a comment group
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for i, ch := range p.buffer {
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// ignore any blanks before a comment
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// respect existing tabs - important
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// for proper formatting of commented structs
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// apply pending indentation
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// make sure there is at least one separator
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if pos.Line == next.Line {
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// next item is on the same line as the comment
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// (which must be a /*-style comment): separate
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// with a blank instead of a tab
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// comment on a different line:
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// separate with at least one line break
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// first comment of a comment group
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for i, ch := range p.buffer {
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// ignore any horizontal whitespace before line breaks
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// apply pending indentation
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// if the next token is a keyword, apply the outdent
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// if it appears that the comment is aligned with the
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// keyword; otherwise assume the outdent is part of a
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// closing block and stop (this scenario appears with
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// comments before a case label where the comments
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// apply to the next case instead of the current one)
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if isKeyword && pos.Column == next.Column {
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case newline, formfeed:
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// TODO(gri): may want to keep formfeed info in some cases
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// use formfeeds to break columns before a comment;
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// this is analogous to using formfeeds to separate
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// individual lines of /*-style comments - but make
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// sure there is at least one line break if the previous
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// comment was a line comment
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n := pos.Line - p.last.Line // if !pos.IsValid(), pos.Line == 0, and n will be 0
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if n <= 0 && prev != nil && prev.Text[1] == '/' {
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p.writeNewlines(n, true)
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func (p *printer) writeCommentLine(comment *ast.Comment, pos token.Position, line []byte) {
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// line must pass through unchanged, bracket it with tabwriter.Escape
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line = bytes.Join([][]byte{esc, line, esc}, nil)
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p.writeItem(pos, line)
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// Split comment text into lines
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func split(text []byte) [][]byte {
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// count lines (comment text never ends in a newline)
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for _, c := range text {
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lines := make([][]byte, n)
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for j, c := range text {
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lines[n] = text[i:j] // exclude newline
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i = j + 1 // discard newline
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func isBlank(s []byte) bool {
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for _, b := range s {
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func commonPrefix(a, b []byte) []byte {
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for i < len(a) && i < len(b) && a[i] == b[i] && (a[i] <= ' ' || a[i] == '*') {
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func stripCommonPrefix(lines [][]byte) {
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return // at most one line - nothing to do
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// The heuristic in this function tries to handle a few
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// common patterns of /*-style comments: Comments where
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// the opening /* and closing */ are aligned and the
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// rest of the comment text is aligned and indented with
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// blanks or tabs, cases with a vertical "line of stars"
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// on the left, and cases where the closing */ is on the
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// same line as the last comment text.
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// Compute maximum common white prefix of all but the first,
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// last, and blank lines, and replace blank lines with empty
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// lines (the first line starts with /* and has no prefix).
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// In case of two-line comments, consider the last line for
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// the prefix computation since otherwise the prefix would
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// Note that the first and last line are never empty (they
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// contain the opening /* and closing */ respectively) and
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// thus they can be ignored by the blank line check.
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for i, line := range lines[1 : len(lines)-1] {
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lines[1+i] = nil // range starts at line 1
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prefix = commonPrefix(line, line)
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prefix = commonPrefix(prefix, line)
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} else { // len(lines) == 2
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prefix = commonPrefix(line, line)
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* Check for vertical "line of stars" and correct prefix accordingly.
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if i := bytes.Index(prefix, []byte{'*'}); i >= 0 {
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// Line of stars present.
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if i > 0 && prefix[i-1] == ' ' {
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i-- // remove trailing blank from prefix so stars remain aligned
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// No line of stars present.
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// Determine the white space on the first line after the /*
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// and before the beginning of the comment text, assume two
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// blanks instead of the /* unless the first character after
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// the /* is a tab. If the first comment line is empty but
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// for the opening /*, assume up to 3 blanks or a tab. This
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// whitespace may be found as suffix in the common prefix.
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if isBlank(first[2:]) {
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// no comment text on the first line:
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// reduce prefix by up to 3 blanks or a tab
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// if present - this keeps comment text indented
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// relative to the /* and */'s if it was indented
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// in the first place
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for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
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if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
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// comment text on the first line
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suffix := make([]byte, len(first))
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n := 2 // start after opening /*
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for n < len(first) && first[n] <= ' ' {
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if n > 2 && suffix[2] == '\t' {
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// assume the '\t' compensates for the /*
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// otherwise assume two blanks
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suffix[0], suffix[1] = ' ', ' '
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// Shorten the computed common prefix by the length of
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// suffix, if it is found as suffix of the prefix.
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if bytes.HasSuffix(prefix, suffix) {
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prefix = prefix[0 : len(prefix)-len(suffix)]
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// Handle last line: If it only contains a closing */, align it
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// with the opening /*, otherwise align the text with the other
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last := lines[len(lines)-1]
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closing := []byte("*/")
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i := bytes.Index(last, closing)
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if isBlank(last[0:i]) {
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// last line only contains closing */
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// insert an aligning blank
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lines[len(lines)-1] = bytes.Join([][]byte{prefix, closing}, sep)
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// last line contains more comment text - assume
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// it is aligned like the other lines
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prefix = commonPrefix(prefix, last)
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// Remove the common prefix from all but the first and empty lines.
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for i, line := range lines[1:] {
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lines[1+i] = line[len(prefix):] // range starts at line 1
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func (p *printer) writeComment(comment *ast.Comment) {
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// shortcut common case of //-style comments
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p.writeCommentLine(comment, p.fset.Position(comment.Pos()), text)
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// for /*-style comments, print line by line and let the
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// write function take care of the proper indentation
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stripCommonPrefix(lines)
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// write comment lines, separated by formfeed,
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// without a line break after the last line
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linebreak := formfeeds[0:1]
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pos := p.fset.Position(comment.Pos())
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for i, line := range lines {
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p.writeCommentLine(comment, pos, line)
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// writeCommentSuffix writes a line break after a comment if indicated
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// and processes any leftover indentation information. If a line break
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// is needed, the kind of break (newline vs formfeed) depends on the
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// pending whitespace. writeCommentSuffix returns true if a pending
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// formfeed was dropped from the whitespace buffer.
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func (p *printer) writeCommentSuffix(needsLinebreak bool) (droppedFF bool) {
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for i, ch := range p.buffer {
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// ignore trailing whitespace
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case indent, unindent:
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// don't loose indentation information
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case newline, formfeed:
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// if we need a line break, keep exactly one
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// but remember if we dropped any formfeeds
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needsLinebreak = false
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p.writeWhitespace(len(p.buffer))
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// make sure we have a line break
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p.write([]byte{'\n'})
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// intersperseComments consumes all comments that appear before the next token
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// tok and prints it together with the buffered whitespace (i.e., the whitespace
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// that needs to be written before the next token). A heuristic is used to mix
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// the comments and whitespace. intersperseComments returns true if a pending
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// formfeed was dropped from the whitespace buffer.
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func (p *printer) intersperseComments(next token.Position, tok token.Token) (droppedFF bool) {
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var last *ast.Comment
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for ; p.commentBefore(next); p.cindex++ {
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for _, c := range p.comments[p.cindex].List {
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p.writeCommentPrefix(p.fset.Position(c.Pos()), next, last, tok.IsKeyword())
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if last.Text[1] == '*' && p.fset.Position(last.Pos()).Line == next.Line {
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// the last comment is a /*-style comment and the next item
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// follows on the same line: separate with an extra blank
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// ensure that there is a line break after a //-style comment,
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// before a closing '}' unless explicitly disabled, or at eof
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last.Text[1] == '/' ||
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tok == token.RBRACE && p.mode&noExtraLinebreak == 0 ||
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return p.writeCommentSuffix(needsLinebreak)
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// no comment was written - we should never reach here since
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// intersperseComments should not be called in that case
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p.internalError("intersperseComments called without pending comments")
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// whiteWhitespace writes the first n whitespace entries.
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func (p *printer) writeWhitespace(n int) {
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for i := 0; i < n; i++ {
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switch ch := p.buffer[i]; ch {
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p.internalError("negative indentation:", p.indent)
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case newline, formfeed:
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// A line break immediately followed by a "correcting"
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// unindent is swapped with the unindent - this permits
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// proper label positioning. If a comment is between
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// the line break and the label, the unindent is not
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// part of the comment whitespace prefix and the comment
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// will be positioned correctly indented.
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if i+1 < n && p.buffer[i+1] == unindent {
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// Use a formfeed to terminate the current section.
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// Otherwise, a long label name on the next line leading
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// to a wide column may increase the indentation column
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// of lines before the label; effectively leading to wrong
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p.buffer[i], p.buffer[i+1] = unindent, formfeed
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// shift remaining entries down
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for ; n < len(p.buffer); n++ {
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p.buffer[i] = p.buffer[n]
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p.buffer = p.buffer[0:i]
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// ----------------------------------------------------------------------------
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// Printing interface
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func mayCombine(prev token.Token, next byte) (b bool) {
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b = next == '.' // 1.
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b = next == '+' // ++
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b = next == '-' // --
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b = next == '*' // /*
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b = next == '-' || next == '<' // <- or <<
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b = next == '&' || next == '^' // && or &^
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// print prints a list of "items" (roughly corresponding to syntactic
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// tokens, but also including whitespace and formatting information).
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// It is the only print function that should be called directly from
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// any of the AST printing functions in nodes.go.
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// Whitespace is accumulated until a non-whitespace token appears. Any
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// comments that need to appear before that token are printed first,
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// taking into account the amount and structure of any pending white-
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// space for best comment placement. Then, any leftover whitespace is
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// printed, followed by the actual token.
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func (p *printer) print(args ...interface{}) {
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for _, f := range args {
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next := p.pos // estimated position of next item
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switch x := f.(type) {
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// toggle printer mode
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// don't add ignore's to the buffer; they
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// may screw up "correcting" unindents (see
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if i == cap(p.buffer) {
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// Whitespace sequences are very short so this should
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// never happen. Handle gracefully (but possibly with
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// bad comment placement) if it does happen.
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p.buffer = p.buffer[0 : i+1]
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data = []byte(x.Name)
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// escape all literals so they pass through unchanged
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// (note that valid Go programs cannot contain
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// tabwriter.Escape bytes since they do not appear in
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// legal UTF-8 sequences)
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data = make([]byte, 0, len(x.Value)+2)
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data = append(data, tabwriter.Escape)
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data = append(data, x.Value...)
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data = append(data, tabwriter.Escape)
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// If we have a raw string that spans multiple lines and
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// the opening quote (`) is on a line preceded only by
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// indentation, we don't want to write that indentation
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// because the following lines of the raw string are not
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// indented. It's easiest to correct the output at the end
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// via the trimmer (because of the complex handling of
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// Mark multi-line raw strings by replacing the opening
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// quote with esc2 and have the trimmer take care of fixing
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// it up. (Do this _after_ making a copy of data!)
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if data[1] == '`' && bytes.IndexByte(data, '\n') > 0 {
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if mayCombine(p.lastTok, s[0]) {
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// the previous and the current token must be
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// separated by a blank otherwise they combine
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// into a different incorrect token sequence
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// (except for token.INT followed by a '.' this
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// should never happen because it is taken care
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// of via binary expression formatting)
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if len(p.buffer) != 0 {
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p.internalError("whitespace buffer not empty")
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p.buffer = p.buffer[0:1]
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next = p.fset.Position(x) // accurate position of next item
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fmt.Fprintf(os.Stderr, "print: unsupported argument type %T\n", f)
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panic("go/printer type")
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droppedFF := p.flush(next, tok)
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// intersperse extra newlines if present in the source
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// (don't do this in flush as it will cause extra newlines
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// at the end of a file) - use formfeeds if we dropped one
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p.writeNewlines(next.Line-p.pos.Line, droppedFF)
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p.writeItem(next, data)
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// commentBefore returns true iff the current comment occurs
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// before the next position in the source code.
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func (p *printer) commentBefore(next token.Position) bool {
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return p.cindex < len(p.comments) && p.fset.Position(p.comments[p.cindex].List[0].Pos()).Offset < next.Offset
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// Flush prints any pending comments and whitespace occurring
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// textually before the position of the next token tok. Flush
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// returns true if a pending formfeed character was dropped
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// from the whitespace buffer as a result of interspersing
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func (p *printer) flush(next token.Position, tok token.Token) (droppedFF bool) {
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if p.commentBefore(next) {
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// if there are comments before the next item, intersperse them
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droppedFF = p.intersperseComments(next, tok)
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// otherwise, write any leftover whitespace
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p.writeWhitespace(len(p.buffer))
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// ----------------------------------------------------------------------------
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// A trimmer is an io.Writer filter for stripping tabwriter.Escape
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// characters, trailing blanks and tabs, and for converting formfeed
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// and vtab characters into newlines and htabs (in case no tabwriter
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// is used). Text bracketed by tabwriter.Escape characters is passed
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// through unchanged.
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type trimmer struct {
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// trimmer is implemented as a state machine.
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// It can be in one of the following states:
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inSpace = iota // inside space
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atEscape // inside space and the last char was an opening tabwriter.Escape
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inEscape // inside text bracketed by tabwriter.Escapes
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inText // inside text
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var backquote = []byte{'`'}
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// Design note: It is tempting to eliminate extra blanks occurring in
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// whitespace in this function as it could simplify some
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// of the blanks logic in the node printing functions.
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// However, this would mess up any formatting done by
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func (p *trimmer) Write(data []byte) (n int, err os.Error) {
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// p.state == inSpace, atEscape:
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// p.space is unwritten
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// p.hasText indicates if there is any text on this line
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// p.state == inEscape, inText:
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// data[m:n] is unwritten
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for n, b = range data {
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b = '\t' // convert to htab
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p.space.WriteByte(b) // WriteByte returns no errors
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p.space.Reset() // discard trailing space
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_, err = p.output.Write(newlines[0:1]) // write newline
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case tabwriter.Escape:
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_, err = p.output.Write(p.space.Bytes())
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// discard indentation if we have a multi-line raw string
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// (see printer.print for details)
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if b != esc2 || p.hasText {
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_, err = p.output.Write(p.space.Bytes())
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_, err = p.output.Write(backquote) // convert back
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if b == tabwriter.Escape {
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_, err = p.output.Write(data[m:n])
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_, err = p.output.Write(data[m:n])
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p.space.WriteByte(b) // WriteByte returns no errors
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_, err = p.output.Write(data[m:n])
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_, err = p.output.Write(newlines[0:1]) // write newline
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case tabwriter.Escape:
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_, err = p.output.Write(data[m:n])
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case inEscape, inText:
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_, err = p.output.Write(data[m:n])
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// ----------------------------------------------------------------------------
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// General printing is controlled with these Config.Mode flags.
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RawFormat uint = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
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TabIndent // use tabs for indentation independent of UseSpaces
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UseSpaces // use spaces instead of tabs for alignment
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// A Config node controls the output of Fprint.
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Mode uint // default: 0
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Tabwidth int // default: 8
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// Fprint "pretty-prints" an AST node to output and returns the number
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// of bytes written and an error (if any) for a given configuration cfg.
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// Position information is interpreted relative to the file set fset.
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// The node type must be *ast.File, or assignment-compatible to ast.Expr,
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// ast.Decl, ast.Spec, or ast.Stmt.
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func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) (int, os.Error) {
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// redirect output through a trimmer to eliminate trailing whitespace
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// (Input to a tabwriter must be untrimmed since trailing tabs provide
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// formatting information. The tabwriter could provide trimming
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// functionality but no tabwriter is used when RawFormat is set.)
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output = &trimmer{output: output}
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// setup tabwriter if needed and redirect output
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var tw *tabwriter.Writer
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if cfg.Mode&RawFormat == 0 {
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minwidth := cfg.Tabwidth
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padchar := byte('\t')
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if cfg.Mode&UseSpaces != 0 {
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twmode := tabwriter.DiscardEmptyColumns
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if cfg.Mode&TabIndent != 0 {
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twmode |= tabwriter.TabIndent
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tw = tabwriter.NewWriter(output, minwidth, cfg.Tabwidth, 1, padchar, twmode)
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// setup printer and print node
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p.init(output, cfg, fset)
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switch n := node.(type) {
1037
p.useNodeComments = true
1038
p.expr(n, ignoreMultiLine)
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p.useNodeComments = true
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// A labeled statement will un-indent to position the
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// label. Set indent to 1 so we don't get indent "underflow".
1044
if _, labeledStmt := n.(*ast.LabeledStmt); labeledStmt {
1047
p.stmt(n, false, ignoreMultiLine)
1050
p.useNodeComments = true
1051
p.decl(n, ignoreMultiLine)
1054
p.useNodeComments = true
1055
p.spec(n, 1, false, ignoreMultiLine)
1058
p.comments = n.Comments
1059
p.useNodeComments = n.Comments == nil
1062
p.errors <- fmt.Errorf("printer.Fprint: unsupported node type %T", n)
1065
p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)
1066
p.errors <- nil // no errors
1068
err := <-p.errors // wait for completion of goroutine
1070
// flush tabwriter, if any
1072
tw.Flush() // ignore errors
1075
return p.written, err
1079
// Fprint "pretty-prints" an AST node to output.
1080
// It calls Config.Fprint with default settings.
1082
func Fprint(output io.Writer, fset *token.FileSet, node interface{}) os.Error {
1083
_, err := (&Config{Tabwidth: 8}).Fprint(output, fset, node) // don't care about number of bytes written
added
removed
src/pkg/go/printer/printer.go
