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Source file src/text/template/parse/lex.go

Documentation: text/template/parse

     1  // Copyright 2011 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 parse
     6  
     7  import (
     8  	"fmt"
     9  	"strings"
    10  	"unicode"
    11  	"unicode/utf8"
    12  )
    13  
    14  // item represents a token or text string returned from the scanner.
    15  type item struct {
    16  	typ  itemType // The type of this item.
    17  	pos  Pos      // The starting position, in bytes, of this item in the input string.
    18  	val  string   // The value of this item.
    19  	line int      // The line number at the start of this item.
    20  }
    21  
    22  func (i item) String() string {
    23  	switch {
    24  	case i.typ == itemEOF:
    25  		return "EOF"
    26  	case i.typ == itemError:
    27  		return i.val
    28  	case i.typ > itemKeyword:
    29  		return fmt.Sprintf("<%s>", i.val)
    30  	case len(i.val) > 10:
    31  		return fmt.Sprintf("%.10q...", i.val)
    32  	}
    33  	return fmt.Sprintf("%q", i.val)
    34  }
    35  
    36  // itemType identifies the type of lex items.
    37  type itemType int
    38  
    39  const (
    40  	itemError        itemType = iota // error occurred; value is text of error
    41  	itemBool                         // boolean constant
    42  	itemChar                         // printable ASCII character; grab bag for comma etc.
    43  	itemCharConstant                 // character constant
    44  	itemComplex                      // complex constant (1+2i); imaginary is just a number
    45  	itemColonEquals                  // colon-equals (':=') introducing a declaration
    46  	itemEOF
    47  	itemField      // alphanumeric identifier starting with '.'
    48  	itemIdentifier // alphanumeric identifier not starting with '.'
    49  	itemLeftDelim  // left action delimiter
    50  	itemLeftParen  // '(' inside action
    51  	itemNumber     // simple number, including imaginary
    52  	itemPipe       // pipe symbol
    53  	itemRawString  // raw quoted string (includes quotes)
    54  	itemRightDelim // right action delimiter
    55  	itemRightParen // ')' inside action
    56  	itemSpace      // run of spaces separating arguments
    57  	itemString     // quoted string (includes quotes)
    58  	itemText       // plain text
    59  	itemVariable   // variable starting with '$', such as '$' or  '$1' or '$hello'
    60  	// Keywords appear after all the rest.
    61  	itemKeyword  // used only to delimit the keywords
    62  	itemBlock    // block keyword
    63  	itemDot      // the cursor, spelled '.'
    64  	itemDefine   // define keyword
    65  	itemElse     // else keyword
    66  	itemEnd      // end keyword
    67  	itemIf       // if keyword
    68  	itemNil      // the untyped nil constant, easiest to treat as a keyword
    69  	itemRange    // range keyword
    70  	itemTemplate // template keyword
    71  	itemWith     // with keyword
    72  )
    73  
    74  var key = map[string]itemType{
    75  	".":        itemDot,
    76  	"block":    itemBlock,
    77  	"define":   itemDefine,
    78  	"else":     itemElse,
    79  	"end":      itemEnd,
    80  	"if":       itemIf,
    81  	"range":    itemRange,
    82  	"nil":      itemNil,
    83  	"template": itemTemplate,
    84  	"with":     itemWith,
    85  }
    86  
    87  const eof = -1
    88  
    89  // Trimming spaces.
    90  // If the action begins "{{- " rather than "{{", then all space/tab/newlines
    91  // preceding the action are trimmed; conversely if it ends " -}}" the
    92  // leading spaces are trimmed. This is done entirely in the lexer; the
    93  // parser never sees it happen. We require an ASCII space to be
    94  // present to avoid ambiguity with things like "{{-3}}". It reads
    95  // better with the space present anyway. For simplicity, only ASCII
    96  // space does the job.
    97  const (
    98  	spaceChars      = " \t\r\n" // These are the space characters defined by Go itself.
    99  	leftTrimMarker  = "- "      // Attached to left delimiter, trims trailing spaces from preceding text.
   100  	rightTrimMarker = " -"      // Attached to right delimiter, trims leading spaces from following text.
   101  	trimMarkerLen   = Pos(len(leftTrimMarker))
   102  )
   103  
   104  // stateFn represents the state of the scanner as a function that returns the next state.
   105  type stateFn func(*lexer) stateFn
   106  
   107  // lexer holds the state of the scanner.
   108  type lexer struct {
   109  	name       string    // the name of the input; used only for error reports
   110  	input      string    // the string being scanned
   111  	leftDelim  string    // start of action
   112  	rightDelim string    // end of action
   113  	pos        Pos       // current position in the input
   114  	start      Pos       // start position of this item
   115  	width      Pos       // width of last rune read from input
   116  	items      chan item // channel of scanned items
   117  	parenDepth int       // nesting depth of ( ) exprs
   118  	line       int       // 1+number of newlines seen
   119  }
   120  
   121  // next returns the next rune in the input.
   122  func (l *lexer) next() rune {
   123  	if int(l.pos) >= len(l.input) {
   124  		l.width = 0
   125  		return eof
   126  	}
   127  	r, w := utf8.DecodeRuneInString(l.input[l.pos:])
   128  	l.width = Pos(w)
   129  	l.pos += l.width
   130  	if r == '\n' {
   131  		l.line++
   132  	}
   133  	return r
   134  }
   135  
   136  // peek returns but does not consume the next rune in the input.
   137  func (l *lexer) peek() rune {
   138  	r := l.next()
   139  	l.backup()
   140  	return r
   141  }
   142  
   143  // backup steps back one rune. Can only be called once per call of next.
   144  func (l *lexer) backup() {
   145  	l.pos -= l.width
   146  	// Correct newline count.
   147  	if l.width == 1 && l.input[l.pos] == '\n' {
   148  		l.line--
   149  	}
   150  }
   151  
   152  // emit passes an item back to the client.
   153  func (l *lexer) emit(t itemType) {
   154  	l.items <- item{t, l.start, l.input[l.start:l.pos], l.line}
   155  	// Some items contain text internally. If so, count their newlines.
   156  	switch t {
   157  	case itemText, itemRawString, itemLeftDelim, itemRightDelim:
   158  		l.line += strings.Count(l.input[l.start:l.pos], "\n")
   159  	}
   160  	l.start = l.pos
   161  }
   162  
   163  // ignore skips over the pending input before this point.
   164  func (l *lexer) ignore() {
   165  	l.line += strings.Count(l.input[l.start:l.pos], "\n")
   166  	l.start = l.pos
   167  }
   168  
   169  // accept consumes the next rune if it's from the valid set.
   170  func (l *lexer) accept(valid string) bool {
   171  	if strings.ContainsRune(valid, l.next()) {
   172  		return true
   173  	}
   174  	l.backup()
   175  	return false
   176  }
   177  
   178  // acceptRun consumes a run of runes from the valid set.
   179  func (l *lexer) acceptRun(valid string) {
   180  	for strings.ContainsRune(valid, l.next()) {
   181  	}
   182  	l.backup()
   183  }
   184  
   185  // errorf returns an error token and terminates the scan by passing
   186  // back a nil pointer that will be the next state, terminating l.nextItem.
   187  func (l *lexer) errorf(format string, args ...interface{}) stateFn {
   188  	l.items <- item{itemError, l.start, fmt.Sprintf(format, args...), l.line}
   189  	return nil
   190  }
   191  
   192  // nextItem returns the next item from the input.
   193  // Called by the parser, not in the lexing goroutine.
   194  func (l *lexer) nextItem() item {
   195  	return <-l.items
   196  }
   197  
   198  // drain drains the output so the lexing goroutine will exit.
   199  // Called by the parser, not in the lexing goroutine.
   200  func (l *lexer) drain() {
   201  	for range l.items {
   202  	}
   203  }
   204  
   205  // lex creates a new scanner for the input string.
   206  func lex(name, input, left, right string) *lexer {
   207  	if left == "" {
   208  		left = leftDelim
   209  	}
   210  	if right == "" {
   211  		right = rightDelim
   212  	}
   213  	l := &lexer{
   214  		name:       name,
   215  		input:      input,
   216  		leftDelim:  left,
   217  		rightDelim: right,
   218  		items:      make(chan item),
   219  		line:       1,
   220  	}
   221  	go l.run()
   222  	return l
   223  }
   224  
   225  // run runs the state machine for the lexer.
   226  func (l *lexer) run() {
   227  	for state := lexText; state != nil; {
   228  		state = state(l)
   229  	}
   230  	close(l.items)
   231  }
   232  
   233  // state functions
   234  
   235  const (
   236  	leftDelim    = "{{"
   237  	rightDelim   = "}}"
   238  	leftComment  = "/*"
   239  	rightComment = "*/"
   240  )
   241  
   242  // lexText scans until an opening action delimiter, "{{".
   243  func lexText(l *lexer) stateFn {
   244  	l.width = 0
   245  	if x := strings.Index(l.input[l.pos:], l.leftDelim); x >= 0 {
   246  		ldn := Pos(len(l.leftDelim))
   247  		l.pos += Pos(x)
   248  		trimLength := Pos(0)
   249  		if strings.HasPrefix(l.input[l.pos+ldn:], leftTrimMarker) {
   250  			trimLength = rightTrimLength(l.input[l.start:l.pos])
   251  		}
   252  		l.pos -= trimLength
   253  		if l.pos > l.start {
   254  			l.emit(itemText)
   255  		}
   256  		l.pos += trimLength
   257  		l.ignore()
   258  		return lexLeftDelim
   259  	} else {
   260  		l.pos = Pos(len(l.input))
   261  	}
   262  	// Correctly reached EOF.
   263  	if l.pos > l.start {
   264  		l.emit(itemText)
   265  	}
   266  	l.emit(itemEOF)
   267  	return nil
   268  }
   269  
   270  // rightTrimLength returns the length of the spaces at the end of the string.
   271  func rightTrimLength(s string) Pos {
   272  	return Pos(len(s) - len(strings.TrimRight(s, spaceChars)))
   273  }
   274  
   275  // atRightDelim reports whether the lexer is at a right delimiter, possibly preceded by a trim marker.
   276  func (l *lexer) atRightDelim() (delim, trimSpaces bool) {
   277  	if strings.HasPrefix(l.input[l.pos:], l.rightDelim) {
   278  		return true, false
   279  	}
   280  	// The right delim might have the marker before.
   281  	if strings.HasPrefix(l.input[l.pos:], rightTrimMarker) &&
   282  		strings.HasPrefix(l.input[l.pos+trimMarkerLen:], l.rightDelim) {
   283  		return true, true
   284  	}
   285  	return false, false
   286  }
   287  
   288  // leftTrimLength returns the length of the spaces at the beginning of the string.
   289  func leftTrimLength(s string) Pos {
   290  	return Pos(len(s) - len(strings.TrimLeft(s, spaceChars)))
   291  }
   292  
   293  // lexLeftDelim scans the left delimiter, which is known to be present, possibly with a trim marker.
   294  func lexLeftDelim(l *lexer) stateFn {
   295  	l.pos += Pos(len(l.leftDelim))
   296  	trimSpace := strings.HasPrefix(l.input[l.pos:], leftTrimMarker)
   297  	afterMarker := Pos(0)
   298  	if trimSpace {
   299  		afterMarker = trimMarkerLen
   300  	}
   301  	if strings.HasPrefix(l.input[l.pos+afterMarker:], leftComment) {
   302  		l.pos += afterMarker
   303  		l.ignore()
   304  		return lexComment
   305  	}
   306  	l.emit(itemLeftDelim)
   307  	l.pos += afterMarker
   308  	l.ignore()
   309  	l.parenDepth = 0
   310  	return lexInsideAction
   311  }
   312  
   313  // lexComment scans a comment. The left comment marker is known to be present.
   314  func lexComment(l *lexer) stateFn {
   315  	l.pos += Pos(len(leftComment))
   316  	i := strings.Index(l.input[l.pos:], rightComment)
   317  	if i < 0 {
   318  		return l.errorf("unclosed comment")
   319  	}
   320  	l.pos += Pos(i + len(rightComment))
   321  	delim, trimSpace := l.atRightDelim()
   322  	if !delim {
   323  		return l.errorf("comment ends before closing delimiter")
   324  	}
   325  	if trimSpace {
   326  		l.pos += trimMarkerLen
   327  	}
   328  	l.pos += Pos(len(l.rightDelim))
   329  	if trimSpace {
   330  		l.pos += leftTrimLength(l.input[l.pos:])
   331  	}
   332  	l.ignore()
   333  	return lexText
   334  }
   335  
   336  // lexRightDelim scans the right delimiter, which is known to be present, possibly with a trim marker.
   337  func lexRightDelim(l *lexer) stateFn {
   338  	trimSpace := strings.HasPrefix(l.input[l.pos:], rightTrimMarker)
   339  	if trimSpace {
   340  		l.pos += trimMarkerLen
   341  		l.ignore()
   342  	}
   343  	l.pos += Pos(len(l.rightDelim))
   344  	l.emit(itemRightDelim)
   345  	if trimSpace {
   346  		l.pos += leftTrimLength(l.input[l.pos:])
   347  		l.ignore()
   348  	}
   349  	return lexText
   350  }
   351  
   352  // lexInsideAction scans the elements inside action delimiters.
   353  func lexInsideAction(l *lexer) stateFn {
   354  	// Either number, quoted string, or identifier.
   355  	// Spaces separate arguments; runs of spaces turn into itemSpace.
   356  	// Pipe symbols separate and are emitted.
   357  	delim, _ := l.atRightDelim()
   358  	if delim {
   359  		if l.parenDepth == 0 {
   360  			return lexRightDelim
   361  		}
   362  		return l.errorf("unclosed left paren")
   363  	}
   364  	switch r := l.next(); {
   365  	case r == eof || isEndOfLine(r):
   366  		return l.errorf("unclosed action")
   367  	case isSpace(r):
   368  		return lexSpace
   369  	case r == ':':
   370  		if l.next() != '=' {
   371  			return l.errorf("expected :=")
   372  		}
   373  		l.emit(itemColonEquals)
   374  	case r == '|':
   375  		l.emit(itemPipe)
   376  	case r == '"':
   377  		return lexQuote
   378  	case r == '`':
   379  		return lexRawQuote
   380  	case r == '$':
   381  		return lexVariable
   382  	case r == '\'':
   383  		return lexChar
   384  	case r == '.':
   385  		// special look-ahead for ".field" so we don't break l.backup().
   386  		if l.pos < Pos(len(l.input)) {
   387  			r := l.input[l.pos]
   388  			if r < '0' || '9' < r {
   389  				return lexField
   390  			}
   391  		}
   392  		fallthrough // '.' can start a number.
   393  	case r == '+' || r == '-' || ('0' <= r && r <= '9'):
   394  		l.backup()
   395  		return lexNumber
   396  	case isAlphaNumeric(r):
   397  		l.backup()
   398  		return lexIdentifier
   399  	case r == '(':
   400  		l.emit(itemLeftParen)
   401  		l.parenDepth++
   402  	case r == ')':
   403  		l.emit(itemRightParen)
   404  		l.parenDepth--
   405  		if l.parenDepth < 0 {
   406  			return l.errorf("unexpected right paren %#U", r)
   407  		}
   408  	case r <= unicode.MaxASCII && unicode.IsPrint(r):
   409  		l.emit(itemChar)
   410  		return lexInsideAction
   411  	default:
   412  		return l.errorf("unrecognized character in action: %#U", r)
   413  	}
   414  	return lexInsideAction
   415  }
   416  
   417  // lexSpace scans a run of space characters.
   418  // One space has already been seen.
   419  func lexSpace(l *lexer) stateFn {
   420  	for isSpace(l.peek()) {
   421  		l.next()
   422  	}
   423  	l.emit(itemSpace)
   424  	return lexInsideAction
   425  }
   426  
   427  // lexIdentifier scans an alphanumeric.
   428  func lexIdentifier(l *lexer) stateFn {
   429  Loop:
   430  	for {
   431  		switch r := l.next(); {
   432  		case isAlphaNumeric(r):
   433  			// absorb.
   434  		default:
   435  			l.backup()
   436  			word := l.input[l.start:l.pos]
   437  			if !l.atTerminator() {
   438  				return l.errorf("bad character %#U", r)
   439  			}
   440  			switch {
   441  			case key[word] > itemKeyword:
   442  				l.emit(key[word])
   443  			case word[0] == '.':
   444  				l.emit(itemField)
   445  			case word == "true", word == "false":
   446  				l.emit(itemBool)
   447  			default:
   448  				l.emit(itemIdentifier)
   449  			}
   450  			break Loop
   451  		}
   452  	}
   453  	return lexInsideAction
   454  }
   455  
   456  // lexField scans a field: .Alphanumeric.
   457  // The . has been scanned.
   458  func lexField(l *lexer) stateFn {
   459  	return lexFieldOrVariable(l, itemField)
   460  }
   461  
   462  // lexVariable scans a Variable: $Alphanumeric.
   463  // The $ has been scanned.
   464  func lexVariable(l *lexer) stateFn {
   465  	if l.atTerminator() { // Nothing interesting follows -> "$".
   466  		l.emit(itemVariable)
   467  		return lexInsideAction
   468  	}
   469  	return lexFieldOrVariable(l, itemVariable)
   470  }
   471  
   472  // lexVariable scans a field or variable: [.$]Alphanumeric.
   473  // The . or $ has been scanned.
   474  func lexFieldOrVariable(l *lexer, typ itemType) stateFn {
   475  	if l.atTerminator() { // Nothing interesting follows -> "." or "$".
   476  		if typ == itemVariable {
   477  			l.emit(itemVariable)
   478  		} else {
   479  			l.emit(itemDot)
   480  		}
   481  		return lexInsideAction
   482  	}
   483  	var r rune
   484  	for {
   485  		r = l.next()
   486  		if !isAlphaNumeric(r) {
   487  			l.backup()
   488  			break
   489  		}
   490  	}
   491  	if !l.atTerminator() {
   492  		return l.errorf("bad character %#U", r)
   493  	}
   494  	l.emit(typ)
   495  	return lexInsideAction
   496  }
   497  
   498  // atTerminator reports whether the input is at valid termination character to
   499  // appear after an identifier. Breaks .X.Y into two pieces. Also catches cases
   500  // like "$x+2" not being acceptable without a space, in case we decide one
   501  // day to implement arithmetic.
   502  func (l *lexer) atTerminator() bool {
   503  	r := l.peek()
   504  	if isSpace(r) || isEndOfLine(r) {
   505  		return true
   506  	}
   507  	switch r {
   508  	case eof, '.', ',', '|', ':', ')', '(':
   509  		return true
   510  	}
   511  	// Does r start the delimiter? This can be ambiguous (with delim=="//", $x/2 will
   512  	// succeed but should fail) but only in extremely rare cases caused by willfully
   513  	// bad choice of delimiter.
   514  	if rd, _ := utf8.DecodeRuneInString(l.rightDelim); rd == r {
   515  		return true
   516  	}
   517  	return false
   518  }
   519  
   520  // lexChar scans a character constant. The initial quote is already
   521  // scanned. Syntax checking is done by the parser.
   522  func lexChar(l *lexer) stateFn {
   523  Loop:
   524  	for {
   525  		switch l.next() {
   526  		case '\\':
   527  			if r := l.next(); r != eof && r != '\n' {
   528  				break
   529  			}
   530  			fallthrough
   531  		case eof, '\n':
   532  			return l.errorf("unterminated character constant")
   533  		case '\'':
   534  			break Loop
   535  		}
   536  	}
   537  	l.emit(itemCharConstant)
   538  	return lexInsideAction
   539  }
   540  
   541  // lexNumber scans a number: decimal, octal, hex, float, or imaginary. This
   542  // isn't a perfect number scanner - for instance it accepts "." and "0x0.2"
   543  // and "089" - but when it's wrong the input is invalid and the parser (via
   544  // strconv) will notice.
   545  func lexNumber(l *lexer) stateFn {
   546  	if !l.scanNumber() {
   547  		return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
   548  	}
   549  	if sign := l.peek(); sign == '+' || sign == '-' {
   550  		// Complex: 1+2i. No spaces, must end in 'i'.
   551  		if !l.scanNumber() || l.input[l.pos-1] != 'i' {
   552  			return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
   553  		}
   554  		l.emit(itemComplex)
   555  	} else {
   556  		l.emit(itemNumber)
   557  	}
   558  	return lexInsideAction
   559  }
   560  
   561  func (l *lexer) scanNumber() bool {
   562  	// Optional leading sign.
   563  	l.accept("+-")
   564  	// Is it hex?
   565  	digits := "0123456789"
   566  	if l.accept("0") && l.accept("xX") {
   567  		digits = "0123456789abcdefABCDEF"
   568  	}
   569  	l.acceptRun(digits)
   570  	if l.accept(".") {
   571  		l.acceptRun(digits)
   572  	}
   573  	if l.accept("eE") {
   574  		l.accept("+-")
   575  		l.acceptRun("0123456789")
   576  	}
   577  	// Is it imaginary?
   578  	l.accept("i")
   579  	// Next thing mustn't be alphanumeric.
   580  	if isAlphaNumeric(l.peek()) {
   581  		l.next()
   582  		return false
   583  	}
   584  	return true
   585  }
   586  
   587  // lexQuote scans a quoted string.
   588  func lexQuote(l *lexer) stateFn {
   589  Loop:
   590  	for {
   591  		switch l.next() {
   592  		case '\\':
   593  			if r := l.next(); r != eof && r != '\n' {
   594  				break
   595  			}
   596  			fallthrough
   597  		case eof, '\n':
   598  			return l.errorf("unterminated quoted string")
   599  		case '"':
   600  			break Loop
   601  		}
   602  	}
   603  	l.emit(itemString)
   604  	return lexInsideAction
   605  }
   606  
   607  // lexRawQuote scans a raw quoted string.
   608  func lexRawQuote(l *lexer) stateFn {
   609  	startLine := l.line
   610  Loop:
   611  	for {
   612  		switch l.next() {
   613  		case eof:
   614  			// Restore line number to location of opening quote.
   615  			// We will error out so it's ok just to overwrite the field.
   616  			l.line = startLine
   617  			return l.errorf("unterminated raw quoted string")
   618  		case '`':
   619  			break Loop
   620  		}
   621  	}
   622  	l.emit(itemRawString)
   623  	return lexInsideAction
   624  }
   625  
   626  // isSpace reports whether r is a space character.
   627  func isSpace(r rune) bool {
   628  	return r == ' ' || r == '\t'
   629  }
   630  
   631  // isEndOfLine reports whether r is an end-of-line character.
   632  func isEndOfLine(r rune) bool {
   633  	return r == '\r' || r == '\n'
   634  }
   635  
   636  // isAlphaNumeric reports whether r is an alphabetic, digit, or underscore.
   637  func isAlphaNumeric(r rune) bool {
   638  	return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
   639  }
   640  

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