...
Run Format

Source file src/go/printer/printer.go

Documentation: go/printer

     1  // Copyright 2009 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 printer implements printing of AST nodes.
     6  package printer
     7  
     8  import (
     9  	"fmt"
    10  	"go/ast"
    11  	"go/token"
    12  	"io"
    13  	"os"
    14  	"strconv"
    15  	"strings"
    16  	"text/tabwriter"
    17  	"unicode"
    18  )
    19  
    20  const (
    21  	maxNewlines = 2     // max. number of newlines between source text
    22  	debug       = false // enable for debugging
    23  	infinity    = 1 << 30
    24  )
    25  
    26  type whiteSpace byte
    27  
    28  const (
    29  	ignore   = whiteSpace(0)
    30  	blank    = whiteSpace(' ')
    31  	vtab     = whiteSpace('\v')
    32  	newline  = whiteSpace('\n')
    33  	formfeed = whiteSpace('\f')
    34  	indent   = whiteSpace('>')
    35  	unindent = whiteSpace('<')
    36  )
    37  
    38  // A pmode value represents the current printer mode.
    39  type pmode int
    40  
    41  const (
    42  	noExtraBlank     pmode = 1 << iota // disables extra blank after /*-style comment
    43  	noExtraLinebreak                   // disables extra line break after /*-style comment
    44  )
    45  
    46  type commentInfo struct {
    47  	cindex         int               // current comment index
    48  	comment        *ast.CommentGroup // = printer.comments[cindex]; or nil
    49  	commentOffset  int               // = printer.posFor(printer.comments[cindex].List[0].Pos()).Offset; or infinity
    50  	commentNewline bool              // true if the comment group contains newlines
    51  }
    52  
    53  type printer struct {
    54  	// Configuration (does not change after initialization)
    55  	Config
    56  	fset *token.FileSet
    57  
    58  	// Current state
    59  	output      []byte       // raw printer result
    60  	indent      int          // current indentation
    61  	level       int          // level == 0: outside composite literal; level > 0: inside composite literal
    62  	mode        pmode        // current printer mode
    63  	impliedSemi bool         // if set, a linebreak implies a semicolon
    64  	lastTok     token.Token  // last token printed (token.ILLEGAL if it's whitespace)
    65  	prevOpen    token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
    66  	wsbuf       []whiteSpace // delayed white space
    67  
    68  	// Positions
    69  	// The out position differs from the pos position when the result
    70  	// formatting differs from the source formatting (in the amount of
    71  	// white space). If there's a difference and SourcePos is set in
    72  	// ConfigMode, //line directives are used in the output to restore
    73  	// original source positions for a reader.
    74  	pos     token.Position // current position in AST (source) space
    75  	out     token.Position // current position in output space
    76  	last    token.Position // value of pos after calling writeString
    77  	linePtr *int           // if set, record out.Line for the next token in *linePtr
    78  
    79  	// The list of all source comments, in order of appearance.
    80  	comments        []*ast.CommentGroup // may be nil
    81  	useNodeComments bool                // if not set, ignore lead and line comments of nodes
    82  
    83  	// Information about p.comments[p.cindex]; set up by nextComment.
    84  	commentInfo
    85  
    86  	// Cache of already computed node sizes.
    87  	nodeSizes map[ast.Node]int
    88  
    89  	// Cache of most recently computed line position.
    90  	cachedPos  token.Pos
    91  	cachedLine int // line corresponding to cachedPos
    92  }
    93  
    94  func (p *printer) init(cfg *Config, fset *token.FileSet, nodeSizes map[ast.Node]int) {
    95  	p.Config = *cfg
    96  	p.fset = fset
    97  	p.pos = token.Position{Line: 1, Column: 1}
    98  	p.out = token.Position{Line: 1, Column: 1}
    99  	p.wsbuf = make([]whiteSpace, 0, 16) // whitespace sequences are short
   100  	p.nodeSizes = nodeSizes
   101  	p.cachedPos = -1
   102  }
   103  
   104  func (p *printer) internalError(msg ...interface{}) {
   105  	if debug {
   106  		fmt.Print(p.pos.String() + ": ")
   107  		fmt.Println(msg...)
   108  		panic("go/printer")
   109  	}
   110  }
   111  
   112  // commentsHaveNewline reports whether a list of comments belonging to
   113  // an *ast.CommentGroup contains newlines. Because the position information
   114  // may only be partially correct, we also have to read the comment text.
   115  func (p *printer) commentsHaveNewline(list []*ast.Comment) bool {
   116  	// len(list) > 0
   117  	line := p.lineFor(list[0].Pos())
   118  	for i, c := range list {
   119  		if i > 0 && p.lineFor(list[i].Pos()) != line {
   120  			// not all comments on the same line
   121  			return true
   122  		}
   123  		if t := c.Text; len(t) >= 2 && (t[1] == '/' || strings.Contains(t, "\n")) {
   124  			return true
   125  		}
   126  	}
   127  	_ = line
   128  	return false
   129  }
   130  
   131  func (p *printer) nextComment() {
   132  	for p.cindex < len(p.comments) {
   133  		c := p.comments[p.cindex]
   134  		p.cindex++
   135  		if list := c.List; len(list) > 0 {
   136  			p.comment = c
   137  			p.commentOffset = p.posFor(list[0].Pos()).Offset
   138  			p.commentNewline = p.commentsHaveNewline(list)
   139  			return
   140  		}
   141  		// we should not reach here (correct ASTs don't have empty
   142  		// ast.CommentGroup nodes), but be conservative and try again
   143  	}
   144  	// no more comments
   145  	p.commentOffset = infinity
   146  }
   147  
   148  // commentBefore reports whether the current comment group occurs
   149  // before the next position in the source code and printing it does
   150  // not introduce implicit semicolons.
   151  //
   152  func (p *printer) commentBefore(next token.Position) bool {
   153  	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
   154  }
   155  
   156  // commentSizeBefore returns the estimated size of the
   157  // comments on the same line before the next position.
   158  //
   159  func (p *printer) commentSizeBefore(next token.Position) int {
   160  	// save/restore current p.commentInfo (p.nextComment() modifies it)
   161  	defer func(info commentInfo) {
   162  		p.commentInfo = info
   163  	}(p.commentInfo)
   164  
   165  	size := 0
   166  	for p.commentBefore(next) {
   167  		for _, c := range p.comment.List {
   168  			size += len(c.Text)
   169  		}
   170  		p.nextComment()
   171  	}
   172  	return size
   173  }
   174  
   175  // recordLine records the output line number for the next non-whitespace
   176  // token in *linePtr. It is used to compute an accurate line number for a
   177  // formatted construct, independent of pending (not yet emitted) whitespace
   178  // or comments.
   179  //
   180  func (p *printer) recordLine(linePtr *int) {
   181  	p.linePtr = linePtr
   182  }
   183  
   184  // linesFrom returns the number of output lines between the current
   185  // output line and the line argument, ignoring any pending (not yet
   186  // emitted) whitespace or comments. It is used to compute an accurate
   187  // size (in number of lines) for a formatted construct.
   188  //
   189  func (p *printer) linesFrom(line int) int {
   190  	return p.out.Line - line
   191  }
   192  
   193  func (p *printer) posFor(pos token.Pos) token.Position {
   194  	// not used frequently enough to cache entire token.Position
   195  	return p.fset.Position(pos)
   196  }
   197  
   198  func (p *printer) lineFor(pos token.Pos) int {
   199  	if pos != p.cachedPos {
   200  		p.cachedPos = pos
   201  		p.cachedLine = p.fset.Position(pos).Line
   202  	}
   203  	return p.cachedLine
   204  }
   205  
   206  // writeLineDirective writes a //line directive if necessary.
   207  func (p *printer) writeLineDirective(pos token.Position) {
   208  	if pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
   209  		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
   210  		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
   211  		p.output = append(p.output, tabwriter.Escape)
   212  		// p.out must match the //line directive
   213  		p.out.Filename = pos.Filename
   214  		p.out.Line = pos.Line
   215  	}
   216  }
   217  
   218  // writeIndent writes indentation.
   219  func (p *printer) writeIndent() {
   220  	// use "hard" htabs - indentation columns
   221  	// must not be discarded by the tabwriter
   222  	n := p.Config.Indent + p.indent // include base indentation
   223  	for i := 0; i < n; i++ {
   224  		p.output = append(p.output, '\t')
   225  	}
   226  
   227  	// update positions
   228  	p.pos.Offset += n
   229  	p.pos.Column += n
   230  	p.out.Column += n
   231  }
   232  
   233  // writeByte writes ch n times to p.output and updates p.pos.
   234  // Only used to write formatting (white space) characters.
   235  func (p *printer) writeByte(ch byte, n int) {
   236  	if p.out.Column == 1 {
   237  		// no need to write line directives before white space
   238  		p.writeIndent()
   239  	}
   240  
   241  	for i := 0; i < n; i++ {
   242  		p.output = append(p.output, ch)
   243  	}
   244  
   245  	// update positions
   246  	p.pos.Offset += n
   247  	if ch == '\n' || ch == '\f' {
   248  		p.pos.Line += n
   249  		p.out.Line += n
   250  		p.pos.Column = 1
   251  		p.out.Column = 1
   252  		return
   253  	}
   254  	p.pos.Column += n
   255  	p.out.Column += n
   256  }
   257  
   258  // writeString writes the string s to p.output and updates p.pos, p.out,
   259  // and p.last. If isLit is set, s is escaped w/ tabwriter.Escape characters
   260  // to protect s from being interpreted by the tabwriter.
   261  //
   262  // Note: writeString is only used to write Go tokens, literals, and
   263  // comments, all of which must be written literally. Thus, it is correct
   264  // to always set isLit = true. However, setting it explicitly only when
   265  // needed (i.e., when we don't know that s contains no tabs or line breaks)
   266  // avoids processing extra escape characters and reduces run time of the
   267  // printer benchmark by up to 10%.
   268  //
   269  func (p *printer) writeString(pos token.Position, s string, isLit bool) {
   270  	if p.out.Column == 1 {
   271  		if p.Config.Mode&SourcePos != 0 {
   272  			p.writeLineDirective(pos)
   273  		}
   274  		p.writeIndent()
   275  	}
   276  
   277  	if pos.IsValid() {
   278  		// update p.pos (if pos is invalid, continue with existing p.pos)
   279  		// Note: Must do this after handling line beginnings because
   280  		// writeIndent updates p.pos if there's indentation, but p.pos
   281  		// is the position of s.
   282  		p.pos = pos
   283  	}
   284  
   285  	if isLit {
   286  		// Protect s such that is passes through the tabwriter
   287  		// unchanged. Note that valid Go programs cannot contain
   288  		// tabwriter.Escape bytes since they do not appear in legal
   289  		// UTF-8 sequences.
   290  		p.output = append(p.output, tabwriter.Escape)
   291  	}
   292  
   293  	if debug {
   294  		p.output = append(p.output, fmt.Sprintf("/*%s*/", pos)...) // do not update p.pos!
   295  	}
   296  	p.output = append(p.output, s...)
   297  
   298  	// update positions
   299  	nlines := 0
   300  	var li int // index of last newline; valid if nlines > 0
   301  	for i := 0; i < len(s); i++ {
   302  		// Go tokens cannot contain '\f' - no need to look for it
   303  		if s[i] == '\n' {
   304  			nlines++
   305  			li = i
   306  		}
   307  	}
   308  	p.pos.Offset += len(s)
   309  	if nlines > 0 {
   310  		p.pos.Line += nlines
   311  		p.out.Line += nlines
   312  		c := len(s) - li
   313  		p.pos.Column = c
   314  		p.out.Column = c
   315  	} else {
   316  		p.pos.Column += len(s)
   317  		p.out.Column += len(s)
   318  	}
   319  
   320  	if isLit {
   321  		p.output = append(p.output, tabwriter.Escape)
   322  	}
   323  
   324  	p.last = p.pos
   325  }
   326  
   327  // writeCommentPrefix writes the whitespace before a comment.
   328  // If there is any pending whitespace, it consumes as much of
   329  // it as is likely to help position the comment nicely.
   330  // pos is the comment position, next the position of the item
   331  // after all pending comments, prev is the previous comment in
   332  // a group of comments (or nil), and tok is the next token.
   333  //
   334  func (p *printer) writeCommentPrefix(pos, next token.Position, prev *ast.Comment, tok token.Token) {
   335  	if len(p.output) == 0 {
   336  		// the comment is the first item to be printed - don't write any whitespace
   337  		return
   338  	}
   339  
   340  	if pos.IsValid() && pos.Filename != p.last.Filename {
   341  		// comment in a different file - separate with newlines
   342  		p.writeByte('\f', maxNewlines)
   343  		return
   344  	}
   345  
   346  	if pos.Line == p.last.Line && (prev == nil || prev.Text[1] != '/') {
   347  		// comment on the same line as last item:
   348  		// separate with at least one separator
   349  		hasSep := false
   350  		if prev == nil {
   351  			// first comment of a comment group
   352  			j := 0
   353  			for i, ch := range p.wsbuf {
   354  				switch ch {
   355  				case blank:
   356  					// ignore any blanks before a comment
   357  					p.wsbuf[i] = ignore
   358  					continue
   359  				case vtab:
   360  					// respect existing tabs - important
   361  					// for proper formatting of commented structs
   362  					hasSep = true
   363  					continue
   364  				case indent:
   365  					// apply pending indentation
   366  					continue
   367  				}
   368  				j = i
   369  				break
   370  			}
   371  			p.writeWhitespace(j)
   372  		}
   373  		// make sure there is at least one separator
   374  		if !hasSep {
   375  			sep := byte('\t')
   376  			if pos.Line == next.Line {
   377  				// next item is on the same line as the comment
   378  				// (which must be a /*-style comment): separate
   379  				// with a blank instead of a tab
   380  				sep = ' '
   381  			}
   382  			p.writeByte(sep, 1)
   383  		}
   384  
   385  	} else {
   386  		// comment on a different line:
   387  		// separate with at least one line break
   388  		droppedLinebreak := false
   389  		j := 0
   390  		for i, ch := range p.wsbuf {
   391  			switch ch {
   392  			case blank, vtab:
   393  				// ignore any horizontal whitespace before line breaks
   394  				p.wsbuf[i] = ignore
   395  				continue
   396  			case indent:
   397  				// apply pending indentation
   398  				continue
   399  			case unindent:
   400  				// if this is not the last unindent, apply it
   401  				// as it is (likely) belonging to the last
   402  				// construct (e.g., a multi-line expression list)
   403  				// and is not part of closing a block
   404  				if i+1 < len(p.wsbuf) && p.wsbuf[i+1] == unindent {
   405  					continue
   406  				}
   407  				// if the next token is not a closing }, apply the unindent
   408  				// if it appears that the comment is aligned with the
   409  				// token; otherwise assume the unindent is part of a
   410  				// closing block and stop (this scenario appears with
   411  				// comments before a case label where the comments
   412  				// apply to the next case instead of the current one)
   413  				if tok != token.RBRACE && pos.Column == next.Column {
   414  					continue
   415  				}
   416  			case newline, formfeed:
   417  				p.wsbuf[i] = ignore
   418  				droppedLinebreak = prev == nil // record only if first comment of a group
   419  			}
   420  			j = i
   421  			break
   422  		}
   423  		p.writeWhitespace(j)
   424  
   425  		// determine number of linebreaks before the comment
   426  		n := 0
   427  		if pos.IsValid() && p.last.IsValid() {
   428  			n = pos.Line - p.last.Line
   429  			if n < 0 { // should never happen
   430  				n = 0
   431  			}
   432  		}
   433  
   434  		// at the package scope level only (p.indent == 0),
   435  		// add an extra newline if we dropped one before:
   436  		// this preserves a blank line before documentation
   437  		// comments at the package scope level (issue 2570)
   438  		if p.indent == 0 && droppedLinebreak {
   439  			n++
   440  		}
   441  
   442  		// make sure there is at least one line break
   443  		// if the previous comment was a line comment
   444  		if n == 0 && prev != nil && prev.Text[1] == '/' {
   445  			n = 1
   446  		}
   447  
   448  		if n > 0 {
   449  			// use formfeeds to break columns before a comment;
   450  			// this is analogous to using formfeeds to separate
   451  			// individual lines of /*-style comments
   452  			p.writeByte('\f', nlimit(n))
   453  		}
   454  	}
   455  }
   456  
   457  // Returns true if s contains only white space
   458  // (only tabs and blanks can appear in the printer's context).
   459  //
   460  func isBlank(s string) bool {
   461  	for i := 0; i < len(s); i++ {
   462  		if s[i] > ' ' {
   463  			return false
   464  		}
   465  	}
   466  	return true
   467  }
   468  
   469  // commonPrefix returns the common prefix of a and b.
   470  func commonPrefix(a, b string) string {
   471  	i := 0
   472  	for i < len(a) && i < len(b) && a[i] == b[i] && (a[i] <= ' ' || a[i] == '*') {
   473  		i++
   474  	}
   475  	return a[0:i]
   476  }
   477  
   478  // trimRight returns s with trailing whitespace removed.
   479  func trimRight(s string) string {
   480  	return strings.TrimRightFunc(s, unicode.IsSpace)
   481  }
   482  
   483  // stripCommonPrefix removes a common prefix from /*-style comment lines (unless no
   484  // comment line is indented, all but the first line have some form of space prefix).
   485  // The prefix is computed using heuristics such that is likely that the comment
   486  // contents are nicely laid out after re-printing each line using the printer's
   487  // current indentation.
   488  //
   489  func stripCommonPrefix(lines []string) {
   490  	if len(lines) <= 1 {
   491  		return // at most one line - nothing to do
   492  	}
   493  	// len(lines) > 1
   494  
   495  	// The heuristic in this function tries to handle a few
   496  	// common patterns of /*-style comments: Comments where
   497  	// the opening /* and closing */ are aligned and the
   498  	// rest of the comment text is aligned and indented with
   499  	// blanks or tabs, cases with a vertical "line of stars"
   500  	// on the left, and cases where the closing */ is on the
   501  	// same line as the last comment text.
   502  
   503  	// Compute maximum common white prefix of all but the first,
   504  	// last, and blank lines, and replace blank lines with empty
   505  	// lines (the first line starts with /* and has no prefix).
   506  	// In cases where only the first and last lines are not blank,
   507  	// such as two-line comments, or comments where all inner lines
   508  	// are blank, consider the last line for the prefix computation
   509  	// since otherwise the prefix would be empty.
   510  	//
   511  	// Note that the first and last line are never empty (they
   512  	// contain the opening /* and closing */ respectively) and
   513  	// thus they can be ignored by the blank line check.
   514  	prefix := ""
   515  	prefixSet := false
   516  	if len(lines) > 2 {
   517  		for i, line := range lines[1 : len(lines)-1] {
   518  			if isBlank(line) {
   519  				lines[1+i] = "" // range starts with lines[1]
   520  			} else {
   521  				if !prefixSet {
   522  					prefix = line
   523  					prefixSet = true
   524  				}
   525  				prefix = commonPrefix(prefix, line)
   526  			}
   527  
   528  		}
   529  	}
   530  	// If we don't have a prefix yet, consider the last line.
   531  	if !prefixSet {
   532  		line := lines[len(lines)-1]
   533  		prefix = commonPrefix(line, line)
   534  	}
   535  
   536  	/*
   537  	 * Check for vertical "line of stars" and correct prefix accordingly.
   538  	 */
   539  	lineOfStars := false
   540  	if i := strings.Index(prefix, "*"); i >= 0 {
   541  		// Line of stars present.
   542  		if i > 0 && prefix[i-1] == ' ' {
   543  			i-- // remove trailing blank from prefix so stars remain aligned
   544  		}
   545  		prefix = prefix[0:i]
   546  		lineOfStars = true
   547  	} else {
   548  		// No line of stars present.
   549  		// Determine the white space on the first line after the /*
   550  		// and before the beginning of the comment text, assume two
   551  		// blanks instead of the /* unless the first character after
   552  		// the /* is a tab. If the first comment line is empty but
   553  		// for the opening /*, assume up to 3 blanks or a tab. This
   554  		// whitespace may be found as suffix in the common prefix.
   555  		first := lines[0]
   556  		if isBlank(first[2:]) {
   557  			// no comment text on the first line:
   558  			// reduce prefix by up to 3 blanks or a tab
   559  			// if present - this keeps comment text indented
   560  			// relative to the /* and */'s if it was indented
   561  			// in the first place
   562  			i := len(prefix)
   563  			for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
   564  				i--
   565  			}
   566  			if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
   567  				i--
   568  			}
   569  			prefix = prefix[0:i]
   570  		} else {
   571  			// comment text on the first line
   572  			suffix := make([]byte, len(first))
   573  			n := 2 // start after opening /*
   574  			for n < len(first) && first[n] <= ' ' {
   575  				suffix[n] = first[n]
   576  				n++
   577  			}
   578  			if n > 2 && suffix[2] == '\t' {
   579  				// assume the '\t' compensates for the /*
   580  				suffix = suffix[2:n]
   581  			} else {
   582  				// otherwise assume two blanks
   583  				suffix[0], suffix[1] = ' ', ' '
   584  				suffix = suffix[0:n]
   585  			}
   586  			// Shorten the computed common prefix by the length of
   587  			// suffix, if it is found as suffix of the prefix.
   588  			prefix = strings.TrimSuffix(prefix, string(suffix))
   589  		}
   590  	}
   591  
   592  	// Handle last line: If it only contains a closing */, align it
   593  	// with the opening /*, otherwise align the text with the other
   594  	// lines.
   595  	last := lines[len(lines)-1]
   596  	closing := "*/"
   597  	i := strings.Index(last, closing) // i >= 0 (closing is always present)
   598  	if isBlank(last[0:i]) {
   599  		// last line only contains closing */
   600  		if lineOfStars {
   601  			closing = " */" // add blank to align final star
   602  		}
   603  		lines[len(lines)-1] = prefix + closing
   604  	} else {
   605  		// last line contains more comment text - assume
   606  		// it is aligned like the other lines and include
   607  		// in prefix computation
   608  		prefix = commonPrefix(prefix, last)
   609  	}
   610  
   611  	// Remove the common prefix from all but the first and empty lines.
   612  	for i, line := range lines {
   613  		if i > 0 && line != "" {
   614  			lines[i] = line[len(prefix):]
   615  		}
   616  	}
   617  }
   618  
   619  func (p *printer) writeComment(comment *ast.Comment) {
   620  	text := comment.Text
   621  	pos := p.posFor(comment.Pos())
   622  
   623  	const linePrefix = "//line "
   624  	if strings.HasPrefix(text, linePrefix) && (!pos.IsValid() || pos.Column == 1) {
   625  		// possibly a line directive
   626  		ldir := strings.TrimSpace(text[len(linePrefix):])
   627  		if i := strings.LastIndex(ldir, ":"); i >= 0 {
   628  			if line, err := strconv.Atoi(ldir[i+1:]); err == nil && line > 0 {
   629  				// The line directive we are about to print changed
   630  				// the Filename and Line number used for subsequent
   631  				// tokens. We have to update our AST-space position
   632  				// accordingly and suspend indentation temporarily.
   633  				indent := p.indent
   634  				p.indent = 0
   635  				defer func() {
   636  					p.pos.Filename = ldir[:i]
   637  					p.pos.Line = line
   638  					p.pos.Column = 1
   639  					p.indent = indent
   640  				}()
   641  			}
   642  		}
   643  	}
   644  
   645  	// shortcut common case of //-style comments
   646  	if text[1] == '/' {
   647  		p.writeString(pos, trimRight(text), true)
   648  		return
   649  	}
   650  
   651  	// for /*-style comments, print line by line and let the
   652  	// write function take care of the proper indentation
   653  	lines := strings.Split(text, "\n")
   654  
   655  	// The comment started in the first column but is going
   656  	// to be indented. For an idempotent result, add indentation
   657  	// to all lines such that they look like they were indented
   658  	// before - this will make sure the common prefix computation
   659  	// is the same independent of how many times formatting is
   660  	// applied (was issue 1835).
   661  	if pos.IsValid() && pos.Column == 1 && p.indent > 0 {
   662  		for i, line := range lines[1:] {
   663  			lines[1+i] = "   " + line
   664  		}
   665  	}
   666  
   667  	stripCommonPrefix(lines)
   668  
   669  	// write comment lines, separated by formfeed,
   670  	// without a line break after the last line
   671  	for i, line := range lines {
   672  		if i > 0 {
   673  			p.writeByte('\f', 1)
   674  			pos = p.pos
   675  		}
   676  		if len(line) > 0 {
   677  			p.writeString(pos, trimRight(line), true)
   678  		}
   679  	}
   680  }
   681  
   682  // writeCommentSuffix writes a line break after a comment if indicated
   683  // and processes any leftover indentation information. If a line break
   684  // is needed, the kind of break (newline vs formfeed) depends on the
   685  // pending whitespace. The writeCommentSuffix result indicates if a
   686  // newline was written or if a formfeed was dropped from the whitespace
   687  // buffer.
   688  //
   689  func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, droppedFF bool) {
   690  	for i, ch := range p.wsbuf {
   691  		switch ch {
   692  		case blank, vtab:
   693  			// ignore trailing whitespace
   694  			p.wsbuf[i] = ignore
   695  		case indent, unindent:
   696  			// don't lose indentation information
   697  		case newline, formfeed:
   698  			// if we need a line break, keep exactly one
   699  			// but remember if we dropped any formfeeds
   700  			if needsLinebreak {
   701  				needsLinebreak = false
   702  				wroteNewline = true
   703  			} else {
   704  				if ch == formfeed {
   705  					droppedFF = true
   706  				}
   707  				p.wsbuf[i] = ignore
   708  			}
   709  		}
   710  	}
   711  	p.writeWhitespace(len(p.wsbuf))
   712  
   713  	// make sure we have a line break
   714  	if needsLinebreak {
   715  		p.writeByte('\n', 1)
   716  		wroteNewline = true
   717  	}
   718  
   719  	return
   720  }
   721  
   722  // containsLinebreak reports whether the whitespace buffer contains any line breaks.
   723  func (p *printer) containsLinebreak() bool {
   724  	for _, ch := range p.wsbuf {
   725  		if ch == newline || ch == formfeed {
   726  			return true
   727  		}
   728  	}
   729  	return false
   730  }
   731  
   732  // intersperseComments consumes all comments that appear before the next token
   733  // tok and prints it together with the buffered whitespace (i.e., the whitespace
   734  // that needs to be written before the next token). A heuristic is used to mix
   735  // the comments and whitespace. The intersperseComments result indicates if a
   736  // newline was written or if a formfeed was dropped from the whitespace buffer.
   737  //
   738  func (p *printer) intersperseComments(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
   739  	var last *ast.Comment
   740  	for p.commentBefore(next) {
   741  		for _, c := range p.comment.List {
   742  			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, tok)
   743  			p.writeComment(c)
   744  			last = c
   745  		}
   746  		p.nextComment()
   747  	}
   748  
   749  	if last != nil {
   750  		// If the last comment is a /*-style comment and the next item
   751  		// follows on the same line but is not a comma, and not a "closing"
   752  		// token immediately following its corresponding "opening" token,
   753  		// add an extra separator unless explicitly disabled. Use a blank
   754  		// as separator unless we have pending linebreaks, they are not
   755  		// disabled, and we are outside a composite literal, in which case
   756  		// we want a linebreak (issue 15137).
   757  		// TODO(gri) This has become overly complicated. We should be able
   758  		// to track whether we're inside an expression or statement and
   759  		// use that information to decide more directly.
   760  		needsLinebreak := false
   761  		if p.mode&noExtraBlank == 0 &&
   762  			last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line &&
   763  			tok != token.COMMA &&
   764  			(tok != token.RPAREN || p.prevOpen == token.LPAREN) &&
   765  			(tok != token.RBRACK || p.prevOpen == token.LBRACK) {
   766  			if p.containsLinebreak() && p.mode&noExtraLinebreak == 0 && p.level == 0 {
   767  				needsLinebreak = true
   768  			} else {
   769  				p.writeByte(' ', 1)
   770  			}
   771  		}
   772  		// Ensure that there is a line break after a //-style comment,
   773  		// before EOF, and before a closing '}' unless explicitly disabled.
   774  		if last.Text[1] == '/' ||
   775  			tok == token.EOF ||
   776  			tok == token.RBRACE && p.mode&noExtraLinebreak == 0 {
   777  			needsLinebreak = true
   778  		}
   779  		return p.writeCommentSuffix(needsLinebreak)
   780  	}
   781  
   782  	// no comment was written - we should never reach here since
   783  	// intersperseComments should not be called in that case
   784  	p.internalError("intersperseComments called without pending comments")
   785  	return
   786  }
   787  
   788  // whiteWhitespace writes the first n whitespace entries.
   789  func (p *printer) writeWhitespace(n int) {
   790  	// write entries
   791  	for i := 0; i < n; i++ {
   792  		switch ch := p.wsbuf[i]; ch {
   793  		case ignore:
   794  			// ignore!
   795  		case indent:
   796  			p.indent++
   797  		case unindent:
   798  			p.indent--
   799  			if p.indent < 0 {
   800  				p.internalError("negative indentation:", p.indent)
   801  				p.indent = 0
   802  			}
   803  		case newline, formfeed:
   804  			// A line break immediately followed by a "correcting"
   805  			// unindent is swapped with the unindent - this permits
   806  			// proper label positioning. If a comment is between
   807  			// the line break and the label, the unindent is not
   808  			// part of the comment whitespace prefix and the comment
   809  			// will be positioned correctly indented.
   810  			if i+1 < n && p.wsbuf[i+1] == unindent {
   811  				// Use a formfeed to terminate the current section.
   812  				// Otherwise, a long label name on the next line leading
   813  				// to a wide column may increase the indentation column
   814  				// of lines before the label; effectively leading to wrong
   815  				// indentation.
   816  				p.wsbuf[i], p.wsbuf[i+1] = unindent, formfeed
   817  				i-- // do it again
   818  				continue
   819  			}
   820  			fallthrough
   821  		default:
   822  			p.writeByte(byte(ch), 1)
   823  		}
   824  	}
   825  
   826  	// shift remaining entries down
   827  	l := copy(p.wsbuf, p.wsbuf[n:])
   828  	p.wsbuf = p.wsbuf[:l]
   829  }
   830  
   831  // ----------------------------------------------------------------------------
   832  // Printing interface
   833  
   834  // nlines limits n to maxNewlines.
   835  func nlimit(n int) int {
   836  	if n > maxNewlines {
   837  		n = maxNewlines
   838  	}
   839  	return n
   840  }
   841  
   842  func mayCombine(prev token.Token, next byte) (b bool) {
   843  	switch prev {
   844  	case token.INT:
   845  		b = next == '.' // 1.
   846  	case token.ADD:
   847  		b = next == '+' // ++
   848  	case token.SUB:
   849  		b = next == '-' // --
   850  	case token.QUO:
   851  		b = next == '*' // /*
   852  	case token.LSS:
   853  		b = next == '-' || next == '<' // <- or <<
   854  	case token.AND:
   855  		b = next == '&' || next == '^' // && or &^
   856  	}
   857  	return
   858  }
   859  
   860  // print prints a list of "items" (roughly corresponding to syntactic
   861  // tokens, but also including whitespace and formatting information).
   862  // It is the only print function that should be called directly from
   863  // any of the AST printing functions in nodes.go.
   864  //
   865  // Whitespace is accumulated until a non-whitespace token appears. Any
   866  // comments that need to appear before that token are printed first,
   867  // taking into account the amount and structure of any pending white-
   868  // space for best comment placement. Then, any leftover whitespace is
   869  // printed, followed by the actual token.
   870  //
   871  func (p *printer) print(args ...interface{}) {
   872  	for _, arg := range args {
   873  		// information about the current arg
   874  		var data string
   875  		var isLit bool
   876  		var impliedSemi bool // value for p.impliedSemi after this arg
   877  
   878  		// record previous opening token, if any
   879  		switch p.lastTok {
   880  		case token.ILLEGAL:
   881  			// ignore (white space)
   882  		case token.LPAREN, token.LBRACK:
   883  			p.prevOpen = p.lastTok
   884  		default:
   885  			// other tokens followed any opening token
   886  			p.prevOpen = token.ILLEGAL
   887  		}
   888  
   889  		switch x := arg.(type) {
   890  		case pmode:
   891  			// toggle printer mode
   892  			p.mode ^= x
   893  			continue
   894  
   895  		case whiteSpace:
   896  			if x == ignore {
   897  				// don't add ignore's to the buffer; they
   898  				// may screw up "correcting" unindents (see
   899  				// LabeledStmt)
   900  				continue
   901  			}
   902  			i := len(p.wsbuf)
   903  			if i == cap(p.wsbuf) {
   904  				// Whitespace sequences are very short so this should
   905  				// never happen. Handle gracefully (but possibly with
   906  				// bad comment placement) if it does happen.
   907  				p.writeWhitespace(i)
   908  				i = 0
   909  			}
   910  			p.wsbuf = p.wsbuf[0 : i+1]
   911  			p.wsbuf[i] = x
   912  			if x == newline || x == formfeed {
   913  				// newlines affect the current state (p.impliedSemi)
   914  				// and not the state after printing arg (impliedSemi)
   915  				// because comments can be interspersed before the arg
   916  				// in this case
   917  				p.impliedSemi = false
   918  			}
   919  			p.lastTok = token.ILLEGAL
   920  			continue
   921  
   922  		case *ast.Ident:
   923  			data = x.Name
   924  			impliedSemi = true
   925  			p.lastTok = token.IDENT
   926  
   927  		case *ast.BasicLit:
   928  			data = x.Value
   929  			isLit = true
   930  			impliedSemi = true
   931  			p.lastTok = x.Kind
   932  
   933  		case token.Token:
   934  			s := x.String()
   935  			if mayCombine(p.lastTok, s[0]) {
   936  				// the previous and the current token must be
   937  				// separated by a blank otherwise they combine
   938  				// into a different incorrect token sequence
   939  				// (except for token.INT followed by a '.' this
   940  				// should never happen because it is taken care
   941  				// of via binary expression formatting)
   942  				if len(p.wsbuf) != 0 {
   943  					p.internalError("whitespace buffer not empty")
   944  				}
   945  				p.wsbuf = p.wsbuf[0:1]
   946  				p.wsbuf[0] = ' '
   947  			}
   948  			data = s
   949  			// some keywords followed by a newline imply a semicolon
   950  			switch x {
   951  			case token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN,
   952  				token.INC, token.DEC, token.RPAREN, token.RBRACK, token.RBRACE:
   953  				impliedSemi = true
   954  			}
   955  			p.lastTok = x
   956  
   957  		case token.Pos:
   958  			if x.IsValid() {
   959  				p.pos = p.posFor(x) // accurate position of next item
   960  			}
   961  			continue
   962  
   963  		case string:
   964  			// incorrect AST - print error message
   965  			data = x
   966  			isLit = true
   967  			impliedSemi = true
   968  			p.lastTok = token.STRING
   969  
   970  		default:
   971  			fmt.Fprintf(os.Stderr, "print: unsupported argument %v (%T)\n", arg, arg)
   972  			panic("go/printer type")
   973  		}
   974  		// data != ""
   975  
   976  		next := p.pos // estimated/accurate position of next item
   977  		wroteNewline, droppedFF := p.flush(next, p.lastTok)
   978  
   979  		// intersperse extra newlines if present in the source and
   980  		// if they don't cause extra semicolons (don't do this in
   981  		// flush as it will cause extra newlines at the end of a file)
   982  		if !p.impliedSemi {
   983  			n := nlimit(next.Line - p.pos.Line)
   984  			// don't exceed maxNewlines if we already wrote one
   985  			if wroteNewline && n == maxNewlines {
   986  				n = maxNewlines - 1
   987  			}
   988  			if n > 0 {
   989  				ch := byte('\n')
   990  				if droppedFF {
   991  					ch = '\f' // use formfeed since we dropped one before
   992  				}
   993  				p.writeByte(ch, n)
   994  				impliedSemi = false
   995  			}
   996  		}
   997  
   998  		// the next token starts now - record its line number if requested
   999  		if p.linePtr != nil {
  1000  			*p.linePtr = p.out.Line
  1001  			p.linePtr = nil
  1002  		}
  1003  
  1004  		p.writeString(next, data, isLit)
  1005  		p.impliedSemi = impliedSemi
  1006  	}
  1007  }
  1008  
  1009  // flush prints any pending comments and whitespace occurring textually
  1010  // before the position of the next token tok. The flush result indicates
  1011  // if a newline was written or if a formfeed was dropped from the whitespace
  1012  // buffer.
  1013  //
  1014  func (p *printer) flush(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
  1015  	if p.commentBefore(next) {
  1016  		// if there are comments before the next item, intersperse them
  1017  		wroteNewline, droppedFF = p.intersperseComments(next, tok)
  1018  	} else {
  1019  		// otherwise, write any leftover whitespace
  1020  		p.writeWhitespace(len(p.wsbuf))
  1021  	}
  1022  	return
  1023  }
  1024  
  1025  // getNode returns the ast.CommentGroup associated with n, if any.
  1026  func getDoc(n ast.Node) *ast.CommentGroup {
  1027  	switch n := n.(type) {
  1028  	case *ast.Field:
  1029  		return n.Doc
  1030  	case *ast.ImportSpec:
  1031  		return n.Doc
  1032  	case *ast.ValueSpec:
  1033  		return n.Doc
  1034  	case *ast.TypeSpec:
  1035  		return n.Doc
  1036  	case *ast.GenDecl:
  1037  		return n.Doc
  1038  	case *ast.FuncDecl:
  1039  		return n.Doc
  1040  	case *ast.File:
  1041  		return n.Doc
  1042  	}
  1043  	return nil
  1044  }
  1045  
  1046  func getLastComment(n ast.Node) *ast.CommentGroup {
  1047  	switch n := n.(type) {
  1048  	case *ast.Field:
  1049  		return n.Comment
  1050  	case *ast.ImportSpec:
  1051  		return n.Comment
  1052  	case *ast.ValueSpec:
  1053  		return n.Comment
  1054  	case *ast.TypeSpec:
  1055  		return n.Comment
  1056  	case *ast.GenDecl:
  1057  		if len(n.Specs) > 0 {
  1058  			return getLastComment(n.Specs[len(n.Specs)-1])
  1059  		}
  1060  	case *ast.File:
  1061  		if len(n.Comments) > 0 {
  1062  			return n.Comments[len(n.Comments)-1]
  1063  		}
  1064  	}
  1065  	return nil
  1066  }
  1067  
  1068  func (p *printer) printNode(node interface{}) error {
  1069  	// unpack *CommentedNode, if any
  1070  	var comments []*ast.CommentGroup
  1071  	if cnode, ok := node.(*CommentedNode); ok {
  1072  		node = cnode.Node
  1073  		comments = cnode.Comments
  1074  	}
  1075  
  1076  	if comments != nil {
  1077  		// commented node - restrict comment list to relevant range
  1078  		n, ok := node.(ast.Node)
  1079  		if !ok {
  1080  			goto unsupported
  1081  		}
  1082  		beg := n.Pos()
  1083  		end := n.End()
  1084  		// if the node has associated documentation,
  1085  		// include that commentgroup in the range
  1086  		// (the comment list is sorted in the order
  1087  		// of the comment appearance in the source code)
  1088  		if doc := getDoc(n); doc != nil {
  1089  			beg = doc.Pos()
  1090  		}
  1091  		if com := getLastComment(n); com != nil {
  1092  			if e := com.End(); e > end {
  1093  				end = e
  1094  			}
  1095  		}
  1096  		// token.Pos values are global offsets, we can
  1097  		// compare them directly
  1098  		i := 0
  1099  		for i < len(comments) && comments[i].End() < beg {
  1100  			i++
  1101  		}
  1102  		j := i
  1103  		for j < len(comments) && comments[j].Pos() < end {
  1104  			j++
  1105  		}
  1106  		if i < j {
  1107  			p.comments = comments[i:j]
  1108  		}
  1109  	} else if n, ok := node.(*ast.File); ok {
  1110  		// use ast.File comments, if any
  1111  		p.comments = n.Comments
  1112  	}
  1113  
  1114  	// if there are no comments, use node comments
  1115  	p.useNodeComments = p.comments == nil
  1116  
  1117  	// get comments ready for use
  1118  	p.nextComment()
  1119  
  1120  	// format node
  1121  	switch n := node.(type) {
  1122  	case ast.Expr:
  1123  		p.expr(n)
  1124  	case ast.Stmt:
  1125  		// A labeled statement will un-indent to position the label.
  1126  		// Set p.indent to 1 so we don't get indent "underflow".
  1127  		if _, ok := n.(*ast.LabeledStmt); ok {
  1128  			p.indent = 1
  1129  		}
  1130  		p.stmt(n, false)
  1131  	case ast.Decl:
  1132  		p.decl(n)
  1133  	case ast.Spec:
  1134  		p.spec(n, 1, false)
  1135  	case []ast.Stmt:
  1136  		// A labeled statement will un-indent to position the label.
  1137  		// Set p.indent to 1 so we don't get indent "underflow".
  1138  		for _, s := range n {
  1139  			if _, ok := s.(*ast.LabeledStmt); ok {
  1140  				p.indent = 1
  1141  			}
  1142  		}
  1143  		p.stmtList(n, 0, false)
  1144  	case []ast.Decl:
  1145  		p.declList(n)
  1146  	case *ast.File:
  1147  		p.file(n)
  1148  	default:
  1149  		goto unsupported
  1150  	}
  1151  
  1152  	return nil
  1153  
  1154  unsupported:
  1155  	return fmt.Errorf("go/printer: unsupported node type %T", node)
  1156  }
  1157  
  1158  // ----------------------------------------------------------------------------
  1159  // Trimmer
  1160  
  1161  // A trimmer is an io.Writer filter for stripping tabwriter.Escape
  1162  // characters, trailing blanks and tabs, and for converting formfeed
  1163  // and vtab characters into newlines and htabs (in case no tabwriter
  1164  // is used). Text bracketed by tabwriter.Escape characters is passed
  1165  // through unchanged.
  1166  //
  1167  type trimmer struct {
  1168  	output io.Writer
  1169  	state  int
  1170  	space  []byte
  1171  }
  1172  
  1173  // trimmer is implemented as a state machine.
  1174  // It can be in one of the following states:
  1175  const (
  1176  	inSpace  = iota // inside space
  1177  	inEscape        // inside text bracketed by tabwriter.Escapes
  1178  	inText          // inside text
  1179  )
  1180  
  1181  func (p *trimmer) resetSpace() {
  1182  	p.state = inSpace
  1183  	p.space = p.space[0:0]
  1184  }
  1185  
  1186  // Design note: It is tempting to eliminate extra blanks occurring in
  1187  //              whitespace in this function as it could simplify some
  1188  //              of the blanks logic in the node printing functions.
  1189  //              However, this would mess up any formatting done by
  1190  //              the tabwriter.
  1191  
  1192  var aNewline = []byte("\n")
  1193  
  1194  func (p *trimmer) Write(data []byte) (n int, err error) {
  1195  	// invariants:
  1196  	// p.state == inSpace:
  1197  	//	p.space is unwritten
  1198  	// p.state == inEscape, inText:
  1199  	//	data[m:n] is unwritten
  1200  	m := 0
  1201  	var b byte
  1202  	for n, b = range data {
  1203  		if b == '\v' {
  1204  			b = '\t' // convert to htab
  1205  		}
  1206  		switch p.state {
  1207  		case inSpace:
  1208  			switch b {
  1209  			case '\t', ' ':
  1210  				p.space = append(p.space, b)
  1211  			case '\n', '\f':
  1212  				p.resetSpace() // discard trailing space
  1213  				_, err = p.output.Write(aNewline)
  1214  			case tabwriter.Escape:
  1215  				_, err = p.output.Write(p.space)
  1216  				p.state = inEscape
  1217  				m = n + 1 // +1: skip tabwriter.Escape
  1218  			default:
  1219  				_, err = p.output.Write(p.space)
  1220  				p.state = inText
  1221  				m = n
  1222  			}
  1223  		case inEscape:
  1224  			if b == tabwriter.Escape {
  1225  				_, err = p.output.Write(data[m:n])
  1226  				p.resetSpace()
  1227  			}
  1228  		case inText:
  1229  			switch b {
  1230  			case '\t', ' ':
  1231  				_, err = p.output.Write(data[m:n])
  1232  				p.resetSpace()
  1233  				p.space = append(p.space, b)
  1234  			case '\n', '\f':
  1235  				_, err = p.output.Write(data[m:n])
  1236  				p.resetSpace()
  1237  				if err == nil {
  1238  					_, err = p.output.Write(aNewline)
  1239  				}
  1240  			case tabwriter.Escape:
  1241  				_, err = p.output.Write(data[m:n])
  1242  				p.state = inEscape
  1243  				m = n + 1 // +1: skip tabwriter.Escape
  1244  			}
  1245  		default:
  1246  			panic("unreachable")
  1247  		}
  1248  		if err != nil {
  1249  			return
  1250  		}
  1251  	}
  1252  	n = len(data)
  1253  
  1254  	switch p.state {
  1255  	case inEscape, inText:
  1256  		_, err = p.output.Write(data[m:n])
  1257  		p.resetSpace()
  1258  	}
  1259  
  1260  	return
  1261  }
  1262  
  1263  // ----------------------------------------------------------------------------
  1264  // Public interface
  1265  
  1266  // A Mode value is a set of flags (or 0). They control printing.
  1267  type Mode uint
  1268  
  1269  const (
  1270  	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
  1271  	TabIndent                  // use tabs for indentation independent of UseSpaces
  1272  	UseSpaces                  // use spaces instead of tabs for alignment
  1273  	SourcePos                  // emit //line directives to preserve original source positions
  1274  )
  1275  
  1276  // A Config node controls the output of Fprint.
  1277  type Config struct {
  1278  	Mode     Mode // default: 0
  1279  	Tabwidth int  // default: 8
  1280  	Indent   int  // default: 0 (all code is indented at least by this much)
  1281  }
  1282  
  1283  // fprint implements Fprint and takes a nodesSizes map for setting up the printer state.
  1284  func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{}, nodeSizes map[ast.Node]int) (err error) {
  1285  	// print node
  1286  	var p printer
  1287  	p.init(cfg, fset, nodeSizes)
  1288  	if err = p.printNode(node); err != nil {
  1289  		return
  1290  	}
  1291  	// print outstanding comments
  1292  	p.impliedSemi = false // EOF acts like a newline
  1293  	p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)
  1294  
  1295  	// redirect output through a trimmer to eliminate trailing whitespace
  1296  	// (Input to a tabwriter must be untrimmed since trailing tabs provide
  1297  	// formatting information. The tabwriter could provide trimming
  1298  	// functionality but no tabwriter is used when RawFormat is set.)
  1299  	output = &trimmer{output: output}
  1300  
  1301  	// redirect output through a tabwriter if necessary
  1302  	if cfg.Mode&RawFormat == 0 {
  1303  		minwidth := cfg.Tabwidth
  1304  
  1305  		padchar := byte('\t')
  1306  		if cfg.Mode&UseSpaces != 0 {
  1307  			padchar = ' '
  1308  		}
  1309  
  1310  		twmode := tabwriter.DiscardEmptyColumns
  1311  		if cfg.Mode&TabIndent != 0 {
  1312  			minwidth = 0
  1313  			twmode |= tabwriter.TabIndent
  1314  		}
  1315  
  1316  		output = tabwriter.NewWriter(output, minwidth, cfg.Tabwidth, 1, padchar, twmode)
  1317  	}
  1318  
  1319  	// write printer result via tabwriter/trimmer to output
  1320  	if _, err = output.Write(p.output); err != nil {
  1321  		return
  1322  	}
  1323  
  1324  	// flush tabwriter, if any
  1325  	if tw, _ := output.(*tabwriter.Writer); tw != nil {
  1326  		err = tw.Flush()
  1327  	}
  1328  
  1329  	return
  1330  }
  1331  
  1332  // A CommentedNode bundles an AST node and corresponding comments.
  1333  // It may be provided as argument to any of the Fprint functions.
  1334  //
  1335  type CommentedNode struct {
  1336  	Node     interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
  1337  	Comments []*ast.CommentGroup
  1338  }
  1339  
  1340  // Fprint "pretty-prints" an AST node to output for a given configuration cfg.
  1341  // Position information is interpreted relative to the file set fset.
  1342  // The node type must be *ast.File, *CommentedNode, []ast.Decl, []ast.Stmt,
  1343  // or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.
  1344  //
  1345  func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
  1346  	return cfg.fprint(output, fset, node, make(map[ast.Node]int))
  1347  }
  1348  
  1349  // Fprint "pretty-prints" an AST node to output.
  1350  // It calls Config.Fprint with default settings.
  1351  // Note that gofmt uses tabs for indentation but spaces for alignment;
  1352  // use format.Node (package go/format) for output that matches gofmt.
  1353  //
  1354  func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
  1355  	return (&Config{Tabwidth: 8}).Fprint(output, fset, node)
  1356  }
  1357  

View as plain text