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ast.go
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ast.go
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// Copyright 2019 The Scriggo Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package ast declares the types used to define program and template trees.
//
// For example, the source in a template file named "articles.html":
//
// {% for article in articles %}
// <div>{{ article.Title }}</div>
// {% end %}
//
// is represented with the tree:
//
// ast.NewTree("articles.txt", []ast.Node{
// ast.NewForIn(
// &ast.Position{Line: 1, Column: 1, Start: 0, End: 69},
// ast.NewIdentifier(&ast.Position{Line: 1, Column: 8, Start: 7, End: 13}, "article"),
// ast.NewIdentifier(&ast.Position{Line: 1, Column: 19, Start: 18, End: 25}, "articles"),
// []ast.Node{
// ast.NewText(&ast.Position{Line: 1, Column: 30, Start: 29, End: 34}, []byte("\n<div>"), ast.Cut{1, 0}),
// ast.NewShow(
// &ast.Position{Line: 2, Column: 6, Start: 35, End: 53},
// []ast.Expression{
// ast.NewSelector(
// &ast.Position{Line: 2, Column: 16, Start: 38, End: 50},
// ast.NewIdentifier(&ast.Position{Line: 2, Column: 9, Start: 38, End: 44}, "article"),
// "Title"),
// },
// ast.ContextHTML),
// ast.NewText(&ast.Position{Line: 2, Column: 25, Start: 54, End: 60}, []byte("</div>\n"), ast.Cut{}),
// },
// ),
// }, ast.FormatHTML)
package ast
import (
"fmt"
"reflect"
"strconv"
"strings"
)
// expandedPrint is set to true in tests to print completely a composite
// literal.
var expandedPrint = false
// OperatorType represents an operator type in a unary and binary expression.
type OperatorType int
// Operators.
const (
OperatorEqual OperatorType = iota // ==
OperatorNotEqual // !=
OperatorLess // <
OperatorLessEqual // <=
OperatorGreater // >
OperatorGreaterEqual // >=
OperatorNot // !
OperatorBitAnd // &
OperatorBitOr // |
OperatorAnd // &&
OperatorOr // ||
OperatorAddition // +
OperatorSubtraction // -
OperatorMultiplication // *
OperatorDivision // /
OperatorModulo // %
OperatorXor // ^
OperatorAndNot // &^
OperatorLeftShift // <<
OperatorRightShift // >>
OperatorContains // contains
OperatorNotContains // not contains
OperatorReceive // <-
OperatorAddress // &
OperatorPointer // *
OperatorExtendedAnd // and
OperatorExtendedOr // or
OperatorExtendedNot // not
)
// String returns the string representation of the operator type.
func (op OperatorType) String() string {
// The last empty operators are compiler.internalOperatorZero and
// compiler.internalOperatorNotZero.
return []string{"==", "!=", "<", "<=", ">", ">=", "!", "&", "|", "&&", "||",
"+", "-", "*", "/", "%", "^", "&^", "<<", ">>", "contains", "not contains",
"<-", "&", "*", "and", "or", "not", "", ""}[op]
}
// AssignmentType represents a type of assignment.
type AssignmentType int
const (
AssignmentSimple AssignmentType = iota // =
AssignmentDeclaration // :=
AssignmentAddition // +=
AssignmentSubtraction // -=
AssignmentMultiplication // *=
AssignmentDivision // /=
AssignmentModulo // %=
AssignmentAnd // &=
AssignmentOr // |=
AssignmentXor // ^=
AssignmentAndNot // &^=
AssignmentLeftShift // <<=
AssignmentRightShift // >>=
AssignmentIncrement // ++
AssignmentDecrement // --
)
// A Format represents a content format.
type Format int
// Formats.
const (
FormatText Format = iota
FormatHTML
FormatCSS
FormatJS
FormatJSON
FormatMarkdown
)
// String returns the name of the format.
func (format Format) String() string {
switch format {
case FormatText:
return "text"
case FormatHTML:
return "HTML"
case FormatCSS:
return "CSS"
case FormatJS:
return "JavaScript"
case FormatJSON:
return "JSON"
case FormatMarkdown:
return "Markdown"
}
panic("invalid format")
}
// Context indicates the context in which a show statement is evaluated.
type Context int
const (
ContextText Context = iota
ContextHTML
ContextCSS
ContextJS
ContextJSON
ContextMarkdown
ContextTag
ContextQuotedAttr
ContextUnquotedAttr
ContextCSSString
ContextJSString
ContextJSONString
ContextTabCodeBlock
ContextSpacesCodeBlock
)
// String returns the name of the context.
func (ctx Context) String() string {
switch ctx {
case ContextText:
return "text"
case ContextHTML:
return "HTML"
case ContextCSS:
return "CSS"
case ContextJS:
return "JavaScript"
case ContextJSON:
return "JSON"
case ContextMarkdown:
return "Markdown"
case ContextTag:
return "tag"
case ContextQuotedAttr:
return "quoted attribute"
case ContextUnquotedAttr:
return "unquoted attribute"
case ContextCSSString:
return "CSS string"
case ContextJSString:
return "JavaScript string"
case ContextJSONString:
return "JSON string"
case ContextTabCodeBlock:
return "tab code block"
case ContextSpacesCodeBlock:
return "spaces code block"
}
panic("invalid context")
}
// LiteralType represents the type of a literal.
type LiteralType int
const (
StringLiteral LiteralType = iota
RuneLiteral
IntLiteral
FloatLiteral
ImaginaryLiteral
)
// ChanDirection represents the direction of a channel type.
type ChanDirection int
const (
NoDirection ChanDirection = iota
ReceiveDirection
SendDirection
)
var directionString = [3]string{"no direction", "receive", "send"}
// String returns the name of the direction dir or "no direction" if there is
// no direction.
func (dir ChanDirection) String() string { return directionString[dir] }
// Node is a node of the tree.
type Node interface {
Pos() *Position // position in the original source
}
// Position is a position of a node in the source.
type Position struct {
Line int // line starting from 1
Column int // column in characters starting from 1
Start int // index of the first byte
End int // index of the last byte
}
// Pos returns the position p.
func (p *Position) Pos() *Position {
return p
}
// String returns the line and column separated by a colon, for example "37:18".
func (p Position) String() string {
return strconv.Itoa(p.Line) + ":" + strconv.Itoa(p.Column)
}
// WithEnd returns a copy of the position but with the given end index.
func (p *Position) WithEnd(end int) *Position {
pp := *p
pp.End = end
return &pp
}
// Operator represents an operator expression. It is implemented by the
// UnaryOperator and BinaryOperator nodes.
type Operator interface {
Expression
Operator() OperatorType
Precedence() int
}
// Upvar represents a variable defined outside function body. Even package level
// variables (native or not) are considered upvars.
type Upvar struct {
// NativePkg is the name of the native package which holds a native Upvar.
// If Upvar is not a native Upvar then NativeName is an empty string.
NativePkg string
// NativeName is the name of the native declaration of a native Upvar. If
// Upvar is not a native Upvar then NativeName is an empty string.
NativeName string
// NativeValue is the value of the native variable Upvar. If Upvar is not
// native then Upvar is nil.
NativeValue *reflect.Value
// NativeValueType, in case of Upvar refers to a native variable, contains
// the type of such variable. If not then NativeValueType is nil.
//
// NativeValueType is necessary because the type cannot be stored into
// the NativeValue, as if the upvar is not initialized in the compiler
// then NativeValue contains an invalid reflect.Value.
NativeValueType reflect.Type
// Declaration is the ast node where Upvar is defined. If Upvar is a
// native var then Declaration is nil.
Declaration Node
}
// Expression node represents an expression.
type Expression interface {
Parenthesis() int
SetParenthesis(int)
Node
String() string
}
// expression represents an expression.
type expression struct {
parenthesis int
}
// Parenthesis returns the number of parenthesis around the expression.
func (e *expression) Parenthesis() int {
return e.parenthesis
}
// SetParenthesis sets the number of parenthesis around the expression.
func (e *expression) SetParenthesis(n int) {
e.parenthesis = n
}
// StringWithParenthesis returns the string representation of the given
// expression, surrounding it by parenthesis if necessary.
func StringWithParenthesis(expr Expression) string {
s := expr.String()
n := expr.Parenthesis()
return strings.Repeat("(", n) + s + strings.Repeat(")", n)
}
// Cut indicates, in a Text node, how many bytes should be cut from the left
// and the right of the text before rendering the Text node.
type Cut struct {
Left int
Right int
}
// ArrayType node represents an array type.
type ArrayType struct {
*expression
*Position // position in the source.
Len Expression // length. It is nil for arrays specified with ... notation.
ElementType Expression // element type.
}
// NewArrayType returns a new ArrayType node.
func NewArrayType(pos *Position, len Expression, elementType Expression) *ArrayType {
return &ArrayType{&expression{}, pos, len, elementType}
}
// String returns the string representation of n.
func (n *ArrayType) String() string {
s := "["
if n.Len == nil {
s += "..."
} else {
s += n.Len.String()
}
s += "]" + n.ElementType.String()
return s
}
// Assignment node represents an assignment statement.
type Assignment struct {
*Position // position in the source.
Lhs []Expression // left-hand variables.
Type AssignmentType // type.
Rhs []Expression // assigned values (nil for increment and decrement).
}
// NewAssignment returns a new Assignment node.
func NewAssignment(pos *Position, lhs []Expression, typ AssignmentType, rhs []Expression) *Assignment {
return &Assignment{pos, lhs, typ, rhs}
}
// String returns the string representation of n.
func (n *Assignment) String() string {
var s string
for i, v := range n.Lhs {
if i > 0 {
s += ", "
}
s += v.String()
}
switch n.Type {
case AssignmentSimple:
s += " = "
case AssignmentDeclaration:
s += " := "
case AssignmentAddition:
s += " += "
case AssignmentSubtraction:
s += " -= "
case AssignmentMultiplication:
s += " *= "
case AssignmentDivision:
s += " /= "
case AssignmentModulo:
s += " %= "
case AssignmentIncrement:
s += "++"
case AssignmentDecrement:
s += "--"
}
if n.Rhs != nil {
for i, value := range n.Rhs {
if i > 0 {
s += ", "
}
s += value.String()
}
}
return s
}
// BasicLiteral represents integer, floating-point, imaginary, rune and
// string literals.
type BasicLiteral struct {
expression
*Position // position in the source.
Type LiteralType // type.
Value string // value.
}
// NewBasicLiteral returns a new BasicLiteral node.
func NewBasicLiteral(pos *Position, typ LiteralType, value string) *BasicLiteral {
return &BasicLiteral{expression{}, pos, typ, value}
}
// String returns the string representation of n.
func (n *BasicLiteral) String() string {
return n.Value
}
// BinaryOperator node represents a binary operator expression.
type BinaryOperator struct {
*expression
*Position // position in the source.
Op OperatorType // operator.
Expr1 Expression // first expression.
Expr2 Expression // second expression.
}
// NewBinaryOperator returns a new binary operator.
func NewBinaryOperator(pos *Position, op OperatorType, expr1, expr2 Expression) *BinaryOperator {
return &BinaryOperator{&expression{}, pos, op, expr1, expr2}
}
// String returns the string representation of n.
func (n *BinaryOperator) String() string {
var s string
if e, ok := n.Expr1.(Operator); ok && e.Precedence() <= n.Precedence() {
s += "(" + n.Expr1.String() + ")"
} else {
s += n.Expr1.String()
}
s += " " + n.Op.String() + " "
if e, ok := n.Expr2.(Operator); ok && e.Precedence() <= n.Precedence() {
s += "(" + n.Expr2.String() + ")"
} else {
s += n.Expr2.String()
}
return s
}
// Operator returns the operator type of the expression.
func (n *BinaryOperator) Operator() OperatorType {
return n.Op
}
// Precedence returns a number that represents the precedence of the
// expression.
func (n *BinaryOperator) Precedence() int {
switch n.Op {
case OperatorMultiplication, OperatorDivision, OperatorModulo,
OperatorLeftShift, OperatorRightShift, OperatorBitAnd, OperatorAndNot:
return 5
case OperatorAddition, OperatorSubtraction, OperatorBitOr, OperatorXor:
return 4
case OperatorEqual, OperatorNotEqual, OperatorLess, OperatorLessEqual,
OperatorGreater, OperatorGreaterEqual, OperatorContains, OperatorNotContains:
return 3
case OperatorAnd, OperatorExtendedAnd:
return 2
case OperatorOr, OperatorExtendedOr:
return 1
}
panic("invalid operator type")
}
// Block node represents a block with his own scope.
type Block struct {
*Position
Nodes []Node
}
// NewBlock returns a new block statement.
func NewBlock(pos *Position, nodes []Node) *Block {
return &Block{pos, nodes}
}
// String returns the string representation of n.
func (n *Block) String() string {
return "block statement"
}
// Break node represents a "break" statement.
type Break struct {
*Position // position in the source.
Label *Identifier // label.
}
// NewBreak returns a new Break node.
func NewBreak(pos *Position, label *Identifier) *Break {
return &Break{pos, label}
}
// Call node represents a function call expression.
type Call struct {
*expression
*Position // position in the source.
Func Expression // function.
Args []Expression // arguments.
IsVariadic bool // reports whether it is variadic.
IR struct {
// AppendArg1, in transformed calls to the builtin function 'append',
// is the argument with index 1.
AppendArg1 *Call
}
}
// NewCall returns a new Call node.
func NewCall(pos *Position, fun Expression, args []Expression, isVariadic bool) *Call {
return &Call{expression: &expression{}, Position: pos, Func: fun, Args: args, IsVariadic: isVariadic}
}
// String returns the string representation of n.
func (n *Call) String() string {
s := n.Func.String()
switch fn := n.Func.(type) {
case *UnaryOperator:
if fn.Op == OperatorPointer || fn.Op == OperatorReceive {
s = "(" + s + ")"
}
case *FuncType:
if len(fn.Result) == 0 {
s = "(" + s + ")"
}
case *ChanType:
s = "(" + s + ")"
}
s += "("
for i, arg := range n.Args {
if i > 0 {
s += ", "
}
s += arg.String()
}
if n.IsVariadic {
s += "..."
}
s += ")"
return s
}
// Case node represents "case" and "default" statements.
type Case struct {
*Position
Expressions []Expression
Body []Node
}
// NewCase returns a new Case node.
func NewCase(pos *Position, expressions []Expression, body []Node) *Case {
return &Case{pos, expressions, body}
}
// ChanType node represents a chan type.
type ChanType struct {
*expression
*Position // position in the source.
Direction ChanDirection // direction.
ElementType Expression // type of chan elements.
}
// NewChanType returns a new ChanType node.
func NewChanType(pos *Position, direction ChanDirection, elementType Expression) *ChanType {
return &ChanType{&expression{}, pos, direction, elementType}
}
// String returns the string representation of n.
func (n *ChanType) String() string {
var s string
if n.Direction == ReceiveDirection {
s = "<-"
}
s += "chan"
if n.Direction == SendDirection {
s += "<-"
}
return s + " " + n.ElementType.String()
}
// Comment node represents a comment statement in the form {# ... #}.
type Comment struct {
*Position // position in the source.
Text string // comment text.
}
// NewComment returns a new Comment node.
func NewComment(pos *Position, text string) *Comment {
return &Comment{pos, text}
}
// CompositeLiteral node represents a composite literal.
type CompositeLiteral struct {
*expression
*Position // position in the source.
Type Expression // type of the composite literal. nil for composite literals without type.
KeyValues []KeyValue // nil for empty composite literals.
}
// NewCompositeLiteral returns a new CompositeLiteral node.
func NewCompositeLiteral(pos *Position, typ Expression, keyValues []KeyValue) *CompositeLiteral {
return &CompositeLiteral{&expression{}, pos, typ, keyValues}
}
// String returns the string representation of n.
func (n *CompositeLiteral) String() string {
s := n.Type.String()
if expandedPrint {
s += "{"
for i, kv := range n.KeyValues {
if i > 0 {
s += ", "
}
s += kv.String()
}
s += "}"
return s
}
if len(n.KeyValues) > 0 {
return s + "{...}"
}
return s + "{}"
}
// Const node represents a "const" declaration.
type Const struct {
*Position // position in the source.
Lhs []*Identifier // left-hand side identifiers.
Type Expression // nil for non-typed constant declarations.
Rhs []Expression // nil for implicit-value constant declarations.
Index int // index of the declaration in the constant declaration group or 0 if not in a group.
}
// NewConst returns a new Const node.
func NewConst(pos *Position, lhs []*Identifier, typ Expression, rhs []Expression, index int) *Const {
return &Const{pos, lhs, typ, rhs, index}
}
// Continue node represents a "continue" statement.
type Continue struct {
*Position // position in the source.
Label *Identifier // label.
}
// NewContinue returns a new Continue node.
func NewContinue(pos *Position, label *Identifier) *Continue {
return &Continue{pos, label}
}
// Default node represents a default expression.
type Default struct {
expression
*Position // position in the source.
Expr1 Expression // left hand expression.
Expr2 Expression // right hand expression.
}
// NewDefault returns a new Default node.
func NewDefault(pos *Position, expr1, expr2 Expression) *Default {
return &Default{Position: pos, Expr1: expr1, Expr2: expr2}
}
// String returns the string representation of n.
func (n *Default) String() string {
return n.Expr1.String() + " default " + n.Expr2.String()
}
// Defer node represents a "defer" statement.
type Defer struct {
*Position // position in the source.
Call Expression // function or method call (should be a Call node).
}
// NewDefer returns a new Defer node.
func NewDefer(pos *Position, call Expression) *Defer {
return &Defer{pos, call}
}
// String returns the string representation of n.
func (n *Defer) String() string {
return "defer " + n.Call.String()
}
// Extends node represents an "extends" declaration.
type Extends struct {
*Position // position in the source.
Path string // path to extend.
Format Format // format.
Tree *Tree // expanded tree of extends.
}
// NewExtends returns a new Extends node.
func NewExtends(pos *Position, path string, format Format) *Extends {
return &Extends{Position: pos, Path: path, Format: format}
}
// String returns the string representation of n.
func (n *Extends) String() string {
return fmt.Sprintf("extends %v", strconv.Quote(n.Path))
}
// Fallthrough node represents a "fallthrough" statement.
type Fallthrough struct {
*Position
}
// NewFallthrough returns a new Fallthrough node.
func NewFallthrough(pos *Position) *Fallthrough {
return &Fallthrough{pos}
}
// Field represents a field declaration in a struct type. A field
// declaration can be explicit (having an identifier list and a type) or
// implicit (having a type only).
type Field struct {
Idents []*Identifier // identifiers. If nil is an embedded field.
Type Expression
Tag string
}
// NewField returns a new Field node.
func NewField(idents []*Identifier, typ Expression, tag string) *Field {
return &Field{idents, typ, tag}
}
// String returns the string representation of n.
func (n *Field) String() string {
s := ""
for i, ident := range n.Idents {
s += ident.String()
if i != len(n.Idents)-1 {
s += ","
}
s += " "
}
s += n.Type.String()
if n.Tag != "" {
s += " `" + n.Tag + "`"
}
return s
}
// For node represents a "for" statement.
type For struct {
*Position // position in the source.
Init Node // initialization statement.
Condition Expression // condition expression.
Post Node // post iteration statement.
Body []Node // nodes of the body.
}
// NewFor returns a new For node.
func NewFor(pos *Position, init Node, condition Expression, post Node, body []Node) *For {
if body == nil {
body = []Node{}
}
return &For{pos, init, condition, post, body}
}
// ForIn node represents a "for in" statement.
type ForIn struct {
*Position // position in the source.
Ident *Identifier // identifier.
Expr Expression // range expression.
Body []Node // nodes of the body.
Else *Block // nodes to run if the body is not executed.
}
// NewForIn represents a new ForIn node.
func NewForIn(pos *Position, ident *Identifier, expr Expression, body []Node, els *Block) *ForIn {
if body == nil {
body = []Node{}
}
return &ForIn{pos, ident, expr, body, els}
}
// ForRange node represents the "for range" statement.
type ForRange struct {
*Position // position in the source.
Assignment *Assignment // assignment.
Body []Node // nodes of the body.
Else *Block // nodes to run if the body is not executed.
}
// NewForRange returns a new ForRange node.
func NewForRange(pos *Position, assignment *Assignment, body []Node, els *Block) *ForRange {
if body == nil {
body = []Node{}
}
return &ForRange{pos, assignment, body, els}
}
// Func node represents a function declaration or literal.
type Func struct {
expression
*Position
Ident *Identifier // name, nil for function literals.
Type *FuncType // type.
Body *Block // body.
DistFree bool // reports whether it is distraction free.
Upvars []Upvar // Upvars of func.
Format Format // macro format.
}
// NewFunc returns a new Func node.
func NewFunc(pos *Position, name *Identifier, typ *FuncType, body *Block, distFree bool, format Format) *Func {
return &Func{expression{}, pos, name, typ, body, distFree, nil, format}
}
// String returns the string representation of n.
func (n *Func) String() string {
if n.Type.Macro {
return "macro declaration"
}
if n.Ident == nil {
return "func literal"
}
return "func declaration"
}
// FuncType node represents a function type.
type FuncType struct {
expression
*Position // position in the source.
Macro bool // indicates whether it is declared as macro.
Parameters []*Parameter // parameters.
Result []*Parameter // result.
IsVariadic bool // reports whether it is variadic.
Reflect reflect.Type // reflect type.
}
// NewFuncType returns a new FuncType node.
func NewFuncType(pos *Position, macro bool, parameters []*Parameter, result []*Parameter, isVariadic bool) *FuncType {
return &FuncType{expression{}, pos, macro, parameters, result, isVariadic, nil}
}
// String returns the string representation of n.
func (n *FuncType) String() string {
s := "func("
if n.Macro {
s = "macro("
}
for i, param := range n.Parameters {
if i > 0 {
s += ", "
}
s += param.String()
}
s += ")"
if len(n.Result) > 0 {
if n.Result[0].Ident == nil {
s += " " + n.Result[0].Type.String()
} else {
s += " ("
for i, res := range n.Result {
if i > 0 {
s += ", "
}
s += res.String()
}
s += ")"
}
}
return s
}
// Go node represents a "go" statement.
type Go struct {
*Position // position in the source.
Call Expression // function or method call (should be a Call node).
}
// NewGo returns a new Go node.
func NewGo(pos *Position, call Expression) *Go {
return &Go{pos, call}
}
// String returns the string representation of n.
func (n *Go) String() string {
return "go " + n.Call.String()
}
// Goto node represents a "goto" statement.
type Goto struct {
*Position // position in the source.
Label *Identifier // label.
}
// NewGoto returns a new Goto node.
func NewGoto(pos *Position, label *Identifier) *Goto {
return &Goto{pos, label}
}
// String returns the string representation of n.
func (n *Goto) String() string {
return "goto " + n.Label.String()
}
// Identifier node represents an identifier expression.
type Identifier struct {
expression
*Position // position in the source.
Name string // name.
}
// NewIdentifier returns a new Identifier node.
func NewIdentifier(pos *Position, name string) *Identifier {
return &Identifier{expression{}, pos, name}
}
// String returns the string representation of n.
func (n *Identifier) String() string {
if strings.HasPrefix(n.Name, "$itea") {
return "itea"
}
return n.Name
}
// If node represents an "if" statement.
type If struct {
*Position // position in the source.
Init Node // init simple statement.
Condition Expression // condition that once evaluated returns true or false.
Then *Block // nodes to run if the expression is evaluated to true.
Else Node // nodes to run if the expression is evaluated to false. Can be Block or If.
}
// NewIf returns a new If node.
func NewIf(pos *Position, init Node, cond Expression, then *Block, els Node) *If {
if then == nil {
then = NewBlock(nil, []Node{})
}
return &If{pos, init, cond, then, els}
}
// Import node represents a "import" declaration.
type Import struct {
*Position // position in the source.
Ident *Identifier // name (including "." and "_") or nil.
Path string // path to import.
For []*Identifier // exported identifiers that are enable for access.
Tree *Tree // expanded tree of import.
}
// NewImport returns a new Import node.
func NewImport(pos *Position, ident *Identifier, path string, forIdents []*Identifier) *Import {
return &Import{Position: pos, Ident: ident, Path: path, For: forIdents}
}
// String returns the string representation of n.
func (n *Import) String() string {
var s strings.Builder
s.WriteString("import ")
if n.Ident != nil {
s.WriteString(n.Ident.Name)
s.WriteString(" ")
}
s.WriteString(strconv.Quote(n.Path))
if n.For != nil {
s.WriteString("for ")
for i, ident := range n.For {
if i > 0 {
s.WriteString(", ")
}
s.WriteString(ident.Name)
}
}
return s.String()
}
// Index node represents an index expression.
type Index struct {
*expression
*Position // position in the source.
Expr Expression // expression.
Index Expression // index.
}
// NewIndex returns a new Index node.
func NewIndex(pos *Position, expr Expression, index Expression) *Index {
return &Index{&expression{}, pos, expr, index}
}