parse.go

     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  	lex.breakOK = !t.hasFunction("break")
   228  	lex.continueOK = !t.hasFunction("continue")
   229  }
   230  
   231  // stopParse terminates parsing.
   232  func (t *Tree) stopParse() {
   233  	t.lex = nil
   234  	t.vars = nil
   235  	t.funcs = nil
   236  	t.treeSet = nil
   237  }
   238  
   239  // Parse parses the template definition string to construct a representation of
   240  // the template for execution. If either action delimiter string is empty, the
   241  // default ("{{" or "}}") is used. Embedded template definitions are added to
   242  // the treeSet map.
   243  func (t *Tree) Parse(text, leftDelim, rightDelim string, treeSet map[string]*Tree, funcs ...map[string]any) (tree *Tree, err error) {
   244  	defer t.recover(&err)
   245  	t.ParseName = t.Name
   246  	emitComment := t.Mode&ParseComments != 0
   247  	t.startParse(funcs, lex(t.Name, text, leftDelim, rightDelim, emitComment), treeSet)
   248  	t.text = text
   249  	t.parse()
   250  	t.add()
   251  	t.stopParse()
   252  	return t, nil
   253  }
   254  
   255  // add adds tree to t.treeSet.
   256  func (t *Tree) add() {
   257  	tree := t.treeSet[t.Name]
   258  	if tree == nil || IsEmptyTree(tree.Root) {
   259  		t.treeSet[t.Name] = t
   260  		return
   261  	}
   262  	if !IsEmptyTree(t.Root) {
   263  		t.errorf("template: multiple definition of template %q", t.Name)
   264  	}
   265  }
   266  
   267  // IsEmptyTree reports whether this tree (node) is empty of everything but space or comments.
   268  func IsEmptyTree(n Node) bool {
   269  	switch n := n.(type) {
   270  	case nil:
   271  		return true
   272  	case *ActionNode:
   273  	case *CommentNode:
   274  		return true
   275  	case *IfNode:
   276  	case *ListNode:
   277  		for _, node := range n.Nodes {
   278  			if !IsEmptyTree(node) {
   279  				return false
   280  			}
   281  		}
   282  		return true
   283  	case *RangeNode:
   284  	case *TemplateNode:
   285  	case *TextNode:
   286  		return len(bytes.TrimSpace(n.Text)) == 0
   287  	case *WithNode:
   288  	default:
   289  		panic("unknown node: " + n.String())
   290  	}
   291  	return false
   292  }
   293  
   294  // parse is the top-level parser for a template, essentially the same
   295  // as itemList except it also parses {{define}} actions.
   296  // It runs to EOF.
   297  func (t *Tree) parse() {
   298  	t.Root = t.newList(t.peek().pos)
   299  	for t.peek().typ != itemEOF {
   300  		if t.peek().typ == itemLeftDelim {
   301  			delim := t.next()
   302  			if t.nextNonSpace().typ == itemDefine {
   303  				newT := New("definition") // name will be updated once we know it.
   304  				newT.text = t.text
   305  				newT.Mode = t.Mode
   306  				newT.ParseName = t.ParseName
   307  				newT.startParse(t.funcs, t.lex, t.treeSet)
   308  				newT.parseDefinition()
   309  				continue
   310  			}
   311  			t.backup2(delim)
   312  		}
   313  		switch n := t.textOrAction(); n.Type() {
   314  		case nodeEnd, nodeElse:
   315  			t.errorf("unexpected %s", n)
   316  		default:
   317  			t.Root.append(n)
   318  		}
   319  	}
   320  }
   321  
   322  // parseDefinition parses a {{define}} ...  {{end}} template definition and
   323  // installs the definition in t.treeSet. The "define" keyword has already
   324  // been scanned.
   325  func (t *Tree) parseDefinition() {
   326  	const context = "define clause"
   327  	name := t.expectOneOf(itemString, itemRawString, context)
   328  	var err error
   329  	t.Name, err = strconv.Unquote(name.val)
   330  	if err != nil {
   331  		t.error(err)
   332  	}
   333  	t.expect(itemRightDelim, context)
   334  	var end Node
   335  	t.Root, end = t.itemList()
   336  	if end.Type() != nodeEnd {
   337  		t.errorf("unexpected %s in %s", end, context)
   338  	}
   339  	t.add()
   340  	t.stopParse()
   341  }
   342  
   343  // itemList:
   344  //	textOrAction*
   345  // Terminates at {{end}} or {{else}}, returned separately.
   346  func (t *Tree) itemList() (list *ListNode, next Node) {
   347  	list = t.newList(t.peekNonSpace().pos)
   348  	for t.peekNonSpace().typ != itemEOF {
   349  		n := t.textOrAction()
   350  		switch n.Type() {
   351  		case nodeEnd, nodeElse:
   352  			return list, n
   353  		}
   354  		list.append(n)
   355  	}
   356  	t.errorf("unexpected EOF")
   357  	return
   358  }
   359  
   360  // textOrAction:
   361  //	text | comment | action
   362  func (t *Tree) textOrAction() Node {
   363  	switch token := t.nextNonSpace(); token.typ {
   364  	case itemText:
   365  		return t.newText(token.pos, token.val)
   366  	case itemLeftDelim:
   367  		t.actionLine = token.line
   368  		defer t.clearActionLine()
   369  		return t.action()
   370  	case itemComment:
   371  		return t.newComment(token.pos, token.val)
   372  	default:
   373  		t.unexpected(token, "input")
   374  	}
   375  	return nil
   376  }
   377  
   378  func (t *Tree) clearActionLine() {
   379  	t.actionLine = 0
   380  }
   381  
   382  // Action:
   383  //	control
   384  //	command ("|" command)*
   385  // Left delim is past. Now get actions.
   386  // First word could be a keyword such as range.
   387  func (t *Tree) action() (n Node) {
   388  	switch token := t.nextNonSpace(); token.typ {
   389  	case itemBlock:
   390  		return t.blockControl()
   391  	case itemBreak:
   392  		return t.breakControl(token.pos, token.line)
   393  	case itemContinue:
   394  		return t.continueControl(token.pos, token.line)
   395  	case itemElse:
   396  		return t.elseControl()
   397  	case itemEnd:
   398  		return t.endControl()
   399  	case itemIf:
   400  		return t.ifControl()
   401  	case itemRange:
   402  		return t.rangeControl()
   403  	case itemTemplate:
   404  		return t.templateControl()
   405  	case itemWith:
   406  		return t.withControl()
   407  	}
   408  	t.backup()
   409  	token := t.peek()
   410  	// Do not pop variables; they persist until "end".
   411  	return t.newAction(token.pos, token.line, t.pipeline("command", itemRightDelim))
   412  }
   413  
   414  // Break:
   415  //	{{break}}
   416  // Break keyword is past.
   417  func (t *Tree) breakControl(pos Pos, line int) Node {
   418  	if token := t.nextNonSpace(); token.typ != itemRightDelim {
   419  		t.unexpected(token, "{{break}}")
   420  	}
   421  	if t.rangeDepth == 0 {
   422  		t.errorf("{{break}} outside {{range}}")
   423  	}
   424  	return t.newBreak(pos, line)
   425  }
   426  
   427  // Continue:
   428  //	{{continue}}
   429  // Continue keyword is past.
   430  func (t *Tree) continueControl(pos Pos, line int) Node {
   431  	if token := t.nextNonSpace(); token.typ != itemRightDelim {
   432  		t.unexpected(token, "{{continue}}")
   433  	}
   434  	if t.rangeDepth == 0 {
   435  		t.errorf("{{continue}} outside {{range}}")
   436  	}
   437  	return t.newContinue(pos, line)
   438  }
   439  
   440  // Pipeline:
   441  //	declarations? command ('|' command)*
   442  func (t *Tree) pipeline(context string, end itemType) (pipe *PipeNode) {
   443  	token := t.peekNonSpace()
   444  	pipe = t.newPipeline(token.pos, token.line, nil)
   445  	// Are there declarations or assignments?
   446  decls:
   447  	if v := t.peekNonSpace(); v.typ == itemVariable {
   448  		t.next()
   449  		// Since space is a token, we need 3-token look-ahead here in the worst case:
   450  		// in "$x foo" we need to read "foo" (as opposed to ":=") to know that $x is an
   451  		// argument variable rather than a declaration. So remember the token
   452  		// adjacent to the variable so we can push it back if necessary.
   453  		tokenAfterVariable := t.peek()
   454  		next := t.peekNonSpace()
   455  		switch {
   456  		case next.typ == itemAssign, next.typ == itemDeclare:
   457  			pipe.IsAssign = next.typ == itemAssign
   458  			t.nextNonSpace()
   459  			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
   460  			t.vars = append(t.vars, v.val)
   461  		case next.typ == itemChar && next.val == ",":
   462  			t.nextNonSpace()
   463  			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
   464  			t.vars = append(t.vars, v.val)
   465  			if context == "range" && len(pipe.Decl) < 2 {
   466  				switch t.peekNonSpace().typ {
   467  				case itemVariable, itemRightDelim, itemRightParen:
   468  					// second initialized variable in a range pipeline
   469  					goto decls
   470  				default:
   471  					t.errorf("range can only initialize variables")
   472  				}
   473  			}
   474  			t.errorf("too many declarations in %s", context)
   475  		case tokenAfterVariable.typ == itemSpace:
   476  			t.backup3(v, tokenAfterVariable)
   477  		default:
   478  			t.backup2(v)
   479  		}
   480  	}
   481  	for {
   482  		switch token := t.nextNonSpace(); token.typ {
   483  		case end:
   484  			// At this point, the pipeline is complete
   485  			t.checkPipeline(pipe, context)
   486  			return
   487  		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
   488  			itemNumber, itemNil, itemRawString, itemString, itemVariable, itemLeftParen:
   489  			t.backup()
   490  			pipe.append(t.command())
   491  		default:
   492  			t.unexpected(token, context)
   493  		}
   494  	}
   495  }
   496  
   497  func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
   498  	// Reject empty pipelines
   499  	if len(pipe.Cmds) == 0 {
   500  		t.errorf("missing value for %s", context)
   501  	}
   502  	// Only the first command of a pipeline can start with a non executable operand
   503  	for i, c := range pipe.Cmds[1:] {
   504  		switch c.Args[0].Type() {
   505  		case NodeBool, NodeDot, NodeNil, NodeNumber, NodeString:
   506  			// With A|B|C, pipeline stage 2 is B
   507  			t.errorf("non executable command in pipeline stage %d", i+2)
   508  		}
   509  	}
   510  }
   511  
   512  func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
   513  	defer t.popVars(len(t.vars))
   514  	pipe = t.pipeline(context, itemRightDelim)
   515  	if context == "range" {
   516  		t.rangeDepth++
   517  	}
   518  	var next Node
   519  	list, next = t.itemList()
   520  	if context == "range" {
   521  		t.rangeDepth--
   522  	}
   523  	switch next.Type() {
   524  	case nodeEnd: //done
   525  	case nodeElse:
   526  		if allowElseIf {
   527  			// Special case for "else if". If the "else" is followed immediately by an "if",
   528  			// the elseControl will have left the "if" token pending. Treat
   529  			//	{{if a}}_{{else if b}}_{{end}}
   530  			// as
   531  			//	{{if a}}_{{else}}{{if b}}_{{end}}{{end}}.
   532  			// To do this, parse the if as usual and stop at it {{end}}; the subsequent{{end}}
   533  			// is assumed. This technique works even for long if-else-if chains.
   534  			// TODO: Should we allow else-if in with and range?
   535  			if t.peek().typ == itemIf {
   536  				t.next() // Consume the "if" token.
   537  				elseList = t.newList(next.Position())
   538  				elseList.append(t.ifControl())
   539  				// Do not consume the next item - only one {{end}} required.
   540  				break
   541  			}
   542  		}
   543  		elseList, next = t.itemList()
   544  		if next.Type() != nodeEnd {
   545  			t.errorf("expected end; found %s", next)
   546  		}
   547  	}
   548  	return pipe.Position(), pipe.Line, pipe, list, elseList
   549  }
   550  
   551  // If:
   552  //	{{if pipeline}} itemList {{end}}
   553  //	{{if pipeline}} itemList {{else}} itemList {{end}}
   554  // If keyword is past.
   555  func (t *Tree) ifControl() Node {
   556  	return t.newIf(t.parseControl(true, "if"))
   557  }
   558  
   559  // Range:
   560  //	{{range pipeline}} itemList {{end}}
   561  //	{{range pipeline}} itemList {{else}} itemList {{end}}
   562  // Range keyword is past.
   563  func (t *Tree) rangeControl() Node {
   564  	r := t.newRange(t.parseControl(false, "range"))
   565  	return r
   566  }
   567  
   568  // With:
   569  //	{{with pipeline}} itemList {{end}}
   570  //	{{with pipeline}} itemList {{else}} itemList {{end}}
   571  // If keyword is past.
   572  func (t *Tree) withControl() Node {
   573  	return t.newWith(t.parseControl(false, "with"))
   574  }
   575  
   576  // End:
   577  //	{{end}}
   578  // End keyword is past.
   579  func (t *Tree) endControl() Node {
   580  	return t.newEnd(t.expect(itemRightDelim, "end").pos)
   581  }
   582  
   583  // Else:
   584  //	{{else}}
   585  // Else keyword is past.
   586  func (t *Tree) elseControl() Node {
   587  	// Special case for "else if".
   588  	peek := t.peekNonSpace()
   589  	if peek.typ == itemIf {
   590  		// We see "{{else if ... " but in effect rewrite it to {{else}}{{if ... ".
   591  		return t.newElse(peek.pos, peek.line)
   592  	}
   593  	token := t.expect(itemRightDelim, "else")
   594  	return t.newElse(token.pos, token.line)
   595  }
   596  
   597  // Block:
   598  //	{{block stringValue pipeline}}
   599  // Block keyword is past.
   600  // The name must be something that can evaluate to a string.
   601  // The pipeline is mandatory.
   602  func (t *Tree) blockControl() Node {
   603  	const context = "block clause"
   604  
   605  	token := t.nextNonSpace()
   606  	name := t.parseTemplateName(token, context)
   607  	pipe := t.pipeline(context, itemRightDelim)
   608  
   609  	block := New(name) // name will be updated once we know it.
   610  	block.text = t.text
   611  	block.Mode = t.Mode
   612  	block.ParseName = t.ParseName
   613  	block.startParse(t.funcs, t.lex, t.treeSet)
   614  	var end Node
   615  	block.Root, end = block.itemList()
   616  	if end.Type() != nodeEnd {
   617  		t.errorf("unexpected %s in %s", end, context)
   618  	}
   619  	block.add()
   620  	block.stopParse()
   621  
   622  	return t.newTemplate(token.pos, token.line, name, pipe)
   623  }
   624  
   625  // Template:
   626  //	{{template stringValue pipeline}}
   627  // Template keyword is past. The name must be something that can evaluate
   628  // to a string.
   629  func (t *Tree) templateControl() Node {
   630  	const context = "template clause"
   631  	token := t.nextNonSpace()
   632  	name := t.parseTemplateName(token, context)
   633  	var pipe *PipeNode
   634  	if t.nextNonSpace().typ != itemRightDelim {
   635  		t.backup()
   636  		// Do not pop variables; they persist until "end".
   637  		pipe = t.pipeline(context, itemRightDelim)
   638  	}
   639  	return t.newTemplate(token.pos, token.line, name, pipe)
   640  }
   641  
   642  func (t *Tree) parseTemplateName(token item, context string) (name string) {
   643  	switch token.typ {
   644  	case itemString, itemRawString:
   645  		s, err := strconv.Unquote(token.val)
   646  		if err != nil {
   647  			t.error(err)
   648  		}
   649  		name = s
   650  	default:
   651  		t.unexpected(token, context)
   652  	}
   653  	return
   654  }
   655  
   656  // command:
   657  //	operand (space operand)*
   658  // space-separated arguments up to a pipeline character or right delimiter.
   659  // we consume the pipe character but leave the right delim to terminate the action.
   660  func (t *Tree) command() *CommandNode {
   661  	cmd := t.newCommand(t.peekNonSpace().pos)
   662  	for {
   663  		t.peekNonSpace() // skip leading spaces.
   664  		operand := t.operand()
   665  		if operand != nil {
   666  			cmd.append(operand)
   667  		}
   668  		switch token := t.next(); token.typ {
   669  		case itemSpace:
   670  			continue
   671  		case itemRightDelim, itemRightParen:
   672  			t.backup()
   673  		case itemPipe:
   674  			// nothing here; break loop below
   675  		default:
   676  			t.unexpected(token, "operand")
   677  		}
   678  		break
   679  	}
   680  	if len(cmd.Args) == 0 {
   681  		t.errorf("empty command")
   682  	}
   683  	return cmd
   684  }
   685  
   686  // operand:
   687  //	term .Field*
   688  // An operand is a space-separated component of a command,
   689  // a term possibly followed by field accesses.
   690  // A nil return means the next item is not an operand.
   691  func (t *Tree) operand() Node {
   692  	node := t.term()
   693  	if node == nil {
   694  		return nil
   695  	}
   696  	if t.peek().typ == itemField {
   697  		chain := t.newChain(t.peek().pos, node)
   698  		for t.peek().typ == itemField {
   699  			chain.Add(t.next().val)
   700  		}
   701  		// Compatibility with original API: If the term is of type NodeField
   702  		// or NodeVariable, just put more fields on the original.
   703  		// Otherwise, keep the Chain node.
   704  		// Obvious parsing errors involving literal values are detected here.
   705  		// More complex error cases will have to be handled at execution time.
   706  		switch node.Type() {
   707  		case NodeField:
   708  			node = t.newField(chain.Position(), chain.String())
   709  		case NodeVariable:
   710  			node = t.newVariable(chain.Position(), chain.String())
   711  		case NodeBool, NodeString, NodeNumber, NodeNil, NodeDot:
   712  			t.errorf("unexpected . after term %q", node.String())
   713  		default:
   714  			node = chain
   715  		}
   716  	}
   717  	return node
   718  }
   719  
   720  // term:
   721  //	literal (number, string, nil, boolean)
   722  //	function (identifier)
   723  //	.
   724  //	.Field
   725  //	$
   726  //	'(' pipeline ')'
   727  // A term is a simple "expression".
   728  // A nil return means the next item is not a term.
   729  func (t *Tree) term() Node {
   730  	switch token := t.nextNonSpace(); token.typ {
   731  	case itemIdentifier:
   732  		checkFunc := t.Mode&SkipFuncCheck == 0
   733  		if checkFunc && !t.hasFunction(token.val) {
   734  			t.errorf("function %q not defined", token.val)
   735  		}
   736  		return NewIdentifier(token.val).SetTree(t).SetPos(token.pos)
   737  	case itemDot:
   738  		return t.newDot(token.pos)
   739  	case itemNil:
   740  		return t.newNil(token.pos)
   741  	case itemVariable:
   742  		return t.useVar(token.pos, token.val)
   743  	case itemField:
   744  		return t.newField(token.pos, token.val)
   745  	case itemBool:
   746  		return t.newBool(token.pos, token.val == "true")
   747  	case itemCharConstant, itemComplex, itemNumber:
   748  		number, err := t.newNumber(token.pos, token.val, token.typ)
   749  		if err != nil {
   750  			t.error(err)
   751  		}
   752  		return number
   753  	case itemLeftParen:
   754  		return t.pipeline("parenthesized pipeline", itemRightParen)
   755  	case itemString, itemRawString:
   756  		s, err := strconv.Unquote(token.val)
   757  		if err != nil {
   758  			t.error(err)
   759  		}
   760  		return t.newString(token.pos, token.val, s)
   761  	}
   762  	t.backup()
   763  	return nil
   764  }
   765  
   766  // hasFunction reports if a function name exists in the Tree's maps.
   767  func (t *Tree) hasFunction(name string) bool {
   768  	for _, funcMap := range t.funcs {
   769  		if funcMap == nil {
   770  			continue
   771  		}
   772  		if funcMap[name] != nil {
   773  			return true
   774  		}
   775  	}
   776  	return false
   777  }
   778  
   779  // popVars trims the variable list to the specified length
   780  func (t *Tree) popVars(n int) {
   781  	t.vars = t.vars[:n]
   782  }
   783  
   784  // useVar returns a node for a variable reference. It errors if the
   785  // variable is not defined.
   786  func (t *Tree) useVar(pos Pos, name string) Node {
   787  	v := t.newVariable(pos, name)
   788  	for _, varName := range t.vars {
   789  		if varName == v.Ident[0] {
   790  			return v
   791  		}
   792  	}
   793  	t.errorf("undefined variable %q", v.Ident[0])
   794  	return nil
   795  }
   796
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