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]any
    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  	rangeDepth int
    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]any) (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]any) *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 ...any) {
   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]any, 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]any) (tree *Tree, err error) {
   242  	defer t.recover(&err)
   243  	t.ParseName = t.Name
   244  	emitComment := t.Mode&ParseComments != 0
   245  	breakOK := !t.hasFunction("break")
   246  	continueOK := !t.hasFunction("continue")
   247  	lexer := lex(t.Name, text, leftDelim, rightDelim, emitComment, breakOK, continueOK)
   248  	t.startParse(funcs, lexer, treeSet)
   249  	t.text = text
   250  	t.parse()
   251  	t.add()
   252  	t.stopParse()
   253  	return t, nil
   254  }
   255  
   256  // add adds tree to t.treeSet.
   257  func (t *Tree) add() {
   258  	tree := t.treeSet[t.Name]
   259  	if tree == nil || IsEmptyTree(tree.Root) {
   260  		t.treeSet[t.Name] = t
   261  		return
   262  	}
   263  	if !IsEmptyTree(t.Root) {
   264  		t.errorf("template: multiple definition of template %q", t.Name)
   265  	}
   266  }
   267  
   268  // IsEmptyTree reports whether this tree (node) is empty of everything but space or comments.
   269  func IsEmptyTree(n Node) bool {
   270  	switch n := n.(type) {
   271  	case nil:
   272  		return true
   273  	case *ActionNode:
   274  	case *CommentNode:
   275  		return true
   276  	case *IfNode:
   277  	case *ListNode:
   278  		for _, node := range n.Nodes {
   279  			if !IsEmptyTree(node) {
   280  				return false
   281  			}
   282  		}
   283  		return true
   284  	case *RangeNode:
   285  	case *TemplateNode:
   286  	case *TextNode:
   287  		return len(bytes.TrimSpace(n.Text)) == 0
   288  	case *WithNode:
   289  	default:
   290  		panic("unknown node: " + n.String())
   291  	}
   292  	return false
   293  }
   294  
   295  // parse is the top-level parser for a template, essentially the same
   296  // as itemList except it also parses {{define}} actions.
   297  // It runs to EOF.
   298  func (t *Tree) parse() {
   299  	t.Root = t.newList(t.peek().pos)
   300  	for t.peek().typ != itemEOF {
   301  		if t.peek().typ == itemLeftDelim {
   302  			delim := t.next()
   303  			if t.nextNonSpace().typ == itemDefine {
   304  				newT := New("definition") // name will be updated once we know it.
   305  				newT.text = t.text
   306  				newT.Mode = t.Mode
   307  				newT.ParseName = t.ParseName
   308  				newT.startParse(t.funcs, t.lex, t.treeSet)
   309  				newT.parseDefinition()
   310  				continue
   311  			}
   312  			t.backup2(delim)
   313  		}
   314  		switch n := t.textOrAction(); n.Type() {
   315  		case nodeEnd, nodeElse:
   316  			t.errorf("unexpected %s", n)
   317  		default:
   318  			t.Root.append(n)
   319  		}
   320  	}
   321  }
   322  
   323  // parseDefinition parses a {{define}} ...  {{end}} template definition and
   324  // installs the definition in t.treeSet. The "define" keyword has already
   325  // been scanned.
   326  func (t *Tree) parseDefinition() {
   327  	const context = "define clause"
   328  	name := t.expectOneOf(itemString, itemRawString, context)
   329  	var err error
   330  	t.Name, err = strconv.Unquote(name.val)
   331  	if err != nil {
   332  		t.error(err)
   333  	}
   334  	t.expect(itemRightDelim, context)
   335  	var end Node
   336  	t.Root, end = t.itemList()
   337  	if end.Type() != nodeEnd {
   338  		t.errorf("unexpected %s in %s", end, context)
   339  	}
   340  	t.add()
   341  	t.stopParse()
   342  }
   343  
   344  // itemList:
   345  //
   346  //	textOrAction*
   347  //
   348  // Terminates at {{end}} or {{else}}, returned separately.
   349  func (t *Tree) itemList() (list *ListNode, next Node) {
   350  	list = t.newList(t.peekNonSpace().pos)
   351  	for t.peekNonSpace().typ != itemEOF {
   352  		n := t.textOrAction()
   353  		switch n.Type() {
   354  		case nodeEnd, nodeElse:
   355  			return list, n
   356  		}
   357  		list.append(n)
   358  	}
   359  	t.errorf("unexpected EOF")
   360  	return
   361  }
   362  
   363  // textOrAction:
   364  //
   365  //	text | comment | action
   366  func (t *Tree) textOrAction() Node {
   367  	switch token := t.nextNonSpace(); token.typ {
   368  	case itemText:
   369  		return t.newText(token.pos, token.val)
   370  	case itemLeftDelim:
   371  		t.actionLine = token.line
   372  		defer t.clearActionLine()
   373  		return t.action()
   374  	case itemComment:
   375  		return t.newComment(token.pos, token.val)
   376  	default:
   377  		t.unexpected(token, "input")
   378  	}
   379  	return nil
   380  }
   381  
   382  func (t *Tree) clearActionLine() {
   383  	t.actionLine = 0
   384  }
   385  
   386  // Action:
   387  //
   388  //	control
   389  //	command ("|" command)*
   390  //
   391  // Left delim is past. Now get actions.
   392  // First word could be a keyword such as range.
   393  func (t *Tree) action() (n Node) {
   394  	switch token := t.nextNonSpace(); token.typ {
   395  	case itemBlock:
   396  		return t.blockControl()
   397  	case itemBreak:
   398  		return t.breakControl(token.pos, token.line)
   399  	case itemContinue:
   400  		return t.continueControl(token.pos, token.line)
   401  	case itemElse:
   402  		return t.elseControl()
   403  	case itemEnd:
   404  		return t.endControl()
   405  	case itemIf:
   406  		return t.ifControl()
   407  	case itemRange:
   408  		return t.rangeControl()
   409  	case itemTemplate:
   410  		return t.templateControl()
   411  	case itemWith:
   412  		return t.withControl()
   413  	}
   414  	t.backup()
   415  	token := t.peek()
   416  	// Do not pop variables; they persist until "end".
   417  	return t.newAction(token.pos, token.line, t.pipeline("command", itemRightDelim))
   418  }
   419  
   420  // Break:
   421  //
   422  //	{{break}}
   423  //
   424  // Break keyword is past.
   425  func (t *Tree) breakControl(pos Pos, line int) Node {
   426  	if token := t.nextNonSpace(); token.typ != itemRightDelim {
   427  		t.unexpected(token, "{{break}}")
   428  	}
   429  	if t.rangeDepth == 0 {
   430  		t.errorf("{{break}} outside {{range}}")
   431  	}
   432  	return t.newBreak(pos, line)
   433  }
   434  
   435  // Continue:
   436  //
   437  //	{{continue}}
   438  //
   439  // Continue keyword is past.
   440  func (t *Tree) continueControl(pos Pos, line int) Node {
   441  	if token := t.nextNonSpace(); token.typ != itemRightDelim {
   442  		t.unexpected(token, "{{continue}}")
   443  	}
   444  	if t.rangeDepth == 0 {
   445  		t.errorf("{{continue}} outside {{range}}")
   446  	}
   447  	return t.newContinue(pos, line)
   448  }
   449  
   450  // Pipeline:
   451  //
   452  //	declarations? command ('|' command)*
   453  func (t *Tree) pipeline(context string, end itemType) (pipe *PipeNode) {
   454  	token := t.peekNonSpace()
   455  	pipe = t.newPipeline(token.pos, token.line, nil)
   456  	// Are there declarations or assignments?
   457  decls:
   458  	if v := t.peekNonSpace(); v.typ == itemVariable {
   459  		t.next()
   460  		// Since space is a token, we need 3-token look-ahead here in the worst case:
   461  		// in "$x foo" we need to read "foo" (as opposed to ":=") to know that $x is an
   462  		// argument variable rather than a declaration. So remember the token
   463  		// adjacent to the variable so we can push it back if necessary.
   464  		tokenAfterVariable := t.peek()
   465  		next := t.peekNonSpace()
   466  		switch {
   467  		case next.typ == itemAssign, next.typ == itemDeclare:
   468  			pipe.IsAssign = next.typ == itemAssign
   469  			t.nextNonSpace()
   470  			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
   471  			t.vars = append(t.vars, v.val)
   472  		case next.typ == itemChar && next.val == ",":
   473  			t.nextNonSpace()
   474  			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
   475  			t.vars = append(t.vars, v.val)
   476  			if context == "range" && len(pipe.Decl) < 2 {
   477  				switch t.peekNonSpace().typ {
   478  				case itemVariable, itemRightDelim, itemRightParen:
   479  					// second initialized variable in a range pipeline
   480  					goto decls
   481  				default:
   482  					t.errorf("range can only initialize variables")
   483  				}
   484  			}
   485  			t.errorf("too many declarations in %s", context)
   486  		case tokenAfterVariable.typ == itemSpace:
   487  			t.backup3(v, tokenAfterVariable)
   488  		default:
   489  			t.backup2(v)
   490  		}
   491  	}
   492  	for {
   493  		switch token := t.nextNonSpace(); token.typ {
   494  		case end:
   495  			// At this point, the pipeline is complete
   496  			t.checkPipeline(pipe, context)
   497  			return
   498  		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
   499  			itemNumber, itemNil, itemRawString, itemString, itemVariable, itemLeftParen:
   500  			t.backup()
   501  			pipe.append(t.command())
   502  		default:
   503  			t.unexpected(token, context)
   504  		}
   505  	}
   506  }
   507  
   508  func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
   509  	// Reject empty pipelines
   510  	if len(pipe.Cmds) == 0 {
   511  		t.errorf("missing value for %s", context)
   512  	}
   513  	// Only the first command of a pipeline can start with a non executable operand
   514  	for i, c := range pipe.Cmds[1:] {
   515  		switch c.Args[0].Type() {
   516  		case NodeBool, NodeDot, NodeNil, NodeNumber, NodeString:
   517  			// With A|B|C, pipeline stage 2 is B
   518  			t.errorf("non executable command in pipeline stage %d", i+2)
   519  		}
   520  	}
   521  }
   522  
   523  func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
   524  	defer t.popVars(len(t.vars))
   525  	pipe = t.pipeline(context, itemRightDelim)
   526  	if context == "range" {
   527  		t.rangeDepth++
   528  	}
   529  	var next Node
   530  	list, next = t.itemList()
   531  	if context == "range" {
   532  		t.rangeDepth--
   533  	}
   534  	switch next.Type() {
   535  	case nodeEnd: //done
   536  	case nodeElse:
   537  		if allowElseIf {
   538  			// Special case for "else if". If the "else" is followed immediately by an "if",
   539  			// the elseControl will have left the "if" token pending. Treat
   540  			//	{{if a}}_{{else if b}}_{{end}}
   541  			// as
   542  			//	{{if a}}_{{else}}{{if b}}_{{end}}{{end}}.
   543  			// To do this, parse the if as usual and stop at it {{end}}; the subsequent{{end}}
   544  			// is assumed. This technique works even for long if-else-if chains.
   545  			// TODO: Should we allow else-if in with and range?
   546  			if t.peek().typ == itemIf {
   547  				t.next() // Consume the "if" token.
   548  				elseList = t.newList(next.Position())
   549  				elseList.append(t.ifControl())
   550  				// Do not consume the next item - only one {{end}} required.
   551  				break
   552  			}
   553  		}
   554  		elseList, next = t.itemList()
   555  		if next.Type() != nodeEnd {
   556  			t.errorf("expected end; found %s", next)
   557  		}
   558  	}
   559  	return pipe.Position(), pipe.Line, pipe, list, elseList
   560  }
   561  
   562  // If:
   563  //
   564  //	{{if pipeline}} itemList {{end}}
   565  //	{{if pipeline}} itemList {{else}} itemList {{end}}
   566  //
   567  // If keyword is past.
   568  func (t *Tree) ifControl() Node {
   569  	return t.newIf(t.parseControl(true, "if"))
   570  }
   571  
   572  // Range:
   573  //
   574  //	{{range pipeline}} itemList {{end}}
   575  //	{{range pipeline}} itemList {{else}} itemList {{end}}
   576  //
   577  // Range keyword is past.
   578  func (t *Tree) rangeControl() Node {
   579  	r := t.newRange(t.parseControl(false, "range"))
   580  	return r
   581  }
   582  
   583  // With:
   584  //
   585  //	{{with pipeline}} itemList {{end}}
   586  //	{{with pipeline}} itemList {{else}} itemList {{end}}
   587  //
   588  // If keyword is past.
   589  func (t *Tree) withControl() Node {
   590  	return t.newWith(t.parseControl(false, "with"))
   591  }
   592  
   593  // End:
   594  //
   595  //	{{end}}
   596  //
   597  // End keyword is past.
   598  func (t *Tree) endControl() Node {
   599  	return t.newEnd(t.expect(itemRightDelim, "end").pos)
   600  }
   601  
   602  // Else:
   603  //
   604  //	{{else}}
   605  //
   606  // Else keyword is past.
   607  func (t *Tree) elseControl() Node {
   608  	// Special case for "else if".
   609  	peek := t.peekNonSpace()
   610  	if peek.typ == itemIf {
   611  		// We see "{{else if ... " but in effect rewrite it to {{else}}{{if ... ".
   612  		return t.newElse(peek.pos, peek.line)
   613  	}
   614  	token := t.expect(itemRightDelim, "else")
   615  	return t.newElse(token.pos, token.line)
   616  }
   617  
   618  // Block:
   619  //
   620  //	{{block stringValue pipeline}}
   621  //
   622  // Block keyword is past.
   623  // The name must be something that can evaluate to a string.
   624  // The pipeline is mandatory.
   625  func (t *Tree) blockControl() Node {
   626  	const context = "block clause"
   627  
   628  	token := t.nextNonSpace()
   629  	name := t.parseTemplateName(token, context)
   630  	pipe := t.pipeline(context, itemRightDelim)
   631  
   632  	block := New(name) // name will be updated once we know it.
   633  	block.text = t.text
   634  	block.Mode = t.Mode
   635  	block.ParseName = t.ParseName
   636  	block.startParse(t.funcs, t.lex, t.treeSet)
   637  	var end Node
   638  	block.Root, end = block.itemList()
   639  	if end.Type() != nodeEnd {
   640  		t.errorf("unexpected %s in %s", end, context)
   641  	}
   642  	block.add()
   643  	block.stopParse()
   644  
   645  	return t.newTemplate(token.pos, token.line, name, pipe)
   646  }
   647  
   648  // Template:
   649  //
   650  //	{{template stringValue pipeline}}
   651  //
   652  // Template keyword is past. The name must be something that can evaluate
   653  // to a string.
   654  func (t *Tree) templateControl() Node {
   655  	const context = "template clause"
   656  	token := t.nextNonSpace()
   657  	name := t.parseTemplateName(token, context)
   658  	var pipe *PipeNode
   659  	if t.nextNonSpace().typ != itemRightDelim {
   660  		t.backup()
   661  		// Do not pop variables; they persist until "end".
   662  		pipe = t.pipeline(context, itemRightDelim)
   663  	}
   664  	return t.newTemplate(token.pos, token.line, name, pipe)
   665  }
   666  
   667  func (t *Tree) parseTemplateName(token item, context string) (name string) {
   668  	switch token.typ {
   669  	case itemString, itemRawString:
   670  		s, err := strconv.Unquote(token.val)
   671  		if err != nil {
   672  			t.error(err)
   673  		}
   674  		name = s
   675  	default:
   676  		t.unexpected(token, context)
   677  	}
   678  	return
   679  }
   680  
   681  // command:
   682  //
   683  //	operand (space operand)*
   684  //
   685  // space-separated arguments up to a pipeline character or right delimiter.
   686  // we consume the pipe character but leave the right delim to terminate the action.
   687  func (t *Tree) command() *CommandNode {
   688  	cmd := t.newCommand(t.peekNonSpace().pos)
   689  	for {
   690  		t.peekNonSpace() // skip leading spaces.
   691  		operand := t.operand()
   692  		if operand != nil {
   693  			cmd.append(operand)
   694  		}
   695  		switch token := t.next(); token.typ {
   696  		case itemSpace:
   697  			continue
   698  		case itemRightDelim, itemRightParen:
   699  			t.backup()
   700  		case itemPipe:
   701  			// nothing here; break loop below
   702  		default:
   703  			t.unexpected(token, "operand")
   704  		}
   705  		break
   706  	}
   707  	if len(cmd.Args) == 0 {
   708  		t.errorf("empty command")
   709  	}
   710  	return cmd
   711  }
   712  
   713  // operand:
   714  //
   715  //	term .Field*
   716  //
   717  // An operand is a space-separated component of a command,
   718  // a term possibly followed by field accesses.
   719  // A nil return means the next item is not an operand.
   720  func (t *Tree) operand() Node {
   721  	node := t.term()
   722  	if node == nil {
   723  		return nil
   724  	}
   725  	if t.peek().typ == itemField {
   726  		chain := t.newChain(t.peek().pos, node)
   727  		for t.peek().typ == itemField {
   728  			chain.Add(t.next().val)
   729  		}
   730  		// Compatibility with original API: If the term is of type NodeField
   731  		// or NodeVariable, just put more fields on the original.
   732  		// Otherwise, keep the Chain node.
   733  		// Obvious parsing errors involving literal values are detected here.
   734  		// More complex error cases will have to be handled at execution time.
   735  		switch node.Type() {
   736  		case NodeField:
   737  			node = t.newField(chain.Position(), chain.String())
   738  		case NodeVariable:
   739  			node = t.newVariable(chain.Position(), chain.String())
   740  		case NodeBool, NodeString, NodeNumber, NodeNil, NodeDot:
   741  			t.errorf("unexpected . after term %q", node.String())
   742  		default:
   743  			node = chain
   744  		}
   745  	}
   746  	return node
   747  }
   748  
   749  // term:
   750  //
   751  //	literal (number, string, nil, boolean)
   752  //	function (identifier)
   753  //	.
   754  //	.Field
   755  //	$
   756  //	'(' pipeline ')'
   757  //
   758  // A term is a simple "expression".
   759  // A nil return means the next item is not a term.
   760  func (t *Tree) term() Node {
   761  	switch token := t.nextNonSpace(); token.typ {
   762  	case itemIdentifier:
   763  		checkFunc := t.Mode&SkipFuncCheck == 0
   764  		if checkFunc && !t.hasFunction(token.val) {
   765  			t.errorf("function %q not defined", token.val)
   766  		}
   767  		return NewIdentifier(token.val).SetTree(t).SetPos(token.pos)
   768  	case itemDot:
   769  		return t.newDot(token.pos)
   770  	case itemNil:
   771  		return t.newNil(token.pos)
   772  	case itemVariable:
   773  		return t.useVar(token.pos, token.val)
   774  	case itemField:
   775  		return t.newField(token.pos, token.val)
   776  	case itemBool:
   777  		return t.newBool(token.pos, token.val == "true")
   778  	case itemCharConstant, itemComplex, itemNumber:
   779  		number, err := t.newNumber(token.pos, token.val, token.typ)
   780  		if err != nil {
   781  			t.error(err)
   782  		}
   783  		return number
   784  	case itemLeftParen:
   785  		return t.pipeline("parenthesized pipeline", itemRightParen)
   786  	case itemString, itemRawString:
   787  		s, err := strconv.Unquote(token.val)
   788  		if err != nil {
   789  			t.error(err)
   790  		}
   791  		return t.newString(token.pos, token.val, s)
   792  	}
   793  	t.backup()
   794  	return nil
   795  }
   796  
   797  // hasFunction reports if a function name exists in the Tree's maps.
   798  func (t *Tree) hasFunction(name string) bool {
   799  	for _, funcMap := range t.funcs {
   800  		if funcMap == nil {
   801  			continue
   802  		}
   803  		if funcMap[name] != nil {
   804  			return true
   805  		}
   806  	}
   807  	return false
   808  }
   809  
   810  // popVars trims the variable list to the specified length
   811  func (t *Tree) popVars(n int) {
   812  	t.vars = t.vars[:n]
   813  }
   814  
   815  // useVar returns a node for a variable reference. It errors if the
   816  // variable is not defined.
   817  func (t *Tree) useVar(pos Pos, name string) Node {
   818  	v := t.newVariable(pos, name)
   819  	for _, varName := range t.vars {
   820  		if varName == v.Ident[0] {
   821  			return v
   822  		}
   823  	}
   824  	t.errorf("undefined variable %q", v.Ident[0])
   825  	return nil
   826  }
   827  

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