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  	itemAssign                       // equals ('=') introducing an assignment
    46  	itemDeclare                      // colon-equals (':=') introducing a declaration
    47  	itemEOF
    48  	itemField      // alphanumeric identifier starting with '.'
    49  	itemIdentifier // alphanumeric identifier not starting with '.'
    50  	itemLeftDelim  // left action delimiter
    51  	itemLeftParen  // '(' inside action
    52  	itemNumber     // simple number, including imaginary
    53  	itemPipe       // pipe symbol
    54  	itemRawString  // raw quoted string (includes quotes)
    55  	itemRightDelim // right action delimiter
    56  	itemRightParen // ')' inside action
    57  	itemSpace      // run of spaces separating arguments
    58  	itemString     // quoted string (includes quotes)
    59  	itemText       // plain text
    60  	itemVariable   // variable starting with '$', such as '$' or  '$1' or '$hello'
    61  	// Keywords appear after all the rest.
    62  	itemKeyword  // used only to delimit the keywords
    63  	itemBlock    // block keyword
    64  	itemDot      // the cursor, spelled '.'
    65  	itemDefine   // define keyword
    66  	itemElse     // else keyword
    67  	itemEnd      // end keyword
    68  	itemIf       // if keyword
    69  	itemNil      // the untyped nil constant, easiest to treat as a keyword
    70  	itemRange    // range keyword
    71  	itemTemplate // template keyword
    72  	itemWith     // with keyword
    73  )
    74  
    75  var key = map[string]itemType{
    76  	".":        itemDot,
    77  	"block":    itemBlock,
    78  	"define":   itemDefine,
    79  	"else":     itemElse,
    80  	"end":      itemEnd,
    81  	"if":       itemIf,
    82  	"range":    itemRange,
    83  	"nil":      itemNil,
    84  	"template": itemTemplate,
    85  	"with":     itemWith,
    86  }
    87  
    88  const eof = -1
    89  
    90  // Trimming spaces.
    91  // If the action begins "{{- " rather than "{{", then all space/tab/newlines
    92  // preceding the action are trimmed; conversely if it ends " -}}" the
    93  // leading spaces are trimmed. This is done entirely in the lexer; the
    94  // parser never sees it happen. We require an ASCII space to be
    95  // present to avoid ambiguity with things like "{{-3}}". It reads
    96  // better with the space present anyway. For simplicity, only ASCII
    97  // space does the job.
    98  const (
    99  	spaceChars      = " \t\r\n" // These are the space characters defined by Go itself.
   100  	leftTrimMarker  = "- "      // Attached to left delimiter, trims trailing spaces from preceding text.
   101  	rightTrimMarker = " -"      // Attached to right delimiter, trims leading spaces from following text.
   102  	trimMarkerLen   = Pos(len(leftTrimMarker))
   103  )
   104  
   105  // stateFn represents the state of the scanner as a function that returns the next state.
   106  type stateFn func(*lexer) stateFn
   107  
   108  // lexer holds the state of the scanner.
   109  type lexer struct {
   110  	name           string    // the name of the input; used only for error reports
   111  	input          string    // the string being scanned
   112  	leftDelim      string    // start of action
   113  	rightDelim     string    // end of action
   114  	trimRightDelim string    // end of action with trim marker
   115  	pos            Pos       // current position in the input
   116  	start          Pos       // start position of this item
   117  	width          Pos       // width of last rune read from input
   118  	items          chan item // channel of scanned items
   119  	parenDepth     int       // nesting depth of ( ) exprs
   120  	line           int       // 1+number of newlines seen
   121  	startLine      int       // start line of this item
   122  }
   123  
   124  // next returns the next rune in the input.
   125  func (l *lexer) next() rune {
   126  	if int(l.pos) >= len(l.input) {
   127  		l.width = 0
   128  		return eof
   129  	}
   130  	r, w := utf8.DecodeRuneInString(l.input[l.pos:])
   131  	l.width = Pos(w)
   132  	l.pos += l.width
   133  	if r == '\n' {
   134  		l.line++
   135  	}
   136  	return r
   137  }
   138  
   139  // peek returns but does not consume the next rune in the input.
   140  func (l *lexer) peek() rune {
   141  	r := l.next()
   142  	l.backup()
   143  	return r
   144  }
   145  
   146  // backup steps back one rune. Can only be called once per call of next.
   147  func (l *lexer) backup() {
   148  	l.pos -= l.width
   149  	// Correct newline count.
   150  	if l.width == 1 && l.input[l.pos] == '\n' {
   151  		l.line--
   152  	}
   153  }
   154  
   155  // emit passes an item back to the client.
   156  func (l *lexer) emit(t itemType) {
   157  	l.items <- item{t, l.start, l.input[l.start:l.pos], l.startLine}
   158  	l.start = l.pos
   159  	l.startLine = l.line
   160  }
   161  
   162  // ignore skips over the pending input before this point.
   163  func (l *lexer) ignore() {
   164  	l.line += strings.Count(l.input[l.start:l.pos], "\n")
   165  	l.start = l.pos
   166  	l.startLine = l.line
   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.startLine}
   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  		trimRightDelim: rightTrimMarker + right,
   219  		items:          make(chan item),
   220  		line:           1,
   221  		startLine:      1,
   222  	}
   223  	go l.run()
   224  	return l
   225  }
   226  
   227  // run runs the state machine for the lexer.
   228  func (l *lexer) run() {
   229  	for state := lexText; state != nil; {
   230  		state = state(l)
   231  	}
   232  	close(l.items)
   233  }
   234  
   235  // state functions
   236  
   237  const (
   238  	leftDelim    = "{{"
   239  	rightDelim   = "}}"
   240  	leftComment  = "/*"
   241  	rightComment = "*/"
   242  )
   243  
   244  // lexText scans until an opening action delimiter, "{{".
   245  func lexText(l *lexer) stateFn {
   246  	l.width = 0
   247  	if x := strings.Index(l.input[l.pos:], l.leftDelim); x >= 0 {
   248  		ldn := Pos(len(l.leftDelim))
   249  		l.pos += Pos(x)
   250  		trimLength := Pos(0)
   251  		if strings.HasPrefix(l.input[l.pos+ldn:], leftTrimMarker) {
   252  			trimLength = rightTrimLength(l.input[l.start:l.pos])
   253  		}
   254  		l.pos -= trimLength
   255  		if l.pos > l.start {
   256  			l.line += strings.Count(l.input[l.start:l.pos], "\n")
   257  			l.emit(itemText)
   258  		}
   259  		l.pos += trimLength
   260  		l.ignore()
   261  		return lexLeftDelim
   262  	}
   263  	l.pos = Pos(len(l.input))
   264  	// Correctly reached EOF.
   265  	if l.pos > l.start {
   266  		l.line += strings.Count(l.input[l.start:l.pos], "\n")
   267  		l.emit(itemText)
   268  	}
   269  	l.emit(itemEOF)
   270  	return nil
   271  }
   272  
   273  // rightTrimLength returns the length of the spaces at the end of the string.
   274  func rightTrimLength(s string) Pos {
   275  	return Pos(len(s) - len(strings.TrimRight(s, spaceChars)))
   276  }
   277  
   278  // atRightDelim reports whether the lexer is at a right delimiter, possibly preceded by a trim marker.
   279  func (l *lexer) atRightDelim() (delim, trimSpaces bool) {
   280  	if strings.HasPrefix(l.input[l.pos:], l.trimRightDelim) { // With trim marker.
   281  		return true, true
   282  	}
   283  	if strings.HasPrefix(l.input[l.pos:], l.rightDelim) { // Without trim marker.
   284  		return true, false
   285  	}
   286  	return false, false
   287  }
   288  
   289  // leftTrimLength returns the length of the spaces at the beginning of the string.
   290  func leftTrimLength(s string) Pos {
   291  	return Pos(len(s) - len(strings.TrimLeft(s, spaceChars)))
   292  }
   293  
   294  // lexLeftDelim scans the left delimiter, which is known to be present, possibly with a trim marker.
   295  func lexLeftDelim(l *lexer) stateFn {
   296  	l.pos += Pos(len(l.leftDelim))
   297  	trimSpace := strings.HasPrefix(l.input[l.pos:], leftTrimMarker)
   298  	afterMarker := Pos(0)
   299  	if trimSpace {
   300  		afterMarker = trimMarkerLen
   301  	}
   302  	if strings.HasPrefix(l.input[l.pos+afterMarker:], leftComment) {
   303  		l.pos += afterMarker
   304  		l.ignore()
   305  		return lexComment
   306  	}
   307  	l.emit(itemLeftDelim)
   308  	l.pos += afterMarker
   309  	l.ignore()
   310  	l.parenDepth = 0
   311  	return lexInsideAction
   312  }
   313  
   314  // lexComment scans a comment. The left comment marker is known to be present.
   315  func lexComment(l *lexer) stateFn {
   316  	l.pos += Pos(len(leftComment))
   317  	i := strings.Index(l.input[l.pos:], rightComment)
   318  	if i < 0 {
   319  		return l.errorf("unclosed comment")
   320  	}
   321  	l.pos += Pos(i + len(rightComment))
   322  	delim, trimSpace := l.atRightDelim()
   323  	if !delim {
   324  		return l.errorf("comment ends before closing delimiter")
   325  	}
   326  	if trimSpace {
   327  		l.pos += trimMarkerLen
   328  	}
   329  	l.pos += Pos(len(l.rightDelim))
   330  	if trimSpace {
   331  		l.pos += leftTrimLength(l.input[l.pos:])
   332  	}
   333  	l.ignore()
   334  	return lexText
   335  }
   336  
   337  // lexRightDelim scans the right delimiter, which is known to be present, possibly with a trim marker.
   338  func lexRightDelim(l *lexer) stateFn {
   339  	trimSpace := strings.HasPrefix(l.input[l.pos:], rightTrimMarker)
   340  	if trimSpace {
   341  		l.pos += trimMarkerLen
   342  		l.ignore()
   343  	}
   344  	l.pos += Pos(len(l.rightDelim))
   345  	l.emit(itemRightDelim)
   346  	if trimSpace {
   347  		l.pos += leftTrimLength(l.input[l.pos:])
   348  		l.ignore()
   349  	}
   350  	return lexText
   351  }
   352  
   353  // lexInsideAction scans the elements inside action delimiters.
   354  func lexInsideAction(l *lexer) stateFn {
   355  	// Either number, quoted string, or identifier.
   356  	// Spaces separate arguments; runs of spaces turn into itemSpace.
   357  	// Pipe symbols separate and are emitted.
   358  	delim, _ := l.atRightDelim()
   359  	if delim {
   360  		if l.parenDepth == 0 {
   361  			return lexRightDelim
   362  		}
   363  		return l.errorf("unclosed left paren")
   364  	}
   365  	switch r := l.next(); {
   366  	case r == eof || isEndOfLine(r):
   367  		return l.errorf("unclosed action")
   368  	case isSpace(r):
   369  		l.backup() // Put space back in case we have " -}}".
   370  		return lexSpace
   371  	case r == '=':
   372  		l.emit(itemAssign)
   373  	case r == ':':
   374  		if l.next() != '=' {
   375  			return l.errorf("expected :=")
   376  		}
   377  		l.emit(itemDeclare)
   378  	case r == '|':
   379  		l.emit(itemPipe)
   380  	case r == '"':
   381  		return lexQuote
   382  	case r == '`':
   383  		return lexRawQuote
   384  	case r == '$':
   385  		return lexVariable
   386  	case r == '\'':
   387  		return lexChar
   388  	case r == '.':
   389  		// special look-ahead for ".field" so we don't break l.backup().
   390  		if l.pos < Pos(len(l.input)) {
   391  			r := l.input[l.pos]
   392  			if r < '0' || '9' < r {
   393  				return lexField
   394  			}
   395  		}
   396  		fallthrough // '.' can start a number.
   397  	case r == '+' || r == '-' || ('0' <= r && r <= '9'):
   398  		l.backup()
   399  		return lexNumber
   400  	case isAlphaNumeric(r):
   401  		l.backup()
   402  		return lexIdentifier
   403  	case r == '(':
   404  		l.emit(itemLeftParen)
   405  		l.parenDepth++
   406  	case r == ')':
   407  		l.emit(itemRightParen)
   408  		l.parenDepth--
   409  		if l.parenDepth < 0 {
   410  			return l.errorf("unexpected right paren %#U", r)
   411  		}
   412  	case r <= unicode.MaxASCII && unicode.IsPrint(r):
   413  		l.emit(itemChar)
   414  	default:
   415  		return l.errorf("unrecognized character in action: %#U", r)
   416  	}
   417  	return lexInsideAction
   418  }
   419  
   420  // lexSpace scans a run of space characters.
   421  // We have not consumed the first space, which is known to be present.
   422  // Take care if there is a trim-marked right delimiter, which starts with a space.
   423  func lexSpace(l *lexer) stateFn {
   424  	var r rune
   425  	var numSpaces int
   426  	for {
   427  		r = l.peek()
   428  		if !isSpace(r) {
   429  			break
   430  		}
   431  		l.next()
   432  		numSpaces++
   433  	}
   434  	// Be careful about a trim-marked closing delimiter, which has a minus
   435  	// after a space. We know there is a space, so check for the '-' that might follow.
   436  	if strings.HasPrefix(l.input[l.pos-1:], l.trimRightDelim) {
   437  		l.backup() // Before the space.
   438  		if numSpaces == 1 {
   439  			return lexRightDelim // On the delim, so go right to that.
   440  		}
   441  	}
   442  	l.emit(itemSpace)
   443  	return lexInsideAction
   444  }
   445  
   446  // lexIdentifier scans an alphanumeric.
   447  func lexIdentifier(l *lexer) stateFn {
   448  Loop:
   449  	for {
   450  		switch r := l.next(); {
   451  		case isAlphaNumeric(r):
   452  			// absorb.
   453  		default:
   454  			l.backup()
   455  			word := l.input[l.start:l.pos]
   456  			if !l.atTerminator() {
   457  				return l.errorf("bad character %#U", r)
   458  			}
   459  			switch {
   460  			case key[word] > itemKeyword:
   461  				l.emit(key[word])
   462  			case word[0] == '.':
   463  				l.emit(itemField)
   464  			case word == "true", word == "false":
   465  				l.emit(itemBool)
   466  			default:
   467  				l.emit(itemIdentifier)
   468  			}
   469  			break Loop
   470  		}
   471  	}
   472  	return lexInsideAction
   473  }
   474  
   475  // lexField scans a field: .Alphanumeric.
   476  // The . has been scanned.
   477  func lexField(l *lexer) stateFn {
   478  	return lexFieldOrVariable(l, itemField)
   479  }
   480  
   481  // lexVariable scans a Variable: $Alphanumeric.
   482  // The $ has been scanned.
   483  func lexVariable(l *lexer) stateFn {
   484  	if l.atTerminator() { // Nothing interesting follows -> "$".
   485  		l.emit(itemVariable)
   486  		return lexInsideAction
   487  	}
   488  	return lexFieldOrVariable(l, itemVariable)
   489  }
   490  
   491  // lexVariable scans a field or variable: [.$]Alphanumeric.
   492  // The . or $ has been scanned.
   493  func lexFieldOrVariable(l *lexer, typ itemType) stateFn {
   494  	if l.atTerminator() { // Nothing interesting follows -> "." or "$".
   495  		if typ == itemVariable {
   496  			l.emit(itemVariable)
   497  		} else {
   498  			l.emit(itemDot)
   499  		}
   500  		return lexInsideAction
   501  	}
   502  	var r rune
   503  	for {
   504  		r = l.next()
   505  		if !isAlphaNumeric(r) {
   506  			l.backup()
   507  			break
   508  		}
   509  	}
   510  	if !l.atTerminator() {
   511  		return l.errorf("bad character %#U", r)
   512  	}
   513  	l.emit(typ)
   514  	return lexInsideAction
   515  }
   516  
   517  // atTerminator reports whether the input is at valid termination character to
   518  // appear after an identifier. Breaks .X.Y into two pieces. Also catches cases
   519  // like "$x+2" not being acceptable without a space, in case we decide one
   520  // day to implement arithmetic.
   521  func (l *lexer) atTerminator() bool {
   522  	r := l.peek()
   523  	if isSpace(r) || isEndOfLine(r) {
   524  		return true
   525  	}
   526  	switch r {
   527  	case eof, '.', ',', '|', ':', ')', '(':
   528  		return true
   529  	}
   530  	// Does r start the delimiter? This can be ambiguous (with delim=="//", $x/2 will
   531  	// succeed but should fail) but only in extremely rare cases caused by willfully
   532  	// bad choice of delimiter.
   533  	if rd, _ := utf8.DecodeRuneInString(l.rightDelim); rd == r {
   534  		return true
   535  	}
   536  	return false
   537  }
   538  
   539  // lexChar scans a character constant. The initial quote is already
   540  // scanned. Syntax checking is done by the parser.
   541  func lexChar(l *lexer) stateFn {
   542  Loop:
   543  	for {
   544  		switch l.next() {
   545  		case '\\':
   546  			if r := l.next(); r != eof && r != '\n' {
   547  				break
   548  			}
   549  			fallthrough
   550  		case eof, '\n':
   551  			return l.errorf("unterminated character constant")
   552  		case '\'':
   553  			break Loop
   554  		}
   555  	}
   556  	l.emit(itemCharConstant)
   557  	return lexInsideAction
   558  }
   559  
   560  // lexNumber scans a number: decimal, octal, hex, float, or imaginary. This
   561  // isn't a perfect number scanner - for instance it accepts "." and "0x0.2"
   562  // and "089" - but when it's wrong the input is invalid and the parser (via
   563  // strconv) will notice.
   564  func lexNumber(l *lexer) stateFn {
   565  	if !l.scanNumber() {
   566  		return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
   567  	}
   568  	if sign := l.peek(); sign == '+' || sign == '-' {
   569  		// Complex: 1+2i. No spaces, must end in 'i'.
   570  		if !l.scanNumber() || l.input[l.pos-1] != 'i' {
   571  			return l.errorf("bad number syntax: %q", l.input[l.start:l.pos])
   572  		}
   573  		l.emit(itemComplex)
   574  	} else {
   575  		l.emit(itemNumber)
   576  	}
   577  	return lexInsideAction
   578  }
   579  
   580  func (l *lexer) scanNumber() bool {
   581  	// Optional leading sign.
   582  	l.accept("+-")
   583  	// Is it hex?
   584  	digits := "0123456789_"
   585  	if l.accept("0") {
   586  		// Note: Leading 0 does not mean octal in floats.
   587  		if l.accept("xX") {
   588  			digits = "0123456789abcdefABCDEF_"
   589  		} else if l.accept("oO") {
   590  			digits = "01234567_"
   591  		} else if l.accept("bB") {
   592  			digits = "01_"
   593  		}
   594  	}
   595  	l.acceptRun(digits)
   596  	if l.accept(".") {
   597  		l.acceptRun(digits)
   598  	}
   599  	if len(digits) == 10+1 && l.accept("eE") {
   600  		l.accept("+-")
   601  		l.acceptRun("0123456789_")
   602  	}
   603  	if len(digits) == 16+6+1 && l.accept("pP") {
   604  		l.accept("+-")
   605  		l.acceptRun("0123456789_")
   606  	}
   607  	// Is it imaginary?
   608  	l.accept("i")
   609  	// Next thing mustn't be alphanumeric.
   610  	if isAlphaNumeric(l.peek()) {
   611  		l.next()
   612  		return false
   613  	}
   614  	return true
   615  }
   616  
   617  // lexQuote scans a quoted string.
   618  func lexQuote(l *lexer) stateFn {
   619  Loop:
   620  	for {
   621  		switch l.next() {
   622  		case '\\':
   623  			if r := l.next(); r != eof && r != '\n' {
   624  				break
   625  			}
   626  			fallthrough
   627  		case eof, '\n':
   628  			return l.errorf("unterminated quoted string")
   629  		case '"':
   630  			break Loop
   631  		}
   632  	}
   633  	l.emit(itemString)
   634  	return lexInsideAction
   635  }
   636  
   637  // lexRawQuote scans a raw quoted string.
   638  func lexRawQuote(l *lexer) stateFn {
   639  Loop:
   640  	for {
   641  		switch l.next() {
   642  		case eof:
   643  			return l.errorf("unterminated raw quoted string")
   644  		case '`':
   645  			break Loop
   646  		}
   647  	}
   648  	l.emit(itemRawString)
   649  	return lexInsideAction
   650  }
   651  
   652  // isSpace reports whether r is a space character.
   653  func isSpace(r rune) bool {
   654  	return r == ' ' || r == '\t'
   655  }
   656  
   657  // isEndOfLine reports whether r is an end-of-line character.
   658  func isEndOfLine(r rune) bool {
   659  	return r == '\r' || r == '\n'
   660  }
   661  
   662  // isAlphaNumeric reports whether r is an alphabetic, digit, or underscore.
   663  func isAlphaNumeric(r rune) bool {
   664  	return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
   665  }
   666
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