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Source file src/text/template/parse/parse.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 builds parse trees for templates as defined by text/template
     6  // and html/template. Clients should use those packages to construct templates
     7  // rather than this one, which provides shared internal data structures not
     8  // intended for general use.
     9  package parse
    10  
    11  import (
    12  	"bytes"
    13  	"fmt"
    14  	"runtime"
    15  	"strconv"
    16  	"strings"
    17  )
    18  
    19  // Tree is the representation of a single parsed template.
    20  type Tree struct {
    21  	Name      string    // name of the template represented by the tree.
    22  	ParseName string    // name of the top-level template during parsing, for error messages.
    23  	Root      *ListNode // top-level root of the tree.
    24  	Mode      Mode      // parsing mode.
    25  	text      string    // text parsed to create the template (or its parent)
    26  	// Parsing only; cleared after parse.
    27  	funcs      []map[string]interface{}
    28  	lex        *lexer
    29  	token      [3]item // three-token lookahead for parser.
    30  	peekCount  int
    31  	vars       []string // variables defined at the moment.
    32  	treeSet    map[string]*Tree
    33  	actionLine int // line of left delim starting action
    34  	mode       Mode
    35  }
    36  
    37  // A mode value is a set of flags (or 0). Modes control parser behavior.
    38  type Mode uint
    39  
    40  const (
    41  	ParseComments Mode = 1 << iota // parse comments and add them to AST
    42  	SkipFuncCheck                  // do not check that functions are defined
    43  )
    44  
    45  // Copy returns a copy of the Tree. Any parsing state is discarded.
    46  func (t *Tree) Copy() *Tree {
    47  	if t == nil {
    48  		return nil
    49  	}
    50  	return &Tree{
    51  		Name:      t.Name,
    52  		ParseName: t.ParseName,
    53  		Root:      t.Root.CopyList(),
    54  		text:      t.text,
    55  	}
    56  }
    57  
    58  // Parse returns a map from template name to parse.Tree, created by parsing the
    59  // templates described in the argument string. The top-level template will be
    60  // given the specified name. If an error is encountered, parsing stops and an
    61  // empty map is returned with the error.
    62  func Parse(name, text, leftDelim, rightDelim string, funcs ...map[string]interface{}) (map[string]*Tree, error) {
    63  	treeSet := make(map[string]*Tree)
    64  	t := New(name)
    65  	t.text = text
    66  	_, err := t.Parse(text, leftDelim, rightDelim, treeSet, funcs...)
    67  	return treeSet, err
    68  }
    69  
    70  // next returns the next token.
    71  func (t *Tree) next() item {
    72  	if t.peekCount > 0 {
    73  		t.peekCount--
    74  	} else {
    75  		t.token[0] = t.lex.nextItem()
    76  	}
    77  	return t.token[t.peekCount]
    78  }
    79  
    80  // backup backs the input stream up one token.
    81  func (t *Tree) backup() {
    82  	t.peekCount++
    83  }
    84  
    85  // backup2 backs the input stream up two tokens.
    86  // The zeroth token is already there.
    87  func (t *Tree) backup2(t1 item) {
    88  	t.token[1] = t1
    89  	t.peekCount = 2
    90  }
    91  
    92  // backup3 backs the input stream up three tokens
    93  // The zeroth token is already there.
    94  func (t *Tree) backup3(t2, t1 item) { // Reverse order: we're pushing back.
    95  	t.token[1] = t1
    96  	t.token[2] = t2
    97  	t.peekCount = 3
    98  }
    99  
   100  // peek returns but does not consume the next token.
   101  func (t *Tree) peek() item {
   102  	if t.peekCount > 0 {
   103  		return t.token[t.peekCount-1]
   104  	}
   105  	t.peekCount = 1
   106  	t.token[0] = t.lex.nextItem()
   107  	return t.token[0]
   108  }
   109  
   110  // nextNonSpace returns the next non-space token.
   111  func (t *Tree) nextNonSpace() (token item) {
   112  	for {
   113  		token = t.next()
   114  		if token.typ != itemSpace {
   115  			break
   116  		}
   117  	}
   118  	return token
   119  }
   120  
   121  // peekNonSpace returns but does not consume the next non-space token.
   122  func (t *Tree) peekNonSpace() item {
   123  	token := t.nextNonSpace()
   124  	t.backup()
   125  	return token
   126  }
   127  
   128  // Parsing.
   129  
   130  // New allocates a new parse tree with the given name.
   131  func New(name string, funcs ...map[string]interface{}) *Tree {
   132  	return &Tree{
   133  		Name:  name,
   134  		funcs: funcs,
   135  	}
   136  }
   137  
   138  // ErrorContext returns a textual representation of the location of the node in the input text.
   139  // The receiver is only used when the node does not have a pointer to the tree inside,
   140  // which can occur in old code.
   141  func (t *Tree) ErrorContext(n Node) (location, context string) {
   142  	pos := int(n.Position())
   143  	tree := n.tree()
   144  	if tree == nil {
   145  		tree = t
   146  	}
   147  	text := tree.text[:pos]
   148  	byteNum := strings.LastIndex(text, "\n")
   149  	if byteNum == -1 {
   150  		byteNum = pos // On first line.
   151  	} else {
   152  		byteNum++ // After the newline.
   153  		byteNum = pos - byteNum
   154  	}
   155  	lineNum := 1 + strings.Count(text, "\n")
   156  	context = n.String()
   157  	return fmt.Sprintf("%s:%d:%d", tree.ParseName, lineNum, byteNum), context
   158  }
   159  
   160  // errorf formats the error and terminates processing.
   161  func (t *Tree) errorf(format string, args ...interface{}) {
   162  	t.Root = nil
   163  	format = fmt.Sprintf("template: %s:%d: %s", t.ParseName, t.token[0].line, format)
   164  	panic(fmt.Errorf(format, args...))
   165  }
   166  
   167  // error terminates processing.
   168  func (t *Tree) error(err error) {
   169  	t.errorf("%s", err)
   170  }
   171  
   172  // expect consumes the next token and guarantees it has the required type.
   173  func (t *Tree) expect(expected itemType, context string) item {
   174  	token := t.nextNonSpace()
   175  	if token.typ != expected {
   176  		t.unexpected(token, context)
   177  	}
   178  	return token
   179  }
   180  
   181  // expectOneOf consumes the next token and guarantees it has one of the required types.
   182  func (t *Tree) expectOneOf(expected1, expected2 itemType, context string) item {
   183  	token := t.nextNonSpace()
   184  	if token.typ != expected1 && token.typ != expected2 {
   185  		t.unexpected(token, context)
   186  	}
   187  	return token
   188  }
   189  
   190  // unexpected complains about the token and terminates processing.
   191  func (t *Tree) unexpected(token item, context string) {
   192  	if token.typ == itemError {
   193  		extra := ""
   194  		if t.actionLine != 0 && t.actionLine != token.line {
   195  			extra = fmt.Sprintf(" in action started at %s:%d", t.ParseName, t.actionLine)
   196  			if strings.HasSuffix(token.val, " action") {
   197  				extra = extra[len(" in action"):] // avoid "action in action"
   198  			}
   199  		}
   200  		t.errorf("%s%s", token, extra)
   201  	}
   202  	t.errorf("unexpected %s in %s", token, context)
   203  }
   204  
   205  // recover is the handler that turns panics into returns from the top level of Parse.
   206  func (t *Tree) recover(errp *error) {
   207  	e := recover()
   208  	if e != nil {
   209  		if _, ok := e.(runtime.Error); ok {
   210  			panic(e)
   211  		}
   212  		if t != nil {
   213  			t.lex.drain()
   214  			t.stopParse()
   215  		}
   216  		*errp = e.(error)
   217  	}
   218  }
   219  
   220  // startParse initializes the parser, using the lexer.
   221  func (t *Tree) startParse(funcs []map[string]interface{}, lex *lexer, treeSet map[string]*Tree) {
   222  	t.Root = nil
   223  	t.lex = lex
   224  	t.vars = []string{"$"}
   225  	t.funcs = funcs
   226  	t.treeSet = treeSet
   227  }
   228  
   229  // stopParse terminates parsing.
   230  func (t *Tree) stopParse() {
   231  	t.lex = nil
   232  	t.vars = nil
   233  	t.funcs = nil
   234  	t.treeSet = nil
   235  }
   236  
   237  // Parse parses the template definition string to construct a representation of
   238  // the template for execution. If either action delimiter string is empty, the
   239  // default ("{{" or "}}") is used. Embedded template definitions are added to
   240  // the treeSet map.
   241  func (t *Tree) Parse(text, leftDelim, rightDelim string, treeSet map[string]*Tree, funcs ...map[string]interface{}) (tree *Tree, err error) {
   242  	defer t.recover(&err)
   243  	t.ParseName = t.Name
   244  	emitComment := t.Mode&ParseComments != 0
   245  	t.startParse(funcs, lex(t.Name, text, leftDelim, rightDelim, emitComment), treeSet)
   246  	t.text = text
   247  	t.parse()
   248  	t.add()
   249  	t.stopParse()
   250  	return t, nil
   251  }
   252  
   253  // add adds tree to t.treeSet.
   254  func (t *Tree) add() {
   255  	tree := t.treeSet[t.Name]
   256  	if tree == nil || IsEmptyTree(tree.Root) {
   257  		t.treeSet[t.Name] = t
   258  		return
   259  	}
   260  	if !IsEmptyTree(t.Root) {
   261  		t.errorf("template: multiple definition of template %q", t.Name)
   262  	}
   263  }
   264  
   265  // IsEmptyTree reports whether this tree (node) is empty of everything but space or comments.
   266  func IsEmptyTree(n Node) bool {
   267  	switch n := n.(type) {
   268  	case nil:
   269  		return true
   270  	case *ActionNode:
   271  	case *CommentNode:
   272  		return true
   273  	case *IfNode:
   274  	case *ListNode:
   275  		for _, node := range n.Nodes {
   276  			if !IsEmptyTree(node) {
   277  				return false
   278  			}
   279  		}
   280  		return true
   281  	case *RangeNode:
   282  	case *TemplateNode:
   283  	case *TextNode:
   284  		return len(bytes.TrimSpace(n.Text)) == 0
   285  	case *WithNode:
   286  	default:
   287  		panic("unknown node: " + n.String())
   288  	}
   289  	return false
   290  }
   291  
   292  // parse is the top-level parser for a template, essentially the same
   293  // as itemList except it also parses {{define}} actions.
   294  // It runs to EOF.
   295  func (t *Tree) parse() {
   296  	t.Root = t.newList(t.peek().pos)
   297  	for t.peek().typ != itemEOF {
   298  		if t.peek().typ == itemLeftDelim {
   299  			delim := t.next()
   300  			if t.nextNonSpace().typ == itemDefine {
   301  				newT := New("definition") // name will be updated once we know it.
   302  				newT.text = t.text
   303  				newT.Mode = t.Mode
   304  				newT.ParseName = t.ParseName
   305  				newT.startParse(t.funcs, t.lex, t.treeSet)
   306  				newT.parseDefinition()
   307  				continue
   308  			}
   309  			t.backup2(delim)
   310  		}
   311  		switch n := t.textOrAction(); n.Type() {
   312  		case nodeEnd, nodeElse:
   313  			t.errorf("unexpected %s", n)
   314  		default:
   315  			t.Root.append(n)
   316  		}
   317  	}
   318  }
   319  
   320  // parseDefinition parses a {{define}} ...  {{end}} template definition and
   321  // installs the definition in t.treeSet. The "define" keyword has already
   322  // been scanned.
   323  func (t *Tree) parseDefinition() {
   324  	const context = "define clause"
   325  	name := t.expectOneOf(itemString, itemRawString, context)
   326  	var err error
   327  	t.Name, err = strconv.Unquote(name.val)
   328  	if err != nil {
   329  		t.error(err)
   330  	}
   331  	t.expect(itemRightDelim, context)
   332  	var end Node
   333  	t.Root, end = t.itemList()
   334  	if end.Type() != nodeEnd {
   335  		t.errorf("unexpected %s in %s", end, context)
   336  	}
   337  	t.add()
   338  	t.stopParse()
   339  }
   340  
   341  // itemList:
   342  //	textOrAction*
   343  // Terminates at {{end}} or {{else}}, returned separately.
   344  func (t *Tree) itemList() (list *ListNode, next Node) {
   345  	list = t.newList(t.peekNonSpace().pos)
   346  	for t.peekNonSpace().typ != itemEOF {
   347  		n := t.textOrAction()
   348  		switch n.Type() {
   349  		case nodeEnd, nodeElse:
   350  			return list, n
   351  		}
   352  		list.append(n)
   353  	}
   354  	t.errorf("unexpected EOF")
   355  	return
   356  }
   357  
   358  // textOrAction:
   359  //	text | comment | action
   360  func (t *Tree) textOrAction() Node {
   361  	switch token := t.nextNonSpace(); token.typ {
   362  	case itemText:
   363  		return t.newText(token.pos, token.val)
   364  	case itemLeftDelim:
   365  		t.actionLine = token.line
   366  		defer t.clearActionLine()
   367  		return t.action()
   368  	case itemComment:
   369  		return t.newComment(token.pos, token.val)
   370  	default:
   371  		t.unexpected(token, "input")
   372  	}
   373  	return nil
   374  }
   375  
   376  func (t *Tree) clearActionLine() {
   377  	t.actionLine = 0
   378  }
   379  
   380  // Action:
   381  //	control
   382  //	command ("|" command)*
   383  // Left delim is past. Now get actions.
   384  // First word could be a keyword such as range.
   385  func (t *Tree) action() (n Node) {
   386  	switch token := t.nextNonSpace(); token.typ {
   387  	case itemBlock:
   388  		return t.blockControl()
   389  	case itemElse:
   390  		return t.elseControl()
   391  	case itemEnd:
   392  		return t.endControl()
   393  	case itemIf:
   394  		return t.ifControl()
   395  	case itemRange:
   396  		return t.rangeControl()
   397  	case itemTemplate:
   398  		return t.templateControl()
   399  	case itemWith:
   400  		return t.withControl()
   401  	}
   402  	t.backup()
   403  	token := t.peek()
   404  	// Do not pop variables; they persist until "end".
   405  	return t.newAction(token.pos, token.line, t.pipeline("command", itemRightDelim))
   406  }
   407  
   408  // Pipeline:
   409  //	declarations? command ('|' command)*
   410  func (t *Tree) pipeline(context string, end itemType) (pipe *PipeNode) {
   411  	token := t.peekNonSpace()
   412  	pipe = t.newPipeline(token.pos, token.line, nil)
   413  	// Are there declarations or assignments?
   414  decls:
   415  	if v := t.peekNonSpace(); v.typ == itemVariable {
   416  		t.next()
   417  		// Since space is a token, we need 3-token look-ahead here in the worst case:
   418  		// in "$x foo" we need to read "foo" (as opposed to ":=") to know that $x is an
   419  		// argument variable rather than a declaration. So remember the token
   420  		// adjacent to the variable so we can push it back if necessary.
   421  		tokenAfterVariable := t.peek()
   422  		next := t.peekNonSpace()
   423  		switch {
   424  		case next.typ == itemAssign, next.typ == itemDeclare:
   425  			pipe.IsAssign = next.typ == itemAssign
   426  			t.nextNonSpace()
   427  			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
   428  			t.vars = append(t.vars, v.val)
   429  		case next.typ == itemChar && next.val == ",":
   430  			t.nextNonSpace()
   431  			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
   432  			t.vars = append(t.vars, v.val)
   433  			if context == "range" && len(pipe.Decl) < 2 {
   434  				switch t.peekNonSpace().typ {
   435  				case itemVariable, itemRightDelim, itemRightParen:
   436  					// second initialized variable in a range pipeline
   437  					goto decls
   438  				default:
   439  					t.errorf("range can only initialize variables")
   440  				}
   441  			}
   442  			t.errorf("too many declarations in %s", context)
   443  		case tokenAfterVariable.typ == itemSpace:
   444  			t.backup3(v, tokenAfterVariable)
   445  		default:
   446  			t.backup2(v)
   447  		}
   448  	}
   449  	for {
   450  		switch token := t.nextNonSpace(); token.typ {
   451  		case end:
   452  			// At this point, the pipeline is complete
   453  			t.checkPipeline(pipe, context)
   454  			return
   455  		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
   456  			itemNumber, itemNil, itemRawString, itemString, itemVariable, itemLeftParen:
   457  			t.backup()
   458  			pipe.append(t.command())
   459  		default:
   460  			t.unexpected(token, context)
   461  		}
   462  	}
   463  }
   464  
   465  func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
   466  	// Reject empty pipelines
   467  	if len(pipe.Cmds) == 0 {
   468  		t.errorf("missing value for %s", context)
   469  	}
   470  	// Only the first command of a pipeline can start with a non executable operand
   471  	for i, c := range pipe.Cmds[1:] {
   472  		switch c.Args[0].Type() {
   473  		case NodeBool, NodeDot, NodeNil, NodeNumber, NodeString:
   474  			// With A|B|C, pipeline stage 2 is B
   475  			t.errorf("non executable command in pipeline stage %d", i+2)
   476  		}
   477  	}
   478  }
   479  
   480  func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
   481  	defer t.popVars(len(t.vars))
   482  	pipe = t.pipeline(context, itemRightDelim)
   483  	var next Node
   484  	list, next = t.itemList()
   485  	switch next.Type() {
   486  	case nodeEnd: //done
   487  	case nodeElse:
   488  		if allowElseIf {
   489  			// Special case for "else if". If the "else" is followed immediately by an "if",
   490  			// the elseControl will have left the "if" token pending. Treat
   491  			//	{{if a}}_{{else if b}}_{{end}}
   492  			// as
   493  			//	{{if a}}_{{else}}{{if b}}_{{end}}{{end}}.
   494  			// To do this, parse the if as usual and stop at it {{end}}; the subsequent{{end}}
   495  			// is assumed. This technique works even for long if-else-if chains.
   496  			// TODO: Should we allow else-if in with and range?
   497  			if t.peek().typ == itemIf {
   498  				t.next() // Consume the "if" token.
   499  				elseList = t.newList(next.Position())
   500  				elseList.append(t.ifControl())
   501  				// Do not consume the next item - only one {{end}} required.
   502  				break
   503  			}
   504  		}
   505  		elseList, next = t.itemList()
   506  		if next.Type() != nodeEnd {
   507  			t.errorf("expected end; found %s", next)
   508  		}
   509  	}
   510  	return pipe.Position(), pipe.Line, pipe, list, elseList
   511  }
   512  
   513  // If:
   514  //	{{if pipeline}} itemList {{end}}
   515  //	{{if pipeline}} itemList {{else}} itemList {{end}}
   516  // If keyword is past.
   517  func (t *Tree) ifControl() Node {
   518  	return t.newIf(t.parseControl(true, "if"))
   519  }
   520  
   521  // Range:
   522  //	{{range pipeline}} itemList {{end}}
   523  //	{{range pipeline}} itemList {{else}} itemList {{end}}
   524  // Range keyword is past.
   525  func (t *Tree) rangeControl() Node {
   526  	return t.newRange(t.parseControl(false, "range"))
   527  }
   528  
   529  // With:
   530  //	{{with pipeline}} itemList {{end}}
   531  //	{{with pipeline}} itemList {{else}} itemList {{end}}
   532  // If keyword is past.
   533  func (t *Tree) withControl() Node {
   534  	return t.newWith(t.parseControl(false, "with"))
   535  }
   536  
   537  // End:
   538  //	{{end}}
   539  // End keyword is past.
   540  func (t *Tree) endControl() Node {
   541  	return t.newEnd(t.expect(itemRightDelim, "end").pos)
   542  }
   543  
   544  // Else:
   545  //	{{else}}
   546  // Else keyword is past.
   547  func (t *Tree) elseControl() Node {
   548  	// Special case for "else if".
   549  	peek := t.peekNonSpace()
   550  	if peek.typ == itemIf {
   551  		// We see "{{else if ... " but in effect rewrite it to {{else}}{{if ... ".
   552  		return t.newElse(peek.pos, peek.line)
   553  	}
   554  	token := t.expect(itemRightDelim, "else")
   555  	return t.newElse(token.pos, token.line)
   556  }
   557  
   558  // Block:
   559  //	{{block stringValue pipeline}}
   560  // Block keyword is past.
   561  // The name must be something that can evaluate to a string.
   562  // The pipeline is mandatory.
   563  func (t *Tree) blockControl() Node {
   564  	const context = "block clause"
   565  
   566  	token := t.nextNonSpace()
   567  	name := t.parseTemplateName(token, context)
   568  	pipe := t.pipeline(context, itemRightDelim)
   569  
   570  	block := New(name) // name will be updated once we know it.
   571  	block.text = t.text
   572  	block.Mode = t.Mode
   573  	block.ParseName = t.ParseName
   574  	block.startParse(t.funcs, t.lex, t.treeSet)
   575  	var end Node
   576  	block.Root, end = block.itemList()
   577  	if end.Type() != nodeEnd {
   578  		t.errorf("unexpected %s in %s", end, context)
   579  	}
   580  	block.add()
   581  	block.stopParse()
   582  
   583  	return t.newTemplate(token.pos, token.line, name, pipe)
   584  }
   585  
   586  // Template:
   587  //	{{template stringValue pipeline}}
   588  // Template keyword is past. The name must be something that can evaluate
   589  // to a string.
   590  func (t *Tree) templateControl() Node {
   591  	const context = "template clause"
   592  	token := t.nextNonSpace()
   593  	name := t.parseTemplateName(token, context)
   594  	var pipe *PipeNode
   595  	if t.nextNonSpace().typ != itemRightDelim {
   596  		t.backup()
   597  		// Do not pop variables; they persist until "end".
   598  		pipe = t.pipeline(context, itemRightDelim)
   599  	}
   600  	return t.newTemplate(token.pos, token.line, name, pipe)
   601  }
   602  
   603  func (t *Tree) parseTemplateName(token item, context string) (name string) {
   604  	switch token.typ {
   605  	case itemString, itemRawString:
   606  		s, err := strconv.Unquote(token.val)
   607  		if err != nil {
   608  			t.error(err)
   609  		}
   610  		name = s
   611  	default:
   612  		t.unexpected(token, context)
   613  	}
   614  	return
   615  }
   616  
   617  // command:
   618  //	operand (space operand)*
   619  // space-separated arguments up to a pipeline character or right delimiter.
   620  // we consume the pipe character but leave the right delim to terminate the action.
   621  func (t *Tree) command() *CommandNode {
   622  	cmd := t.newCommand(t.peekNonSpace().pos)
   623  	for {
   624  		t.peekNonSpace() // skip leading spaces.
   625  		operand := t.operand()
   626  		if operand != nil {
   627  			cmd.append(operand)
   628  		}
   629  		switch token := t.next(); token.typ {
   630  		case itemSpace:
   631  			continue
   632  		case itemRightDelim, itemRightParen:
   633  			t.backup()
   634  		case itemPipe:
   635  			// nothing here; break loop below
   636  		default:
   637  			t.unexpected(token, "operand")
   638  		}
   639  		break
   640  	}
   641  	if len(cmd.Args) == 0 {
   642  		t.errorf("empty command")
   643  	}
   644  	return cmd
   645  }
   646  
   647  // operand:
   648  //	term .Field*
   649  // An operand is a space-separated component of a command,
   650  // a term possibly followed by field accesses.
   651  // A nil return means the next item is not an operand.
   652  func (t *Tree) operand() Node {
   653  	node := t.term()
   654  	if node == nil {
   655  		return nil
   656  	}
   657  	if t.peek().typ == itemField {
   658  		chain := t.newChain(t.peek().pos, node)
   659  		for t.peek().typ == itemField {
   660  			chain.Add(t.next().val)
   661  		}
   662  		// Compatibility with original API: If the term is of type NodeField
   663  		// or NodeVariable, just put more fields on the original.
   664  		// Otherwise, keep the Chain node.
   665  		// Obvious parsing errors involving literal values are detected here.
   666  		// More complex error cases will have to be handled at execution time.
   667  		switch node.Type() {
   668  		case NodeField:
   669  			node = t.newField(chain.Position(), chain.String())
   670  		case NodeVariable:
   671  			node = t.newVariable(chain.Position(), chain.String())
   672  		case NodeBool, NodeString, NodeNumber, NodeNil, NodeDot:
   673  			t.errorf("unexpected . after term %q", node.String())
   674  		default:
   675  			node = chain
   676  		}
   677  	}
   678  	return node
   679  }
   680  
   681  // term:
   682  //	literal (number, string, nil, boolean)
   683  //	function (identifier)
   684  //	.
   685  //	.Field
   686  //	$
   687  //	'(' pipeline ')'
   688  // A term is a simple "expression".
   689  // A nil return means the next item is not a term.
   690  func (t *Tree) term() Node {
   691  	switch token := t.nextNonSpace(); token.typ {
   692  	case itemIdentifier:
   693  		checkFunc := t.Mode&SkipFuncCheck == 0
   694  		if checkFunc && !t.hasFunction(token.val) {
   695  			t.errorf("function %q not defined", token.val)
   696  		}
   697  		return NewIdentifier(token.val).SetTree(t).SetPos(token.pos)
   698  	case itemDot:
   699  		return t.newDot(token.pos)
   700  	case itemNil:
   701  		return t.newNil(token.pos)
   702  	case itemVariable:
   703  		return t.useVar(token.pos, token.val)
   704  	case itemField:
   705  		return t.newField(token.pos, token.val)
   706  	case itemBool:
   707  		return t.newBool(token.pos, token.val == "true")
   708  	case itemCharConstant, itemComplex, itemNumber:
   709  		number, err := t.newNumber(token.pos, token.val, token.typ)
   710  		if err != nil {
   711  			t.error(err)
   712  		}
   713  		return number
   714  	case itemLeftParen:
   715  		return t.pipeline("parenthesized pipeline", itemRightParen)
   716  	case itemString, itemRawString:
   717  		s, err := strconv.Unquote(token.val)
   718  		if err != nil {
   719  			t.error(err)
   720  		}
   721  		return t.newString(token.pos, token.val, s)
   722  	}
   723  	t.backup()
   724  	return nil
   725  }
   726  
   727  // hasFunction reports if a function name exists in the Tree's maps.
   728  func (t *Tree) hasFunction(name string) bool {
   729  	for _, funcMap := range t.funcs {
   730  		if funcMap == nil {
   731  			continue
   732  		}
   733  		if funcMap[name] != nil {
   734  			return true
   735  		}
   736  	}
   737  	return false
   738  }
   739  
   740  // popVars trims the variable list to the specified length
   741  func (t *Tree) popVars(n int) {
   742  	t.vars = t.vars[:n]
   743  }
   744  
   745  // useVar returns a node for a variable reference. It errors if the
   746  // variable is not defined.
   747  func (t *Tree) useVar(pos Pos, name string) Node {
   748  	v := t.newVariable(pos, name)
   749  	for _, varName := range t.vars {
   750  		if varName == v.Ident[0] {
   751  			return v
   752  		}
   753  	}
   754  	t.errorf("undefined variable %q", v.Ident[0])
   755  	return nil
   756  }
   757  

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