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