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parser.go
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2782 lines (2423 loc) · 60.6 KB
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// Copyright 2016 The Go 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 syntax
import (
"fmt"
"io"
"strconv"
"strings"
)
const debug = false
const trace = false
type parser struct {
file *PosBase
errh ErrorHandler
mode Mode
pragh PragmaHandler
scanner
base *PosBase // current position base
first error // first error encountered
errcnt int // number of errors encountered
pragma Pragma // pragmas
fnest int // function nesting level (for error handling)
xnest int // expression nesting level (for complit ambiguity resolution)
indent []byte // tracing support
}
func (p *parser) init(file *PosBase, r io.Reader, errh ErrorHandler, pragh PragmaHandler, mode Mode) {
p.file = file
p.errh = errh
p.mode = mode
p.pragh = pragh
p.scanner.init(
r,
// Error and directive handler for scanner.
// Because the (line, col) positions passed to the
// handler is always at or after the current reading
// position, it is safe to use the most recent position
// base to compute the corresponding Pos value.
func(line, col uint, msg string) {
if msg[0] != '/' {
p.errorAt(p.posAt(line, col), msg)
return
}
// otherwise it must be a comment containing a line or go: directive.
// //line directives must be at the start of the line (column colbase).
// /*line*/ directives can be anywhere in the line.
text := commentText(msg)
if (col == colbase || msg[1] == '*') && strings.HasPrefix(text, "line ") {
var pos Pos // position immediately following the comment
if msg[1] == '/' {
// line comment (newline is part of the comment)
pos = MakePos(p.file, line+1, colbase)
} else {
// regular comment
// (if the comment spans multiple lines it's not
// a valid line directive and will be discarded
// by updateBase)
pos = MakePos(p.file, line, col+uint(len(msg)))
}
p.updateBase(pos, line, col+2+5, text[5:]) // +2 to skip over // or /*
return
}
// go: directive (but be conservative and test)
if pragh != nil && strings.HasPrefix(text, "go:") {
p.pragma = pragh(p.posAt(line, col+2), p.scanner.blank, text, p.pragma) // +2 to skip over // or /*
}
},
directives,
)
p.base = file
p.first = nil
p.errcnt = 0
p.pragma = nil
p.fnest = 0
p.xnest = 0
p.indent = nil
}
// takePragma returns the current parsed pragmas
// and clears them from the parser state.
func (p *parser) takePragma() Pragma {
prag := p.pragma
p.pragma = nil
return prag
}
// clearPragma is called at the end of a statement or
// other Go form that does NOT accept a pragma.
// It sends the pragma back to the pragma handler
// to be reported as unused.
func (p *parser) clearPragma() {
if p.pragma != nil {
p.pragh(p.pos(), p.scanner.blank, "", p.pragma)
p.pragma = nil
}
}
// updateBase sets the current position base to a new line base at pos.
// The base's filename, line, and column values are extracted from text
// which is positioned at (tline, tcol) (only needed for error messages).
func (p *parser) updateBase(pos Pos, tline, tcol uint, text string) {
i, n, ok := trailingDigits(text)
if i == 0 {
return // ignore (not a line directive)
}
// i > 0
if !ok {
// text has a suffix :xxx but xxx is not a number
p.errorAt(p.posAt(tline, tcol+i), "invalid line number: "+text[i:])
return
}
var line, col uint
i2, n2, ok2 := trailingDigits(text[:i-1])
if ok2 {
//line filename:line:col
i, i2 = i2, i
line, col = n2, n
if col == 0 || col > PosMax {
p.errorAt(p.posAt(tline, tcol+i2), "invalid column number: "+text[i2:])
return
}
text = text[:i2-1] // lop off ":col"
} else {
//line filename:line
line = n
}
if line == 0 || line > PosMax {
p.errorAt(p.posAt(tline, tcol+i), "invalid line number: "+text[i:])
return
}
// If we have a column (//line filename:line:col form),
// an empty filename means to use the previous filename.
filename := text[:i-1] // lop off ":line"
trimmed := false
if filename == "" && ok2 {
filename = p.base.Filename()
trimmed = p.base.Trimmed()
}
p.base = NewLineBase(pos, filename, trimmed, line, col)
}
func commentText(s string) string {
if s[:2] == "/*" {
return s[2 : len(s)-2] // lop off /* and */
}
// line comment (does not include newline)
// (on Windows, the line comment may end in \r\n)
i := len(s)
if s[i-1] == '\r' {
i--
}
return s[2:i] // lop off //, and \r at end, if any
}
func trailingDigits(text string) (uint, uint, bool) {
// Want to use LastIndexByte below but it's not defined in Go1.4 and bootstrap fails.
i := strings.LastIndex(text, ":") // look from right (Windows filenames may contain ':')
if i < 0 {
return 0, 0, false // no ":"
}
// i >= 0
n, err := strconv.ParseUint(text[i+1:], 10, 0)
return uint(i + 1), uint(n), err == nil
}
func (p *parser) got(tok token) bool {
if p.tok == tok {
p.next()
return true
}
return false
}
func (p *parser) want(tok token) {
if !p.got(tok) {
p.syntaxError("expecting " + tokstring(tok))
p.advance()
}
}
// gotAssign is like got(_Assign) but it also accepts ":="
// (and reports an error) for better parser error recovery.
func (p *parser) gotAssign() bool {
switch p.tok {
case _Define:
p.syntaxError("expecting =")
fallthrough
case _Assign:
p.next()
return true
}
return false
}
// ----------------------------------------------------------------------------
// Error handling
// posAt returns the Pos value for (line, col) and the current position base.
func (p *parser) posAt(line, col uint) Pos {
return MakePos(p.base, line, col)
}
// error reports an error at the given position.
func (p *parser) errorAt(pos Pos, msg string) {
err := Error{pos, msg}
if p.first == nil {
p.first = err
}
p.errcnt++
if p.errh == nil {
panic(p.first)
}
p.errh(err)
}
// syntaxErrorAt reports a syntax error at the given position.
func (p *parser) syntaxErrorAt(pos Pos, msg string) {
if trace {
p.print("syntax error: " + msg)
}
if p.tok == _EOF && p.first != nil {
return // avoid meaningless follow-up errors
}
// add punctuation etc. as needed to msg
switch {
case msg == "":
// nothing to do
case strings.HasPrefix(msg, "in "), strings.HasPrefix(msg, "at "), strings.HasPrefix(msg, "after "):
msg = " " + msg
case strings.HasPrefix(msg, "expecting "):
msg = ", " + msg
default:
// plain error - we don't care about current token
p.errorAt(pos, "syntax error: "+msg)
return
}
// determine token string
var tok string
switch p.tok {
case _Name, _Semi:
tok = p.lit
case _Literal:
tok = "literal " + p.lit
case _Operator:
tok = p.op.String()
case _AssignOp:
tok = p.op.String() + "="
case _IncOp:
tok = p.op.String()
tok += tok
default:
tok = tokstring(p.tok)
}
p.errorAt(pos, "syntax error: unexpected "+tok+msg)
}
// tokstring returns the English word for selected punctuation tokens
// for more readable error messages. Use tokstring (not tok.String())
// for user-facing (error) messages; use tok.String() for debugging
// output.
func tokstring(tok token) string {
switch tok {
case _Comma:
return "comma"
case _Semi:
return "semicolon or newline"
}
return tok.String()
}
// Convenience methods using the current token position.
func (p *parser) pos() Pos { return p.posAt(p.line, p.col) }
func (p *parser) error(msg string) { p.errorAt(p.pos(), msg) }
func (p *parser) syntaxError(msg string) { p.syntaxErrorAt(p.pos(), msg) }
// The stopset contains keywords that start a statement.
// They are good synchronization points in case of syntax
// errors and (usually) shouldn't be skipped over.
const stopset uint64 = 1<<_Break |
1<<_Const |
1<<_Continue |
1<<_Defer |
1<<_Fallthrough |
1<<_For |
1<<_Go |
1<<_Goto |
1<<_If |
1<<_Return |
1<<_Select |
1<<_Switch |
1<<_Type |
1<<_Var
// Advance consumes tokens until it finds a token of the stopset or followlist.
// The stopset is only considered if we are inside a function (p.fnest > 0).
// The followlist is the list of valid tokens that can follow a production;
// if it is empty, exactly one (non-EOF) token is consumed to ensure progress.
func (p *parser) advance(followlist ...token) {
if trace {
p.print(fmt.Sprintf("advance %s", followlist))
}
// compute follow set
// (not speed critical, advance is only called in error situations)
var followset uint64 = 1 << _EOF // don't skip over EOF
if len(followlist) > 0 {
if p.fnest > 0 {
followset |= stopset
}
for _, tok := range followlist {
followset |= 1 << tok
}
}
for !contains(followset, p.tok) {
if trace {
p.print("skip " + p.tok.String())
}
p.next()
if len(followlist) == 0 {
break
}
}
if trace {
p.print("next " + p.tok.String())
}
}
// usage: defer p.trace(msg)()
func (p *parser) trace(msg string) func() {
p.print(msg + " (")
const tab = ". "
p.indent = append(p.indent, tab...)
return func() {
p.indent = p.indent[:len(p.indent)-len(tab)]
if x := recover(); x != nil {
panic(x) // skip print_trace
}
p.print(")")
}
}
func (p *parser) print(msg string) {
fmt.Printf("%5d: %s%s\n", p.line, p.indent, msg)
}
// ----------------------------------------------------------------------------
// Package files
//
// Parse methods are annotated with matching Go productions as appropriate.
// The annotations are intended as guidelines only since a single Go grammar
// rule may be covered by multiple parse methods and vice versa.
//
// Excluding methods returning slices, parse methods named xOrNil may return
// nil; all others are expected to return a valid non-nil node.
// SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .
func (p *parser) fileOrNil() *File {
if trace {
defer p.trace("file")()
}
f := new(File)
f.pos = p.pos()
// PackageClause
if !p.got(_Package) {
p.syntaxError("package statement must be first")
return nil
}
f.Pragma = p.takePragma()
f.PkgName = p.name()
p.want(_Semi)
// don't bother continuing if package clause has errors
if p.first != nil {
return nil
}
// { ImportDecl ";" }
for p.got(_Import) {
f.DeclList = p.appendGroup(f.DeclList, p.importDecl)
p.want(_Semi)
}
// { TopLevelDecl ";" }
for p.tok != _EOF {
switch p.tok {
case _Const:
p.next()
f.DeclList = p.appendGroup(f.DeclList, p.constDecl)
case _Type:
p.next()
f.DeclList = p.appendGroup(f.DeclList, p.typeDecl)
case _Var:
p.next()
f.DeclList = p.appendGroup(f.DeclList, p.varDecl)
case _Func:
p.next()
if d := p.funcDeclOrNil(); d != nil {
f.DeclList = append(f.DeclList, d)
}
default:
if p.tok == _Lbrace && len(f.DeclList) > 0 && isEmptyFuncDecl(f.DeclList[len(f.DeclList)-1]) {
// opening { of function declaration on next line
p.syntaxError("unexpected semicolon or newline before {")
} else {
p.syntaxError("non-declaration statement outside function body")
}
p.advance(_Const, _Type, _Var, _Func)
continue
}
// Reset p.pragma BEFORE advancing to the next token (consuming ';')
// since comments before may set pragmas for the next function decl.
p.clearPragma()
if p.tok != _EOF && !p.got(_Semi) {
p.syntaxError("after top level declaration")
p.advance(_Const, _Type, _Var, _Func)
}
}
// p.tok == _EOF
p.clearPragma()
f.EOF = p.pos()
return f
}
func isEmptyFuncDecl(dcl Decl) bool {
f, ok := dcl.(*FuncDecl)
return ok && f.Body == nil
}
// ----------------------------------------------------------------------------
// Declarations
// list parses a possibly empty, sep-separated list of elements, optionally
// followed by sep, and closed by close (or EOF). sep must be one of _Comma
// or _Semi, and close must be one of _Rparen, _Rbrace, or _Rbrack.
//
// For each list element, f is called. Specifically, unless we're at close
// (or EOF), f is called at least once. After f returns true, no more list
// elements are accepted. list returns the position of the closing token.
//
// list = [ f { sep f } [sep] ] close .
func (p *parser) list(context string, sep, close token, f func() bool) Pos {
if debug && (sep != _Comma && sep != _Semi || close != _Rparen && close != _Rbrace && close != _Rbrack) {
panic("invalid sep or close argument for list")
}
done := false
for p.tok != _EOF && p.tok != close && !done {
done = f()
// sep is optional before close
if !p.got(sep) && p.tok != close {
p.syntaxError(fmt.Sprintf("in %s; possibly missing %s or %s", context, tokstring(sep), tokstring(close)))
p.advance(_Rparen, _Rbrack, _Rbrace)
if p.tok != close {
// position could be better but we had an error so we don't care
return p.pos()
}
}
}
pos := p.pos()
p.want(close)
return pos
}
// appendGroup(f) = f | "(" { f ";" } ")" . // ";" is optional before ")"
func (p *parser) appendGroup(list []Decl, f func(*Group) Decl) []Decl {
if p.tok == _Lparen {
g := new(Group)
p.clearPragma()
p.next() // must consume "(" after calling clearPragma!
p.list("grouped declaration", _Semi, _Rparen, func() bool {
if x := f(g); x != nil {
list = append(list, x)
}
return false
})
} else {
if x := f(nil); x != nil {
list = append(list, x)
}
}
return list
}
// ImportSpec = [ "." | PackageName ] ImportPath .
// ImportPath = string_lit .
func (p *parser) importDecl(group *Group) Decl {
if trace {
defer p.trace("importDecl")()
}
d := new(ImportDecl)
d.pos = p.pos()
d.Group = group
d.Pragma = p.takePragma()
switch p.tok {
case _Name:
d.LocalPkgName = p.name()
case _Dot:
d.LocalPkgName = NewName(p.pos(), ".")
p.next()
}
d.Path = p.oliteral()
if d.Path == nil {
p.syntaxError("missing import path")
p.advance(_Semi, _Rparen)
return d
}
if !d.Path.Bad && d.Path.Kind != StringLit {
p.syntaxError("import path must be a string")
d.Path.Bad = true
}
// d.Path.Bad || d.Path.Kind == StringLit
return d
}
// ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] .
func (p *parser) constDecl(group *Group) Decl {
if trace {
defer p.trace("constDecl")()
}
d := new(ConstDecl)
d.pos = p.pos()
d.Group = group
d.Pragma = p.takePragma()
d.NameList = p.nameList(p.name())
if p.tok != _EOF && p.tok != _Semi && p.tok != _Rparen {
d.Type = p.typeOrNil()
if p.gotAssign() {
d.Values = p.exprList()
}
}
return d
}
// TypeSpec = identifier [ TypeParams ] [ "=" ] Type .
func (p *parser) typeDecl(group *Group) Decl {
if trace {
defer p.trace("typeDecl")()
}
d := new(TypeDecl)
d.pos = p.pos()
d.Group = group
d.Pragma = p.takePragma()
d.Name = p.name()
if p.tok == _Lbrack {
// d.Name "[" ...
// array/slice type or type parameter list
pos := p.pos()
p.next()
switch p.tok {
case _Name:
// We may have an array type or a type parameter list.
// In either case we expect an expression x (which may
// just be a name, or a more complex expression) which
// we can analyze further.
//
// A type parameter list may have a type bound starting
// with a "[" as in: P []E. In that case, simply parsing
// an expression would lead to an error: P[] is invalid.
// But since index or slice expressions are never constant
// and thus invalid array length expressions, if we see a
// "[" following a name it must be the start of an array
// or slice constraint. Only if we don't see a "[" do we
// need to parse a full expression.
var x Expr = p.name()
if p.tok != _Lbrack {
// To parse the expression starting with name, expand
// the call sequence we would get by passing in name
// to parser.expr, and pass in name to parser.pexpr.
p.xnest++
x = p.binaryExpr(p.pexpr(x, false), 0)
p.xnest--
}
// analyze the cases
var pname *Name // pname != nil means pname is the type parameter name
var ptype Expr // ptype != nil means ptype is the type parameter type; pname != nil in this case
switch t := x.(type) {
case *Name:
// Unless we see a "]", we are at the start of a type parameter list.
if p.tok != _Rbrack {
// d.Name "[" name ...
pname = t
// no ptype
}
case *Operation:
// If we have an expression of the form name*T, and T is a (possibly
// parenthesized) type literal or the next token is a comma, we are
// at the start of a type parameter list.
if name, _ := t.X.(*Name); name != nil {
if t.Op == Mul && (isTypeLit(t.Y) || p.tok == _Comma) {
// d.Name "[" name "*" t.Y
// d.Name "[" name "*" t.Y ","
t.X, t.Y = t.Y, nil // convert t into unary *t.Y
pname = name
ptype = t
}
}
case *CallExpr:
// If we have an expression of the form name(T), and T is a (possibly
// parenthesized) type literal or the next token is a comma, we are
// at the start of a type parameter list.
if name, _ := t.Fun.(*Name); name != nil {
if len(t.ArgList) == 1 && !t.HasDots && (isTypeLit(t.ArgList[0]) || p.tok == _Comma) {
// d.Name "[" name "(" t.ArgList[0] ")"
// d.Name "[" name "(" t.ArgList[0] ")" ","
pname = name
ptype = t.ArgList[0]
}
}
}
if pname != nil {
// d.Name "[" pname ...
// d.Name "[" pname ptype ...
// d.Name "[" pname ptype "," ...
d.TParamList = p.paramList(pname, ptype, _Rbrack, true)
d.Alias = p.gotAssign()
d.Type = p.typeOrNil()
} else {
// d.Name "[" x ...
d.Type = p.arrayType(pos, x)
}
case _Rbrack:
// d.Name "[" "]" ...
p.next()
d.Type = p.sliceType(pos)
default:
// d.Name "[" ...
d.Type = p.arrayType(pos, nil)
}
} else {
d.Alias = p.gotAssign()
d.Type = p.typeOrNil()
}
if d.Type == nil {
d.Type = p.badExpr()
p.syntaxError("in type declaration")
p.advance(_Semi, _Rparen)
}
return d
}
// isTypeLit reports whether x is a (possibly parenthesized) type literal.
func isTypeLit(x Expr) bool {
switch x := x.(type) {
case *ArrayType, *StructType, *FuncType, *InterfaceType, *SliceType, *MapType, *ChanType:
return true
case *Operation:
// *T may be a pointer dereferenciation.
// Only consider *T as type literal if T is a type literal.
return x.Op == Mul && x.Y == nil && isTypeLit(x.X)
case *ParenExpr:
return isTypeLit(x.X)
}
return false
}
// VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
func (p *parser) varDecl(group *Group) Decl {
if trace {
defer p.trace("varDecl")()
}
d := new(VarDecl)
d.pos = p.pos()
d.Group = group
d.Pragma = p.takePragma()
d.NameList = p.nameList(p.name())
if p.gotAssign() {
d.Values = p.exprList()
} else {
d.Type = p.type_()
if p.gotAssign() {
d.Values = p.exprList()
}
}
return d
}
// FunctionDecl = "func" FunctionName [ TypeParams ] ( Function | Signature ) .
// FunctionName = identifier .
// Function = Signature FunctionBody .
// MethodDecl = "func" Receiver MethodName ( Function | Signature ) .
// Receiver = Parameters .
func (p *parser) funcDeclOrNil() *FuncDecl {
if trace {
defer p.trace("funcDecl")()
}
f := new(FuncDecl)
f.pos = p.pos()
f.Pragma = p.takePragma()
if p.got(_Lparen) {
rcvr := p.paramList(nil, nil, _Rparen, false)
switch len(rcvr) {
case 0:
p.error("method has no receiver")
default:
p.error("method has multiple receivers")
fallthrough
case 1:
f.Recv = rcvr[0]
}
}
if p.tok != _Name {
p.syntaxError("expecting name or (")
p.advance(_Lbrace, _Semi)
return nil
}
f.Name = p.name()
context := ""
if f.Recv != nil {
context = "method" // don't permit (method) type parameters in funcType
}
f.TParamList, f.Type = p.funcType(context)
if p.tok == _Lbrace {
f.Body = p.funcBody()
}
return f
}
func (p *parser) funcBody() *BlockStmt {
p.fnest++
errcnt := p.errcnt
body := p.blockStmt("")
p.fnest--
// Don't check branches if there were syntax errors in the function
// as it may lead to spurious errors (e.g., see test/switch2.go) or
// possibly crashes due to incomplete syntax trees.
if p.mode&CheckBranches != 0 && errcnt == p.errcnt {
checkBranches(body, p.errh)
}
return body
}
// ----------------------------------------------------------------------------
// Expressions
func (p *parser) expr() Expr {
if trace {
defer p.trace("expr")()
}
return p.binaryExpr(nil, 0)
}
// Expression = UnaryExpr | Expression binary_op Expression .
func (p *parser) binaryExpr(x Expr, prec int) Expr {
// don't trace binaryExpr - only leads to overly nested trace output
if x == nil {
x = p.unaryExpr()
}
for (p.tok == _Operator || p.tok == _Star) && p.prec > prec {
t := new(Operation)
t.pos = p.pos()
t.Op = p.op
tprec := p.prec
p.next()
t.X = x
t.Y = p.binaryExpr(nil, tprec)
x = t
}
return x
}
// UnaryExpr = PrimaryExpr | unary_op UnaryExpr .
func (p *parser) unaryExpr() Expr {
if trace {
defer p.trace("unaryExpr")()
}
switch p.tok {
case _Operator, _Star:
switch p.op {
case Mul, Add, Sub, Not, Xor:
x := new(Operation)
x.pos = p.pos()
x.Op = p.op
p.next()
x.X = p.unaryExpr()
return x
case And:
x := new(Operation)
x.pos = p.pos()
x.Op = And
p.next()
// unaryExpr may have returned a parenthesized composite literal
// (see comment in operand) - remove parentheses if any
x.X = unparen(p.unaryExpr())
return x
}
case _Arrow:
// receive op (<-x) or receive-only channel (<-chan E)
pos := p.pos()
p.next()
// If the next token is _Chan we still don't know if it is
// a channel (<-chan int) or a receive op (<-chan int(ch)).
// We only know once we have found the end of the unaryExpr.
x := p.unaryExpr()
// There are two cases:
//
// <-chan... => <-x is a channel type
// <-x => <-x is a receive operation
//
// In the first case, <- must be re-associated with
// the channel type parsed already:
//
// <-(chan E) => (<-chan E)
// <-(chan<-E) => (<-chan (<-E))
if _, ok := x.(*ChanType); ok {
// x is a channel type => re-associate <-
dir := SendOnly
t := x
for dir == SendOnly {
c, ok := t.(*ChanType)
if !ok {
break
}
dir = c.Dir
if dir == RecvOnly {
// t is type <-chan E but <-<-chan E is not permitted
// (report same error as for "type _ <-<-chan E")
p.syntaxError("unexpected <-, expecting chan")
// already progressed, no need to advance
}
c.Dir = RecvOnly
t = c.Elem
}
if dir == SendOnly {
// channel dir is <- but channel element E is not a channel
// (report same error as for "type _ <-chan<-E")
p.syntaxError(fmt.Sprintf("unexpected %s, expecting chan", String(t)))
// already progressed, no need to advance
}
return x
}
// x is not a channel type => we have a receive op
o := new(Operation)
o.pos = pos
o.Op = Recv
o.X = x
return o
}
// TODO(mdempsky): We need parens here so we can report an
// error for "(x) := true". It should be possible to detect
// and reject that more efficiently though.
return p.pexpr(nil, true)
}
// callStmt parses call-like statements that can be preceded by 'defer' and 'go'.
func (p *parser) callStmt() *CallStmt {
if trace {
defer p.trace("callStmt")()
}
s := new(CallStmt)
s.pos = p.pos()
s.Tok = p.tok // _Defer or _Go
p.next()
x := p.pexpr(nil, p.tok == _Lparen) // keep_parens so we can report error below
if t := unparen(x); t != x {
p.errorAt(x.Pos(), fmt.Sprintf("expression in %s must not be parenthesized", s.Tok))
// already progressed, no need to advance
x = t
}
cx, ok := x.(*CallExpr)
if !ok {
p.errorAt(x.Pos(), fmt.Sprintf("expression in %s must be function call", s.Tok))
// already progressed, no need to advance
cx = new(CallExpr)
cx.pos = x.Pos()
cx.Fun = x // assume common error of missing parentheses (function invocation)
}
s.Call = cx
return s
}
// Operand = Literal | OperandName | MethodExpr | "(" Expression ")" .
// Literal = BasicLit | CompositeLit | FunctionLit .
// BasicLit = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
// OperandName = identifier | QualifiedIdent.
func (p *parser) operand(keep_parens bool) Expr {
if trace {
defer p.trace("operand " + p.tok.String())()
}
switch p.tok {
case _Name:
return p.name()
case _Literal:
return p.oliteral()
case _Lparen:
pos := p.pos()
p.next()
p.xnest++
x := p.expr()
p.xnest--
p.want(_Rparen)
// Optimization: Record presence of ()'s only where needed
// for error reporting. Don't bother in other cases; it is
// just a waste of memory and time.
//
// Parentheses are not permitted around T in a composite
// literal T{}. If the next token is a {, assume x is a
// composite literal type T (it may not be, { could be
// the opening brace of a block, but we don't know yet).
if p.tok == _Lbrace {
keep_parens = true
}
// Parentheses are also not permitted around the expression
// in a go/defer statement. In that case, operand is called
// with keep_parens set.
if keep_parens {
px := new(ParenExpr)
px.pos = pos
px.X = x
x = px
}
return x
case _Func:
pos := p.pos()
p.next()
_, ftyp := p.funcType("function type")
if p.tok == _Lbrace {
p.xnest++
f := new(FuncLit)
f.pos = pos
f.Type = ftyp