From: Robert Griesemer <gri@golang.org>
Date: Thu, 5 Feb 2009 02:28:41 +0000 (-0800)
Subject: today's snapshot: steps towards using interfaces for statements in ast
X-Git-Tag: weekly.2009-11-06~2237
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=1595a1947c047070542b6c8d7478eaf485489bbb;p=gostls13.git

today's snapshot: steps towards using interfaces for statements in ast

R=r
OCL=24380
CL=24380
---

diff --git a/usr/gri/pretty/ast.go b/usr/gri/pretty/ast.go
index cfd345d5a9..c0b3c86278 100644
--- a/usr/gri/pretty/ast.go
+++ b/usr/gri/pretty/ast.go
@@ -18,7 +18,7 @@ type (
 
 	Block struct;
 	Expr interface;
-	Stat struct;
+	StatImpl struct;
 	Decl struct;
 )
 
@@ -350,11 +350,11 @@ func NewBlock(pos, tok int) *Block {
 // Expressions
 
 type (
-	Visitor interface;
+	ExprVisitor interface;
 
 	Expr interface {
 		Pos() int;
-		Visit(v Visitor);
+		Visit(v ExprVisitor);
 	};
 
 	BadExpr struct {
@@ -421,7 +421,7 @@ type (
 )
 
 
-type Visitor interface {
+type ExprVisitor interface {
 	DoBadExpr(x *BadExpr);
 	DoIdent(x *Ident);
 	DoBinaryExpr(x *BinaryExpr);
@@ -451,18 +451,18 @@ func (x *Index) Pos() int { return x.Pos_; }
 func (x *Call) Pos() int { return x.Pos_; }
 
 
-func (x *BadExpr) Visit(v Visitor) { v.DoBadExpr(x); }
-func (x *Ident) Visit(v Visitor) { v.DoIdent(x); }
-func (x *BinaryExpr) Visit(v Visitor) { v.DoBinaryExpr(x); }
-func (x *UnaryExpr) Visit(v Visitor) { v.DoUnaryExpr(x); }
-func (x *BasicLit) Visit(v Visitor) { v.DoBasicLit(x); }
-func (x *FunctionLit) Visit(v Visitor) { v.DoFunctionLit(x); }
-func (x *CompositeLit) Visit(v Visitor) { v.DoCompositeLit(x); }
-func (x *TypeLit) Visit(v Visitor) { v.DoTypeLit(x); }
-func (x *Selector) Visit(v Visitor) { v.DoSelector(x); }
-func (x *TypeGuard) Visit(v Visitor) { v.DoTypeGuard(x); }
-func (x *Index) Visit(v Visitor) { v.DoIndex(x); }
-func (x *Call) Visit(v Visitor) { v.DoCall(x); }
+func (x *BadExpr) Visit(v ExprVisitor) { v.DoBadExpr(x); }
+func (x *Ident) Visit(v ExprVisitor) { v.DoIdent(x); }
+func (x *BinaryExpr) Visit(v ExprVisitor) { v.DoBinaryExpr(x); }
+func (x *UnaryExpr) Visit(v ExprVisitor) { v.DoUnaryExpr(x); }
+func (x *BasicLit) Visit(v ExprVisitor) { v.DoBasicLit(x); }
+func (x *FunctionLit) Visit(v ExprVisitor) { v.DoFunctionLit(x); }
+func (x *CompositeLit) Visit(v ExprVisitor) { v.DoCompositeLit(x); }
+func (x *TypeLit) Visit(v ExprVisitor) { v.DoTypeLit(x); }
+func (x *Selector) Visit(v ExprVisitor) { v.DoSelector(x); }
+func (x *TypeGuard) Visit(v ExprVisitor) { v.DoTypeGuard(x); }
+func (x *Index) Visit(v ExprVisitor) { v.DoIndex(x); }
+func (x *Call) Visit(v ExprVisitor) { v.DoCall(x); }
 
 
 
@@ -518,23 +518,112 @@ func (t *Type) Nfields() int {
 // ----------------------------------------------------------------------------
 // Statements
 
-type Stat struct {
+type (
+	StatVisitor interface;
+
+	Stat interface {
+		Visit(v StatVisitor);
+	};
+	
+	BadStat struct {
+		Pos int;
+	};
+
+	LabelDecl struct {
+		Pos int;  // position of ":"
+		Label *Ident;
+	};
+
+	DeclarationStat struct {
+		Decl *Decl;
+	};
+
+	ExpressionStat struct {
+		Pos int;  // position of Tok
+		Tok int;  // INC, DEC, RETURN, GO, DEFER
+		Expr Expr;
+	};
+	
+	IfStat struct {
+		Pos int;  // position of "if"
+		Init Stat;
+		Cond Expr;
+		Body *Block;
+		Else Stat;
+	};
+	
+	ForStat struct {
+		Pos int;  // position of "for"
+		Init Stat;
+		Cond Expr;
+		Post Stat;
+		Body *Block;
+	};
+	
+	SwitchStat struct {
+		Pos int;  // position of "switch"
+		Init Stat;
+		Tag Expr;
+		Body *Block;
+	};
+	
+	SelectStat struct {
+		Pos int;  // position of "select"
+		Body *Block;
+	};
+	
+	ControlFlowStat struct {
+		Pos int;  // position of Tok
+		Tok int;  // BREAK, CONTINUE, GOTO, FALLTHROUGH
+		Label *Ident;  // if any, or nil
+	};
+)
+
+
+type StatVisitor interface {
+	DoBadStat(s *BadStat);
+	DoLabelDecl(s *LabelDecl);
+	DoDeclarationStat(s *DeclarationStat);
+	DoExpressionStat(s *ExpressionStat);
+	DoIfStat(s *IfStat);
+	DoForStat(s *ForStat);
+	DoSwitchStat(s *SwitchStat);
+	DoSelectStat(s *SelectStat);
+	DoControlFlowStat(s *ControlFlowStat);
+}
+
+
+func (s *BadStat) Visit(v StatVisitor) { v.DoBadStat(s); }
+func (s *LabelDecl) Visit(v StatVisitor) { v.DoLabelDecl(s); }
+func (s *DeclarationStat) Visit(v StatVisitor) { v.DoDeclarationStat(s); }
+func (s *ExpressionStat) Visit(v StatVisitor) { v.DoExpressionStat(s); }
+func (s *IfStat) Visit(v StatVisitor) { v.DoIfStat(s); }
+func (s *ForStat) Visit(v StatVisitor) { v.DoForStat(s); }
+func (s *SwitchStat) Visit(v StatVisitor) { v.DoSwitchStat(s); }
+func (s *SelectStat) Visit(v StatVisitor) { v.DoSelectStat(s); }
+func (s *ControlFlowStat) Visit(v StatVisitor) { v.DoControlFlowStat(s); }
+
+
+// ----------------------------------------------------------------------------
+// Old style statements
+
+type StatImpl struct {
 	Node;
-	Init, Post *Stat;
+	Init, Post *StatImpl;
 	Expr Expr;
 	Body *Block;  // composite statement body
 	Decl *Decl;  // declaration statement
 }
 
 
-func NewStat(pos, tok int) *Stat {
-	s := new(Stat);
+func NewStat(pos, tok int) *StatImpl {
+	s := new(StatImpl);
 	s.Pos, s.Tok = pos, tok;
 	return s;
 }
 
 
-var BadStat = NewStat(0, Scanner.ILLEGAL);
+var OldBadStat = NewStat(0, Scanner.ILLEGAL);
 
 
 // ----------------------------------------------------------------------------
diff --git a/usr/gri/pretty/parser.go b/usr/gri/pretty/parser.go
index ae904a6511..f7e4ccd62c 100644
--- a/usr/gri/pretty/parser.go
+++ b/usr/gri/pretty/parser.go
@@ -5,6 +5,7 @@
 package Parser
 
 import (
+	"flag";
 	"fmt";
 	"array";
 	Scanner "scanner";
@@ -83,6 +84,13 @@ func (P *Parser) Ecart() {
 }
 
 
+func (P *Parser) VerifyIndent(indent uint) {
+	if indent != P.indent {
+		panic("imbalanced tracing code");
+	}
+}
+
+
 func (P *Parser) Next0() {
 	if P.tokchan == nil {
 		P.pos, P.tok, P.val = P.scanner.Scan();
@@ -247,7 +255,8 @@ func (P *Parser) NewBinary(pos, tok int, x, y AST.Expr) *AST.BinaryExpr {
 
 func (P *Parser) TryType() *AST.Type;
 func (P *Parser) ParseExpression(prec int) AST.Expr;
-func (P *Parser) ParseStatement() *AST.Stat;
+func (P *Parser) ParseStatement() AST.Stat;
+func (P *Parser) OldParseStatement() *AST.StatImpl;
 func (P *Parser) ParseDeclaration() *AST.Decl;
 
 
@@ -746,14 +755,24 @@ func (P *Parser) TryType() *AST.Type {
 // ----------------------------------------------------------------------------
 // Blocks
 
+
+var newstat = flag.Bool("newstat", false, "use new statement parsing - work in progress");
+
+
 func (P *Parser) ParseStatementList(list *array.Array) {
 	if P.verbose {
 		P.Trace("StatementList");
 		defer P.Ecart();
+		defer P.VerifyIndent(P.indent);
 	}
 
 	for P.tok != Scanner.CASE && P.tok != Scanner.DEFAULT && P.tok != Scanner.RBRACE && P.tok != Scanner.EOF {
-		s := P.ParseStatement();
+		var s interface{};
+		if *newstat {
+			s = P.ParseStatement();
+		} else {
+			s = P.OldParseStatement();
+		}
 		if s != nil {
 			// not the empty statement
 			list.Push(s);
@@ -1133,31 +1152,93 @@ func (P *Parser) ParseExpression(prec int) AST.Expr {
 	if P.verbose {
 		P.Trace("Expression");
 		defer P.Ecart();
+		defer P.VerifyIndent(P.indent);
 	}
 
-	indent := P.indent;
 	if prec < 0 {
 		panic("precedence must be >= 0");
 	}
-	x := P.NoType(P.ParseBinaryExpr(prec));
-	if indent != P.indent {
-		panic("imbalanced tracing code (Expression)");
-	}
 
-	return x;
+	return P.NoType(P.ParseBinaryExpr(prec));
 }
 
 
 // ----------------------------------------------------------------------------
 // Statements
 
-func (P *Parser) ParseSimpleStat(range_ok bool) *AST.Stat {
+func (P *Parser) ParseSimpleStat(range_ok bool) AST.Stat {
+	if P.verbose {
+		P.Trace("SimpleStat");
+		defer P.Ecart();
+	}
+
+	x := P.ParseExpressionList();
+
+	switch P.tok {
+	case Scanner.COLON:
+		// label declaration
+		pos := P.pos;
+		P.Next();  // consume ":"
+		if AST.ExprLen(x) == 1 {
+			if label, is_ident := x.(*AST.Ident); is_ident {
+				return &AST.LabelDecl{pos, label};
+			}
+		}
+		P.Error(x.Pos(), "illegal label declaration");
+		return nil;
+		
+	case
+		Scanner.DEFINE, Scanner.ASSIGN, Scanner.ADD_ASSIGN,
+		Scanner.SUB_ASSIGN, Scanner.MUL_ASSIGN, Scanner.QUO_ASSIGN,
+		Scanner.REM_ASSIGN, Scanner.AND_ASSIGN, Scanner.OR_ASSIGN,
+		Scanner.XOR_ASSIGN, Scanner.SHL_ASSIGN, Scanner.SHR_ASSIGN:
+		// declaration/assignment
+		pos, tok := P.pos, P.tok;
+		P.Next();
+		var y AST.Expr;
+		if range_ok && P.tok == Scanner.RANGE {
+			range_pos := P.pos;
+			P.Next();
+			y = &AST.UnaryExpr{range_pos, Scanner.RANGE, P.ParseExpression(1)};
+			if tok != Scanner.DEFINE && tok != Scanner.ASSIGN {
+				P.Error(pos, "expected '=' or ':=', found '" + Scanner.TokenString(tok) + "'");
+			}
+		} else {
+			y = P.ParseExpressionList();
+			if xl, yl := AST.ExprLen(x), AST.ExprLen(y); xl > 1 && yl > 1 && xl != yl {
+				P.Error(x.Pos(), "arity of lhs doesn't match rhs");
+			}
+		}
+		// TODO changed ILLEGAL -> NONE
+		return &AST.ExpressionStat{x.Pos(), Scanner.ILLEGAL, P.NewBinary(pos, tok, x, y)};
+		
+	default:
+		if AST.ExprLen(x) != 1 {
+			P.Error(x.Pos(), "only one expression allowed");
+		}
+		
+		if P.tok == Scanner.INC || P.tok == Scanner.DEC {
+			s := &AST.ExpressionStat{P.pos, P.tok, x};
+			P.Next();  // consume "++" or "--"
+			return s;
+		}
+		
+		// TODO changed ILLEGAL -> NONE
+		return &AST.ExpressionStat{x.Pos(), Scanner.ILLEGAL, x};
+	}
+
+	unreachable();
+	return nil;
+}
+
+
+func (P *Parser) OldParseSimpleStat(range_ok bool) *AST.StatImpl {
 	if P.verbose {
 		P.Trace("SimpleStat");
 		defer P.Ecart();
 	}
 
-	s := AST.BadStat;
+	s := AST.OldBadStat;
 	x := P.ParseExpressionList();
 
 	switch P.tok {
@@ -1180,7 +1261,7 @@ func (P *Parser) ParseSimpleStat(range_ok bool) *AST.Stat {
 		pos, tok := P.pos, P.tok;
 		P.Next();
 		var y AST.Expr = &AST.BadExpr{pos};
-		if P.tok == Scanner.RANGE {
+		if range_ok && P.tok == Scanner.RANGE {
 			range_pos := P.pos;
 			P.Next();
 			y = P.ParseExpression(1);
@@ -1197,14 +1278,6 @@ func (P *Parser) ParseSimpleStat(range_ok bool) *AST.Stat {
 		s = AST.NewStat(x.Pos(), Scanner.EXPRSTAT);
 		s.Expr = P.NewBinary(pos, tok, x, y);
 		
-	case Scanner.RANGE:
-		pos := P.pos;
-		P.Next();
-		y := P.ParseExpression(1);
-		y = &AST.UnaryExpr{pos, Scanner.RANGE, y};
-		s = AST.NewStat(x.Pos(), Scanner.EXPRSTAT);
-		s.Expr = P.NewBinary(pos, Scanner.DEFINE, x, y);
-
 	default:
 		var pos, tok int;
 		if P.tok == Scanner.INC || P.tok == Scanner.DEC {
@@ -1225,7 +1298,19 @@ func (P *Parser) ParseSimpleStat(range_ok bool) *AST.Stat {
 }
 
 
-func (P *Parser) ParseInvocationStat(keyword int) *AST.Stat {
+func (P *Parser) ParseInvocationStat(keyword int) *AST.ExpressionStat {
+	if P.verbose {
+		P.Trace("InvocationStat");
+		defer P.Ecart();
+	}
+
+	pos := P.pos;
+	P.Expect(keyword);
+	return &AST.ExpressionStat{pos, keyword, P.ParseExpression(1)};
+}
+
+
+func (P *Parser) OldParseInvocationStat(keyword int) *AST.StatImpl {
 	if P.verbose {
 		P.Trace("InvocationStat");
 		defer P.Ecart();
@@ -1239,7 +1324,24 @@ func (P *Parser) ParseInvocationStat(keyword int) *AST.Stat {
 }
 
 
-func (P *Parser) ParseReturnStat() *AST.Stat {
+func (P *Parser) ParseReturnStat() *AST.ExpressionStat {
+	if P.verbose {
+		P.Trace("ReturnStat");
+		defer P.Ecart();
+	}
+
+	pos := P.pos;
+	P.Expect(Scanner.RETURN);
+	var x AST.Expr;
+	if P.tok != Scanner.SEMICOLON && P.tok != Scanner.RBRACE {
+		x = P.ParseExpressionList();
+	}
+
+	return &AST.ExpressionStat{pos, Scanner.RETURN, x};
+}
+
+
+func (P *Parser) OldParseReturnStat() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("ReturnStat");
 		defer P.Ecart();
@@ -1255,7 +1357,7 @@ func (P *Parser) ParseReturnStat() *AST.Stat {
 }
 
 
-func (P *Parser) ParseControlFlowStat(tok int) *AST.Stat {
+func (P *Parser) ParseControlFlowStat(tok int) *AST.StatImpl {
 	if P.verbose {
 		P.Trace("ControlFlowStat");
 		defer P.Ecart();
@@ -1271,7 +1373,47 @@ func (P *Parser) ParseControlFlowStat(tok int) *AST.Stat {
 }
 
 
-func (P *Parser) ParseControlClause(keyword int) *AST.Stat {
+func (P *Parser) ParseControlClause(isForStat bool) (init AST.Stat, expr AST.Expr, post AST.Stat) {
+	if P.verbose {
+		P.Trace("ControlClause");
+		defer P.Ecart();
+	}
+
+	if P.tok != Scanner.LBRACE {
+		prev_lev := P.expr_lev;
+		P.expr_lev = -1;
+		if P.tok != Scanner.SEMICOLON {
+			init = P.ParseSimpleStat(isForStat);
+			// TODO check for range clause and exit if found
+		}
+		if P.tok == Scanner.SEMICOLON {
+			P.Next();
+			if P.tok != Scanner.SEMICOLON && P.tok != Scanner.LBRACE {
+				expr = P.ParseExpression(1);
+			}
+			if isForStat {
+				P.Expect(Scanner.SEMICOLON);
+				if P.tok != Scanner.LBRACE {
+					post = P.ParseSimpleStat(false);
+				}
+			}
+		} else {
+			if init != nil {  // guard in case of errors
+				if s, is_expr_stat := init.(*AST.ExpressionStat); is_expr_stat {
+					expr, init = s.Expr, nil;
+				} else {
+					P.Error(0, "illegal control clause");
+				}
+			}
+		}
+		P.expr_lev = prev_lev;
+	}
+
+	return init, expr, post;
+}
+
+
+func (P *Parser) OldParseControlClause(keyword int) *AST.StatImpl {
 	if P.verbose {
 		P.Trace("ControlClause");
 		defer P.Ecart();
@@ -1283,7 +1425,7 @@ func (P *Parser) ParseControlClause(keyword int) *AST.Stat {
 		prev_lev := P.expr_lev;
 		P.expr_lev = -1;
 		if P.tok != Scanner.SEMICOLON {
-			s.Init = P.ParseSimpleStat(keyword == Scanner.FOR);
+			s.Init = P.OldParseSimpleStat(keyword == Scanner.FOR);
 			// TODO check for range clause and exit if found
 		}
 		if P.tok == Scanner.SEMICOLON {
@@ -1294,7 +1436,7 @@ func (P *Parser) ParseControlClause(keyword int) *AST.Stat {
 			if keyword == Scanner.FOR {
 				P.Expect(Scanner.SEMICOLON);
 				if P.tok != Scanner.LBRACE {
-					s.Post = P.ParseSimpleStat(false);
+					s.Post = P.OldParseSimpleStat(false);
 				}
 			}
 		} else {
@@ -1309,22 +1451,61 @@ func (P *Parser) ParseControlClause(keyword int) *AST.Stat {
 }
 
 
-func (P *Parser) ParseIfStat() *AST.Stat {
+func (P *Parser) ParseIfStat() *AST.IfStat {
+	if P.verbose {
+		P.Trace("IfStat");
+		defer P.Ecart();
+	}
+
+	P.OpenScope();
+	pos := P.pos;
+	P.Expect(Scanner.IF);
+	init, cond, dummy := P.ParseControlClause(false);
+	body := P.ParseBlock(nil, Scanner.LBRACE);
+	var else_ AST.Stat;
+	if P.tok == Scanner.ELSE {
+		P.Next();
+		if P.tok == Scanner.IF || P.tok == Scanner.LBRACE {
+			else_ = P.ParseStatement();
+		} else if P.sixg {
+			else_ = P.ParseStatement();
+			if else_ != nil {
+				// not the empty statement
+				// wrap in a block since we don't have one
+				panic();
+				/*
+				b := AST.NewStat(s1.Pos, Scanner.LBRACE);
+				b.Body = AST.NewBlock(s1.Pos, Scanner.LBRACE);
+				b.Body.List.Push(s1);
+				s1 = b;
+				*/
+			}
+		} else {
+			P.Error(P.pos, "'if' or '{' expected - illegal 'else' branch");
+		}
+	}
+	P.CloseScope();
+
+	return &AST.IfStat{pos, init, cond, body, else_ };
+}
+
+
+func (P *Parser) OldParseIfStat() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("IfStat");
 		defer P.Ecart();
 	}
 
 	P.OpenScope();
-	s := P.ParseControlClause(Scanner.IF);
+	s := P.OldParseControlClause(Scanner.IF);
 	s.Body = P.ParseBlock(nil, Scanner.LBRACE);
 	if P.tok == Scanner.ELSE {
 		P.Next();
-		s1 := AST.BadStat;
+		s1 := AST.OldBadStat;
 		if P.tok == Scanner.IF || P.tok == Scanner.LBRACE {
-			s1 = P.ParseStatement();
+			s1 = P.OldParseStatement();
 		} else if P.sixg {
-			s1 = P.ParseStatement();
+			s1 = P.OldParseStatement();
 			if s1 != nil {
 				// not the empty statement
 				assert(s1.Tok != Scanner.LBRACE);
@@ -1345,14 +1526,31 @@ func (P *Parser) ParseIfStat() *AST.Stat {
 }
 
 
-func (P *Parser) ParseForStat() *AST.Stat {
+func (P *Parser) ParseForStat() *AST.ForStat {
 	if P.verbose {
 		P.Trace("ForStat");
 		defer P.Ecart();
 	}
 
 	P.OpenScope();
-	s := P.ParseControlClause(Scanner.FOR);
+	pos := P.pos;
+	P.Expect(Scanner.FOR);
+	init, cond, post := P.ParseControlClause(true);
+	body := P.ParseBlock(nil, Scanner.LBRACE);
+	P.CloseScope();
+
+	return &AST.ForStat{pos, init, cond, post, body};
+}
+
+
+func (P *Parser) OldParseForStat() *AST.StatImpl {
+	if P.verbose {
+		P.Trace("ForStat");
+		defer P.Ecart();
+	}
+
+	P.OpenScope();
+	s := P.OldParseControlClause(Scanner.FOR);
 	s.Body = P.ParseBlock(nil, Scanner.LBRACE);
 	P.CloseScope();
 
@@ -1360,7 +1558,7 @@ func (P *Parser) ParseForStat() *AST.Stat {
 }
 
 
-func (P *Parser) ParseSwitchCase() *AST.Stat {
+func (P *Parser) ParseSwitchCase() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("SwitchCase");
 		defer P.Ecart();
@@ -1378,7 +1576,7 @@ func (P *Parser) ParseSwitchCase() *AST.Stat {
 }
 
 
-func (P *Parser) ParseCaseClause() *AST.Stat {
+func (P *Parser) ParseCaseClause() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("CaseClause");
 		defer P.Ecart();
@@ -1391,14 +1589,38 @@ func (P *Parser) ParseCaseClause() *AST.Stat {
 }
 
 
-func (P *Parser) ParseSwitchStat() *AST.Stat {
+func (P *Parser) ParseSwitchStat() *AST.SwitchStat {
+	if P.verbose {
+		P.Trace("SwitchStat");
+		defer P.Ecart();
+	}
+
+	P.OpenScope();
+	pos := P.pos;
+	P.Expect(Scanner.SWITCH);
+	init, tag, post := P.ParseControlClause(false);
+	body := AST.NewBlock(P.pos, Scanner.LBRACE);
+	P.Expect(Scanner.LBRACE);
+	for P.tok != Scanner.RBRACE && P.tok != Scanner.EOF {
+		body.List.Push(P.ParseCaseClause());
+	}
+	body.End = P.pos;
+	P.Expect(Scanner.RBRACE);
+	P.opt_semi = true;
+	P.CloseScope();
+
+	return &AST.SwitchStat{pos, init, tag, body};
+}
+
+
+func (P *Parser) OldParseSwitchStat() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("SwitchStat");
 		defer P.Ecart();
 	}
 
 	P.OpenScope();
-	s := P.ParseControlClause(Scanner.SWITCH);
+	s := P.OldParseControlClause(Scanner.SWITCH);
 	b := AST.NewBlock(P.pos, Scanner.LBRACE);
 	P.Expect(Scanner.LBRACE);
 	for P.tok != Scanner.RBRACE && P.tok != Scanner.EOF {
@@ -1414,7 +1636,7 @@ func (P *Parser) ParseSwitchStat() *AST.Stat {
 }
 
 
-func (P *Parser) ParseCommCase() *AST.Stat {
+func (P *Parser) ParseCommCase() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("CommCase");
 		defer P.Ecart();
@@ -1443,7 +1665,7 @@ func (P *Parser) ParseCommCase() *AST.Stat {
 }
 
 
-func (P *Parser) ParseCommClause() *AST.Stat {
+func (P *Parser) ParseCommClause() *AST.StatImpl {
 	if P.verbose {
 		P.Trace("CommClause");
 		defer P.Ecart();
@@ -1456,12 +1678,36 @@ func (P *Parser) ParseCommClause() *AST.Stat {
 }
 
 
-func (P *Parser) ParseSelectStat() *AST.Stat {
+func (P *Parser) ParseSelectStat() *AST.SelectStat {
 	if P.verbose {
 		P.Trace("SelectStat");
 		defer P.Ecart();
 	}
 
+	P.OpenScope();
+	pos := P.pos;
+	P.Expect(Scanner.SELECT);
+	body := AST.NewBlock(P.pos, Scanner.LBRACE);
+	P.Expect(Scanner.LBRACE);
+	for P.tok != Scanner.RBRACE && P.tok != Scanner.EOF {
+		body.List.Push(P.ParseCommClause());
+	}
+	body.End = P.pos;
+	P.Expect(Scanner.RBRACE);
+	P.opt_semi = true;
+	P.CloseScope();
+
+	return &AST.SelectStat{pos, body};
+}
+
+
+func (P *Parser) OldParseSelectStat() *AST.StatImpl {
+	if P.verbose {
+		P.Trace("SelectStat");
+		defer P.Ecart();
+	}
+
+	P.OpenScope();
 	s := AST.NewStat(P.pos, Scanner.SELECT);
 	P.Expect(Scanner.SELECT);
 	b := AST.NewBlock(P.pos, Scanner.LBRACE);
@@ -1472,20 +1718,66 @@ func (P *Parser) ParseSelectStat() *AST.Stat {
 	b.End = P.pos;
 	P.Expect(Scanner.RBRACE);
 	P.opt_semi = true;
+	P.CloseScope();
 	s.Body = b;
 
 	return s;
 }
 
 
-func (P *Parser) ParseStatement() *AST.Stat {
+func (P *Parser) ParseStatement() AST.Stat {
 	if P.verbose {
 		P.Trace("Statement");
 		defer P.Ecart();
+		defer P.VerifyIndent(P.indent);
+	}
+
+	s := AST.OldBadStat;
+	switch P.tok {
+	case Scanner.CONST, Scanner.TYPE, Scanner.VAR:
+		return &AST.DeclarationStat{P.ParseDeclaration()};
+	case Scanner.FUNC:
+		// for now we do not allow local function declarations,
+		// instead we assume this starts a function literal
+		fallthrough;
+	case
+		// only the tokens that are legal top-level expression starts
+		Scanner.IDENT, Scanner.INT, Scanner.FLOAT, Scanner.STRING, Scanner.LPAREN,  // operand
+		Scanner.LBRACK, Scanner.STRUCT,  // composite type
+		Scanner.MUL, Scanner.AND, Scanner.ARROW:  // unary
+		return P.ParseSimpleStat(false);
+	case Scanner.GO, Scanner.DEFER:
+		return P.ParseInvocationStat(P.tok);
+	case Scanner.RETURN:
+		return P.ParseReturnStat();
+	case Scanner.BREAK, Scanner.CONTINUE, Scanner.GOTO, Scanner.FALLTHROUGH:
+		s = P.ParseControlFlowStat(P.tok);
+	case Scanner.LBRACE:
+		s = AST.NewStat(P.pos, Scanner.LBRACE);
+		s.Body = P.ParseBlock(nil, Scanner.LBRACE);
+	case Scanner.IF:
+		return P.ParseIfStat();
+	case Scanner.FOR:
+		return P.ParseForStat();
+	case Scanner.SWITCH:
+		return P.ParseSwitchStat();
+	case Scanner.SELECT:
+		return P.ParseSelectStat();
+	}
+
+	// empty statement
+	return nil;
+}
+
+
+func (P *Parser) OldParseStatement() *AST.StatImpl {
+	if P.verbose {
+		P.Trace("Statement");
+		defer P.Ecart();
+		defer P.VerifyIndent(P.indent);
 	}
-	indent := P.indent;
 
-	s := AST.BadStat;
+	s := AST.OldBadStat;
 	switch P.tok {
 	case Scanner.CONST, Scanner.TYPE, Scanner.VAR:
 		s = AST.NewStat(P.pos, P.tok);
@@ -1499,32 +1791,29 @@ func (P *Parser) ParseStatement() *AST.Stat {
 		Scanner.IDENT, Scanner.INT, Scanner.FLOAT, Scanner.STRING, Scanner.LPAREN,  // operand
 		Scanner.LBRACK, Scanner.STRUCT,  // composite type
 		Scanner.MUL, Scanner.AND, Scanner.ARROW:  // unary
-		s = P.ParseSimpleStat(false);
+		s = P.OldParseSimpleStat(false);
 	case Scanner.GO, Scanner.DEFER:
-		s = P.ParseInvocationStat(P.tok);
+		s = P.OldParseInvocationStat(P.tok);
 	case Scanner.RETURN:
-		s = P.ParseReturnStat();
+		s = P.OldParseReturnStat();
 	case Scanner.BREAK, Scanner.CONTINUE, Scanner.GOTO, Scanner.FALLTHROUGH:
 		s = P.ParseControlFlowStat(P.tok);
 	case Scanner.LBRACE:
 		s = AST.NewStat(P.pos, Scanner.LBRACE);
 		s.Body = P.ParseBlock(nil, Scanner.LBRACE);
 	case Scanner.IF:
-		s = P.ParseIfStat();
+		s = P.OldParseIfStat();
 	case Scanner.FOR:
-		s = P.ParseForStat();
+		s = P.OldParseForStat();
 	case Scanner.SWITCH:
-		s = P.ParseSwitchStat();
+		s = P.OldParseSwitchStat();
 	case Scanner.SELECT:
-		s = P.ParseSelectStat();
+		s = P.OldParseSelectStat();
 	default:
 		// empty statement
 		s = nil;
 	}
 
-	if indent != P.indent {
-		panic("imbalanced tracing code (Statement)");
-	}
 	return s;
 }
 
@@ -1719,6 +2008,7 @@ func (P *Parser) ParseDeclaration() *AST.Decl {
 	if P.verbose {
 		P.Trace("Declaration");
 		defer P.Ecart();
+		defer P.VerifyIndent(P.indent);
 	}
 	indent := P.indent;
 
@@ -1734,9 +2024,6 @@ func (P *Parser) ParseDeclaration() *AST.Decl {
 		P.Next();  // make progress
 	}
 
-	if indent != P.indent {
-		panic("imbalanced tracing code (Declaration)");
-	}
 	return d;
 }
 
diff --git a/usr/gri/pretty/printer.go b/usr/gri/pretty/printer.go
index 5e3f8a4b74..143886fce7 100644
--- a/usr/gri/pretty/printer.go
+++ b/usr/gri/pretty/printer.go
@@ -371,9 +371,8 @@ func (P *Printer) Token(pos int, tok int) {
 
 
 func (P *Printer) Error(pos int, tok int, msg string) {
-	P.String(0, "<");
-	P.Token(pos, tok);
-	P.String(0, " " + msg + ">");
+	fmt.Printf("\ninternal printing error: pos = %d, tok = %s, %s\n", pos, Scanner.TokenString(tok), msg);
+	panic();
 }
 
 
@@ -723,12 +722,25 @@ func (P *Printer) Expr(x AST.Expr) {
 // ----------------------------------------------------------------------------
 // Statements
 
-func (P *Printer) Stat(s *AST.Stat)
+func (P *Printer) Stat(s AST.Stat) {
+	s.Visit(P);
+}
+
+
+func (P *Printer) StatImpl(s *AST.StatImpl)
 
 func (P *Printer) StatementList(list *array.Array) {
 	for i, n := 0, list.Len(); i < n; i++ {
 		P.newlines = 1;  // for first entry
-		P.Stat(list.At(i).(*AST.Stat));
+		
+		if s, is_StatImpl := list.At(i).(*AST.StatImpl); is_StatImpl {
+			P.StatImpl(s);
+		} else if s, is_Stat := list.At(i).(AST.Stat); is_Stat {
+			s.Visit(P);
+		} else {
+			panic();
+		}
+
 		P.newlines = 1;
 		P.state = inside_list;
 	}
@@ -757,7 +769,7 @@ func (P *Printer) Block(b *AST.Block, indent bool) {
 }
 
 
-func (P *Printer) ControlClause(s *AST.Stat) {
+func (P *Printer) OldControlClause(s *AST.StatImpl) {
 	has_post := s.Tok == Scanner.FOR && s.Post != nil;  // post also used by "if"
 
 	P.separator = blank;
@@ -770,7 +782,7 @@ func (P *Printer) ControlClause(s *AST.Stat) {
 		// all semicolons required
 		// (they are not separators, print them explicitly)
 		if s.Init != nil {
-			P.Stat(s.Init);
+			P.StatImpl(s.Init);
 			P.separator = none;
 		}
 		P.String(0, ";");
@@ -783,7 +795,7 @@ func (P *Printer) ControlClause(s *AST.Stat) {
 			P.String(0, ";");
 			P.separator = blank;
 			if has_post {
-				P.Stat(s.Post);
+				P.StatImpl(s.Post);
 			}
 		}
 	}
@@ -793,7 +805,7 @@ func (P *Printer) ControlClause(s *AST.Stat) {
 
 func (P *Printer) Declaration(d *AST.Decl, parenthesized bool);
 
-func (P *Printer) Stat(s *AST.Stat) {
+func (P *Printer) StatImpl(s *AST.StatImpl) {
 	switch s.Tok {
 	case Scanner.EXPRSTAT:
 		// expression statement
@@ -823,23 +835,23 @@ func (P *Printer) Stat(s *AST.Stat) {
 
 	case Scanner.IF:
 		P.String(s.Pos, "if");
-		P.ControlClause(s);
+		P.OldControlClause(s);
 		P.Block(s.Body, true);
 		if s.Post != nil {
 			P.separator = blank;
 			P.String(0, "else");
 			P.separator = blank;
-			P.Stat(s.Post);
+			P.StatImpl(s.Post);
 		}
 
 	case Scanner.FOR:
 		P.String(s.Pos, "for");
-		P.ControlClause(s);
+		P.OldControlClause(s);
 		P.Block(s.Body, true);
 
 	case Scanner.SWITCH, Scanner.SELECT:
 		P.Token(s.Pos, s.Tok);
-		P.ControlClause(s);
+		P.OldControlClause(s);
 		P.Block(s.Body, false);
 
 	case Scanner.CASE, Scanner.DEFAULT:
@@ -872,6 +884,116 @@ func (P *Printer) Stat(s *AST.Stat) {
 }
 
 
+func (P *Printer) DoBadStat(s *AST.BadStat) {
+	panic();
+}
+
+
+func (P *Printer) DoLabelDecl(s *AST.LabelDecl) {
+	panic();
+}
+
+
+func (P *Printer) DoDeclarationStat(s *AST.DeclarationStat) {
+	P.Declaration(s.Decl, false);
+}
+
+
+func (P *Printer) DoExpressionStat(s *AST.ExpressionStat) {
+	switch s.Tok {
+	case Scanner.ILLEGAL:
+		P.Expr(s.Expr);
+	case Scanner.INC, Scanner.DEC:
+		P.Expr(s.Expr);
+		P.Token(s.Pos, s.Tok);
+	case Scanner.RETURN, Scanner.GO, Scanner.DEFER:
+		P.Token(s.Pos, s.Tok);
+		if s.Expr != nil {
+			P.separator = blank;
+			P.Expr(s.Expr);
+		}
+	default:
+		P.Error(s.Pos, s.Tok, "DoExpressionStat");
+		unreachable();
+	}
+	P.separator = semicolon;
+}
+
+
+func (P *Printer) ControlClause(isForStat bool, init AST.Stat, expr AST.Expr, post AST.Stat) {
+	P.separator = blank;
+	if init == nil && post == nil {
+		// no semicolons required
+		if expr != nil {
+			P.Expr(expr);
+		}
+	} else {
+		// all semicolons required
+		// (they are not separators, print them explicitly)
+		if init != nil {
+			P.Stat(init);
+			P.separator = none;
+		}
+		P.String(0, ";");
+		P.separator = blank;
+		if expr != nil {
+			P.Expr(expr);
+			P.separator = none;
+		}
+		if isForStat {
+			P.String(0, ";");
+			P.separator = blank;
+			if post != nil {
+				P.Stat(post);
+			}
+		}
+	}
+	P.separator = blank;
+}
+
+
+func (P *Printer) DoIfStat(s *AST.IfStat) {
+	P.String(s.Pos, "if");
+	P.ControlClause(false, s.Init, s.Cond, nil);
+	P.Block(s.Body, true);
+	if s.Else != nil {
+		P.separator = blank;
+		P.String(0, "else");
+		P.separator = blank;
+		P.Stat(s.Else);
+	}
+}
+
+
+func (P *Printer) DoForStat(s *AST.ForStat) {
+	P.String(s.Pos, "for");
+	P.ControlClause(true, s.Init, s.Cond, s.Post);
+	P.Block(s.Body, true);
+}
+
+
+func (P *Printer) DoSwitchStat(s *AST.SwitchStat) {
+	P.String(s.Pos, "switch");
+	P.ControlClause(false, s.Init, s.Tag, nil);
+	P.Block(s.Body, false);
+}
+
+
+func (P *Printer) DoSelectStat(s *AST.SelectStat) {
+	panic();
+}
+
+
+func (P *Printer) DoControlFlowStat(s *AST.ControlFlowStat) {
+	P.Token(s.Pos, s.Tok);
+	if s.Label != nil {
+		P.separator = blank;
+		P.Expr(s.Label);
+	}
+	P.separator = semicolon;
+}
+
+
 // ----------------------------------------------------------------------------
 // Declarations