fmt.Scan: renamings, strings, errors

- implement scanning for all renamed types
	(compiler bug stops complex from being renamable,
	so it can't be tested but the code is there)
- %q %x for strings
- error handling now done with panic/recover

R=rsc
CC=golang-dev
http://codereview.appspot.com/1458041

2 files changed, 409 insertions, 192 deletions

diff --git a/src/pkg/fmt/scan.go b/src/pkg/fmt/scan.go
index 9851d4d29..17a08226e 100644
--- a/src/pkg/fmt/scan.go
+++ b/src/pkg/fmt/scan.go
@@ -68,8 +68,7 @@ func Scanln(a ...interface{}) (n int, err os.Error) {
 // is less than the number of arguments, err will report why.
 func Fscan(r io.Reader, a ...interface{}) (n int, err os.Error) {
 	s := newScanState(r, true)
-	n = s.doScan(a)
-	err = s.err
+	n, err = s.doScan(a)
 	s.free()
 	return
 }
@@ -82,8 +81,7 @@ func Fscan(r io.Reader, a ...interface{}) (n int, err os.Error) {
 // number of arguments, err will report why.
 func Fscanln(r io.Reader, a ...interface{}) (n int, err os.Error) {
 	s := newScanState(r, false)
-	n = s.doScan(a)
-	err = s.err
+	n, err = s.doScan(a)
 	s.free()
 	return
 }
@@ -96,19 +94,23 @@ func XXXScanf(format string, a ...interface{}) (n int, err os.Error) {
 // XXXFscanf is incomplete, do not use.
 func XXXFscanf(r io.Reader, format string, a ...interface{}) (n int, err os.Error) {
 	s := newScanState(r, false)
-	n = s.doScanf(format, a)
-	err = s.err
+	n, err = s.doScanf(format, a)
 	s.free()
 	return
 }
 
+// scanError represents an error generated by the scanning software.
+// It's used as a unique signature to identify such errors when recovering.
+type scanError struct {
+	err os.Error
+}
+
 // ss is the internal implementation of ScanState.
 type ss struct {
 	rr        readRuner    // where to read input
 	buf       bytes.Buffer // token accumulator
 	nlIsSpace bool         // whether newline counts as white space
 	peekRune  int          // one-rune lookahead
-	err       os.Error
 }
 
 func (s *ss) GetRune() (rune int, err os.Error) {
@@ -121,13 +123,69 @@ func (s *ss) GetRune() (rune int, err os.Error) {
 	return
 }
 
+const EOF = -1
+
+// The public method returns an error; this private one panics.
+// If getRune reaches EOF, the return value is EOF (-1).
+func (s *ss) getRune() (rune int) {
+	if s.peekRune >= 0 {
+		rune = s.peekRune
+		s.peekRune = -1
+		return
+	}
+	rune, _, err := s.rr.ReadRune()
+	if err != nil {
+		if err == os.EOF {
+			return EOF
+		}
+		s.error(err)
+	}
+	return
+}
+
+// mustGetRune turns os.EOF into a panic(io.ErrUnexpectedEOF).
+// It is called in cases such as string scanning where an EOF is a
+// syntax error.
+func (s *ss) mustGetRune() (rune int) {
+	if s.peekRune >= 0 {
+		rune = s.peekRune
+		s.peekRune = -1
+		return
+	}
+	rune, _, err := s.rr.ReadRune()
+	if err != nil {
+		if err == os.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		s.error(err)
+	}
+	return
+}
+
+
 func (s *ss) UngetRune(rune int) {
 	s.peekRune = rune
 }
 
+func (s *ss) error(err os.Error) {
+	panic(scanError{err})
+}
+
+func (s *ss) errorString(err string) {
+	panic(scanError{os.ErrorString(err)})
+}
+
 func (s *ss) Token() (tok string, err os.Error) {
+	defer func() {
+		if e := recover(); e != nil {
+			if se, ok := e.(scanError); ok {
+				err = se.err
+			} else {
+				panic(e)
+			}
+		}
+	}()
 	tok = s.token()
-	err = s.err
 	return
 }
 
@@ -184,7 +242,6 @@ func newScanState(r io.Reader, nlIsSpace bool) *ss {
 	}
 	s.nlIsSpace = nlIsSpace
 	s.peekRune = -1
-	s.err = nil
 	return s
 }
 
@@ -199,39 +256,38 @@ func (s *ss) free() {
 	_ = ssFree <- s
 }
 
-// token returns the next space-delimited string from the input.
-// For Scanln, it stops at newlines.  For Scan, newlines are treated as
-// spaces.
-func (s *ss) token() string {
+// skipSpace skips spaces and maybe newlines
+func (s *ss) skipSpace() {
 	s.buf.Reset()
-	// skip white space and maybe newline
 	for {
-		rune, err := s.GetRune()
-		if err != nil {
-			s.err = err
-			return ""
+		rune := s.getRune()
+		if rune == EOF {
+			return
 		}
 		if rune == '\n' {
 			if s.nlIsSpace {
 				continue
 			}
-			s.err = os.ErrorString("unexpected newline")
-			return ""
+			s.errorString("unexpected newline")
+			return
 		}
 		if !unicode.IsSpace(rune) {
-			s.buf.WriteRune(rune)
+			s.UngetRune(rune)
 			break
 		}
 	}
+}
+
+// token returns the next space-delimited string from the input.
+// For Scanln, it stops at newlines.  For Scan, newlines are treated as
+// spaces.
+func (s *ss) token() string {
+	s.skipSpace()
 	// read until white space or newline
 	for {
-		rune, err := s.GetRune()
-		if err != nil {
-			if err == os.EOF {
-				break
-			}
-			s.err = err
-			return ""
+		rune := s.getRune()
+		if rune == EOF {
+			break
 		}
 		if unicode.IsSpace(rune) {
 			s.UngetRune(rune)
@@ -242,9 +298,9 @@ func (s *ss) token() string {
 	return s.buf.String()
 }
 
-// typeError sets the error string to an indication that the type of the operand did not match the format
+// typeError indicates that the type of the operand did not match the format
 func (s *ss) typeError(field interface{}, expected string) {
-	s.err = os.ErrorString("expected field of type pointer to " + expected + "; found " + reflect.Typeof(field).String())
+	s.errorString("expected field of type pointer to " + expected + "; found " + reflect.Typeof(field).String())
 }
 
 var intBits = uint(reflect.Typeof(int(0)).Size() * 8)
@@ -253,28 +309,29 @@ var complexError = os.ErrorString("syntax error scanning complex number")
 
 // okVerb verifies that the verb is present in the list, setting s.err appropriately if not.
 func (s *ss) okVerb(verb int, okVerbs, typ string) bool {
-	if s.err != nil { // don't overwrite error
-		return false
-	}
 	for _, v := range okVerbs {
 		if v == verb {
 			return true
 		}
 	}
-	s.err = os.ErrorString("bad verb %" + string(verb) + " for " + typ)
+	s.errorString("bad verb %" + string(verb) + " for " + typ)
 	return false
 }
 
-// scanBool converts the token to a boolean value.
-func (s *ss) scanBool(verb int, tok string) bool {
+// scanBool returns the value of the boolean represented by the next token.
+func (s *ss) scanBool(verb int) bool {
 	if !s.okVerb(verb, "tv", "boolean") {
 		return false
 	}
-	var b bool
-	b, s.err = strconv.Atob(tok)
+	tok := s.token()
+	b, err := strconv.Atob(tok)
+	if err != nil {
+		s.error(err)
+	}
 	return b
 }
 
+// getBase returns the numeric base represented by the verb.
 func (s *ss) getBase(verb int) int {
 	s.okVerb(verb, "bdoxXv", "integer") // sets s.err
 	base := 10
@@ -289,32 +346,34 @@ func (s *ss) getBase(verb int) int {
 	return base
 }
 
-// convertInt returns the value of the integer
-// stored in the token, checking for overflow.  Any error is stored in s.err.
-func (s *ss) convertInt(verb int, tok string, bitSize uint) (i int64) {
+// scanInt returns the value of the integer represented by the next
+// token, checking for overflow.  Any error is stored in s.err.
+func (s *ss) scanInt(verb int, bitSize uint) int64 {
 	base := s.getBase(verb)
-	if s.err != nil {
-		return 0
+	tok := s.token()
+	i, err := strconv.Btoi64(tok, base)
+	if err != nil {
+		s.error(err)
 	}
-	i, s.err = strconv.Btoi64(tok, base)
 	x := (i << (64 - bitSize)) >> (64 - bitSize)
 	if x != i {
-		s.err = os.ErrorString("integer overflow on token " + tok)
+		s.errorString("integer overflow on token " + tok)
 	}
 	return i
 }
 
-// convertUint returns the value of the unsigned integer
-// stored in the token, checking for overflow.  Any error is stored in s.err.
-func (s *ss) convertUint(verb int, tok string, bitSize uint) (i uint64) {
+// scanUint returns the value of the unsigned integer represented
+// by the next token, checking for overflow.  Any error is stored in s.err.
+func (s *ss) scanUint(verb int, bitSize uint) uint64 {
 	base := s.getBase(verb)
-	if s.err != nil {
-		return 0
+	tok := s.token()
+	i, err := strconv.Btoui64(tok, base)
+	if err != nil {
+		s.error(err)
 	}
-	i, s.err = strconv.Btoui64(tok, base)
 	x := (i << (64 - bitSize)) >> (64 - bitSize)
 	if x != i {
-		s.err = os.ErrorString("unsigned integer overflow on token " + tok)
+		s.errorString("unsigned integer overflow on token " + tok)
 	}
 	return i
 }
@@ -327,13 +386,11 @@ func (s *ss) complexParts(str string) (real, imag string) {
 	real, str = floatPart(str)
 	// Must now have a sign.
 	if len(str) == 0 || (str[0] != '+' && str[0] != '-') {
-		s.err = complexError
-		return "", ""
+		s.error(complexError)
 	}
 	imag, str = floatPart(str)
 	if str != "i" {
-		s.err = complexError
-		return "", ""
+		s.error(complexError)
 	}
 	return real, imag
 }
@@ -343,11 +400,11 @@ func (s *ss) complexParts(str string) (real, imag string) {
 func floatPart(str string) (first, last string) {
 	i := 0
 	// leading sign?
-	if len(str) > 0 && (str[0] == '+' || str[0] == '-') {
+	if len(str) > i && (str[0] == '+' || str[0] == '-') {
 		i++
 	}
 	// digits?
-	for len(str) > 0 && '0' <= str[i] && str[i] <= '9' {
+	for len(str) > i && '0' <= str[i] && str[i] <= '9' {
 		i++
 	}
 	// period?
@@ -355,197 +412,323 @@ func floatPart(str string) (first, last string) {
 		i++
 	}
 	// fraction?
-	for len(str) > 0 && '0' <= str[i] && str[i] <= '9' {
+	for len(str) > i && '0' <= str[i] && str[i] <= '9' {
 		i++
 	}
 	// exponent?
-	if len(str) > 0 && (str[i] == 'e' || str[i] == 'E') {
+	if len(str) > i && (str[i] == 'e' || str[i] == 'E') {
 		i++
 		// leading sign?
-		if str[0] == '+' || str[0] == '-' {
+		if str[i] == '+' || str[i] == '-' {
 			i++
 		}
 		// digits?
-		for len(str) > 0 && '0' <= str[i] && str[i] <= '9' {
+		for len(str) > i && '0' <= str[i] && str[i] <= '9' {
 			i++
 		}
 	}
 	return str[0:i], str[i:]
 }
 
-// scanFloat converts the string to a float value.
-func (s *ss) scanFloat(str string) float64 {
-	var f float
-	f, s.err = strconv.Atof(str)
+// convertFloat converts the string to a float value.
+func (s *ss) convertFloat(str string) float64 {
+	f, err := strconv.Atof(str)
+	if err != nil {
+		s.error(err)
+	}
 	return float64(f)
 }
 
-// scanFloat32 converts the string to a float32 value.
-func (s *ss) scanFloat32(str string) float64 {
-	var f float32
-	f, s.err = strconv.Atof32(str)
+// convertFloat32 converts the string to a float32 value.
+func (s *ss) convertFloat32(str string) float64 {
+	f, err := strconv.Atof32(str)
+	if err != nil {
+		s.error(err)
+	}
 	return float64(f)
 }
 
-// scanFloat64 converts the string to a float64 value.
-func (s *ss) scanFloat64(str string) float64 {
-	var f float64
-	f, s.err = strconv.Atof64(str)
+// convertFloat64 converts the string to a float64 value.
+func (s *ss) convertFloat64(str string) float64 {
+	f, err := strconv.Atof64(str)
+	if err != nil {
+		s.error(err)
+	}
 	return f
 }
 
-// scanComplex converts the token to a complex128 value.
+// convertComplex converts the next token to a complex128 value.
 // The atof argument is a type-specific reader for the underlying type.
 // If we're reading complex64, atof will parse float32s and convert them
 // to float64's to avoid reproducing this code for each complex type.
-func (s *ss) scanComplex(tok string, atof func(*ss, string) float64) complex128 {
-	sreal, simag := s.complexParts(tok)
-	if s.err != nil {
+func (s *ss) scanComplex(verb int, atof func(*ss, string) float64) complex128 {
+	if !s.okVerb(verb, floatVerbs, "complex") {
 		return 0
 	}
-	var real, imag float64
-	real = atof(s, sreal)
-	if s.err != nil {
-		return 0
+	tok := s.token()
+	sreal, simag := s.complexParts(tok)
+	real := atof(s, sreal)
+	imag := atof(s, simag)
+	return cmplx(real, imag)
+}
+
+// convertString returns the string represented by the next input characters.
+// The format of the input is determined by the verb.
+func (s *ss) convertString(verb int) string {
+	if !s.okVerb(verb, "svqx", "string") {
+		return ""
 	}
-	imag = atof(s, simag)
-	if s.err != nil {
-		return 0
+	s.skipSpace()
+	switch verb {
+	case 'q':
+		return s.quotedString()
+	case 'x':
+		return s.hexString()
 	}
-	return cmplx(real, imag)
+	return s.token() // %s and %v just return the next word
+}
+
+// quotedString returns the double- or back-quoted string.
+func (s *ss) quotedString() string {
+	quote := s.mustGetRune()
+	switch quote {
+	case '`':
+		// Back-quoted: Anything goes until EOF or back quote.
+		for {
+			rune := s.mustGetRune()
+			if rune == quote {
+				break
+			}
+			s.buf.WriteRune(rune)
+		}
+		return s.buf.String()
+	case '"':
+		// Double-quoted: Include the quotes and let strconv.Unquote do the backslash escapes.
+		s.buf.WriteRune(quote)
+		for {
+			rune := s.mustGetRune()
+			s.buf.WriteRune(rune)
+			if rune == '\\' {
+				// In a legal backslash escape, no matter how long, only the character
+				// immediately after the escape can itself be a backslash or quote.
+				// Thus we only need to protect the first character after the backslash.
+				rune := s.mustGetRune()
+				s.buf.WriteRune(rune)
+			} else if rune == '"' {
+				break
+			}
+		}
+		result, err := strconv.Unquote(s.buf.String())
+		if err != nil {
+			s.error(err)
+		}
+		return result
+	default:
+		s.errorString("expected quoted string")
+	}
+	return ""
+}
+
+// hexDigit returns the value of the hexadecimal digit
+func (s *ss) hexDigit(digit int) int {
+	switch digit {
+	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
+		return digit - '0'
+	case 'a', 'b', 'c', 'd', 'e', 'f':
+		return 10 + digit - 'a'
+	case 'A', 'B', 'C', 'D', 'E', 'F':
+		return 10 + digit - 'A'
+	}
+	s.errorString("Scan: illegal hex digit")
+	return 0
+}
+
+// hexByte returns the next hex-encoded (two-character) byte from the input.
+// There must be either two hexadecimal digits or a space character in the input.
+func (s *ss) hexByte() (b byte, ok bool) {
+	rune1 := s.getRune()
+	if rune1 == EOF {
+		return
+	}
+	if unicode.IsSpace(rune1) {
+		s.UngetRune(rune1)
+		return
+	}
+	rune2 := s.mustGetRune()
+	return byte(s.hexDigit(rune1)<<4 | s.hexDigit(rune2)), true
+}
+
+// hexString returns the space-delimited hexpair-encoded string.
+func (s *ss) hexString() string {
+	for {
+		b, ok := s.hexByte()
+		if !ok {
+			break
+		}
+		s.buf.WriteByte(b)
+	}
+	if s.buf.Len() == 0 {
+		s.errorString("Scan: no hex data for %x string")
+		return ""
+	}
+	return s.buf.String()
 }
 
 const floatVerbs = "eEfFgGv"
 
 // scanOne scans a single value, deriving the scanner from the type of the argument.
 func (s *ss) scanOne(verb int, field interface{}) {
+	var err os.Error
 	// If the parameter has its own Scan method, use that.
 	if v, ok := field.(Scanner); ok {
-		s.err = v.Scan(s)
-		return
-	}
-	tok := s.token()
-	if s.err != nil {
+		err = v.Scan(s)
+		if err != nil {
+			s.error(err)
+		}
 		return
 	}
 	switch v := field.(type) {
 	case *bool:
-		*v = s.scanBool(verb, tok)
+		*v = s.scanBool(verb)
 	case *complex:
-		*v = complex(s.scanComplex(tok, (*ss).scanFloat))
+		*v = complex(s.scanComplex(verb, (*ss).convertFloat))
 	case *complex64:
-		*v = complex64(s.scanComplex(tok, (*ss).scanFloat32))
+		*v = complex64(s.scanComplex(verb, (*ss).convertFloat32))
 	case *complex128:
-		*v = s.scanComplex(tok, (*ss).scanFloat64)
+		*v = s.scanComplex(verb, (*ss).convertFloat64)
 	case *int:
-		*v = int(s.convertInt(verb, tok, intBits))
+		*v = int(s.scanInt(verb, intBits))
 	case *int8:
-		*v = int8(s.convertInt(verb, tok, 8))
+		*v = int8(s.scanInt(verb, 8))
 	case *int16:
-		*v = int16(s.convertInt(verb, tok, 16))
+		*v = int16(s.scanInt(verb, 16))
 	case *int32:
-		*v = int32(s.convertInt(verb, tok, 32))
+		*v = int32(s.scanInt(verb, 32))
 	case *int64:
-		*v = s.convertInt(verb, tok, intBits)
+		*v = s.scanInt(verb, intBits)
 	case *uint:
-		*v = uint(s.convertUint(verb, tok, intBits))
+		*v = uint(s.scanUint(verb, intBits))
 	case *uint8:
-		*v = uint8(s.convertUint(verb, tok, 8))
+		*v = uint8(s.scanUint(verb, 8))
 	case *uint16:
-		*v = uint16(s.convertUint(verb, tok, 16))
+		*v = uint16(s.scanUint(verb, 16))
 	case *uint32:
-		*v = uint32(s.convertUint(verb, tok, 32))
+		*v = uint32(s.scanUint(verb, 32))
 	case *uint64:
-		*v = s.convertUint(verb, tok, 64)
+		*v = s.scanUint(verb, 64)
 	case *uintptr:
-		*v = uintptr(s.convertUint(verb, tok, uintptrBits))
+		*v = uintptr(s.scanUint(verb, uintptrBits))
+	// Floats are tricky because you want to scan in the precision of the result, not
+	// scan in high precision and convert, in order to preserve the correct error condition.
 	case *float:
 		if s.okVerb(verb, floatVerbs, "float") {
-			*v, s.err = strconv.Atof(tok)
+			*v = float(s.convertFloat(s.token()))
 		}
 	case *float32:
 		if s.okVerb(verb, floatVerbs, "float32") {
-			*v, s.err = strconv.Atof32(tok)
+			*v = float32(s.convertFloat32(s.token()))
 		}
 	case *float64:
 		if s.okVerb(verb, floatVerbs, "float64") {
-			*v, s.err = strconv.Atof64(tok)
+			*v = s.convertFloat64(s.token())
 		}
 	case *string:
-		*v = tok
+		*v = s.convertString(verb)
 	default:
 		val := reflect.NewValue(v)
 		ptr, ok := val.(*reflect.PtrValue)
 		if !ok {
-			s.err = os.ErrorString("Scan: type not a pointer: " + val.Type().String())
+			s.errorString("Scan: type not a pointer: " + val.Type().String())
 			return
 		}
 		switch v := ptr.Elem().(type) {
+		case *reflect.BoolValue:
+			v.Set(s.scanBool(verb))
 		case *reflect.IntValue:
-			v.Set(int(s.convertInt(verb, tok, intBits)))
+			v.Set(int(s.scanInt(verb, intBits)))
 		case *reflect.Int8Value:
-			v.Set(int8(s.convertInt(verb, tok, 8)))
+			v.Set(int8(s.scanInt(verb, 8)))
 		case *reflect.Int16Value:
-			v.Set(int16(s.convertInt(verb, tok, 16)))
+			v.Set(int16(s.scanInt(verb, 16)))
 		case *reflect.Int32Value:
-			v.Set(int32(s.convertInt(verb, tok, 32)))
+			v.Set(int32(s.scanInt(verb, 32)))
 		case *reflect.Int64Value:
-			v.Set(s.convertInt(verb, tok, 64))
+			v.Set(s.scanInt(verb, 64))
 		case *reflect.UintValue:
-			v.Set(uint(s.convertUint(verb, tok, intBits)))
+			v.Set(uint(s.scanUint(verb, intBits)))
 		case *reflect.Uint8Value:
-			v.Set(uint8(s.convertUint(verb, tok, 8)))
+			v.Set(uint8(s.scanUint(verb, 8)))
 		case *reflect.Uint16Value:
-			v.Set(uint16(s.convertUint(verb, tok, 16)))
+			v.Set(uint16(s.scanUint(verb, 16)))
 		case *reflect.Uint32Value:
-			v.Set(uint32(s.convertUint(verb, tok, 32)))
+			v.Set(uint32(s.scanUint(verb, 32)))
 		case *reflect.Uint64Value:
-			v.Set(s.convertUint(verb, tok, 64))
+			v.Set(s.scanUint(verb, 64))
 		case *reflect.UintptrValue:
-			v.Set(uintptr(s.convertUint(verb, tok, uintptrBits)))
+			v.Set(uintptr(s.scanUint(verb, uintptrBits)))
+		case *reflect.StringValue:
+			v.Set(s.convertString(verb))
+		case *reflect.FloatValue:
+			v.Set(float(s.convertFloat(s.token())))
+		case *reflect.Float32Value:
+			v.Set(float32(s.convertFloat(s.token())))
+		case *reflect.Float64Value:
+			v.Set(s.convertFloat(s.token()))
+		case *reflect.ComplexValue:
+			v.Set(complex(s.scanComplex(verb, (*ss).convertFloat)))
+		case *reflect.Complex64Value:
+			v.Set(complex64(s.scanComplex(verb, (*ss).convertFloat32)))
+		case *reflect.Complex128Value:
+			v.Set(s.scanComplex(verb, (*ss).convertFloat64))
 		default:
-			s.err = os.ErrorString("Scan: can't handle type: " + val.Type().String())
+			s.errorString("Scan: can't handle type: " + val.Type().String())
+		}
+	}
+}
+
+// errorHandler turns local panics into error returns.  EOFs are benign.
+func errorHandler(errp *os.Error) {
+	if e := recover(); e != nil {
+		if se, ok := e.(scanError); ok { // catch local error
+			if se.err != os.EOF {
+				*errp = se.err
+			}
+		} else {
+			panic(e)
 		}
 	}
 }
 
 // doScan does the real work for scanning without a format string.
 // At the moment, it handles only pointers to basic types.
-func (s *ss) doScan(a []interface{}) int {
-	for fieldnum, field := range a {
+func (s *ss) doScan(a []interface{}) (numProcessed int, err os.Error) {
+	defer errorHandler(&err)
+	for _, field := range a {
 		s.scanOne('v', field)
-		if s.err != nil {
-			return fieldnum
-		}
+		numProcessed++
 	}
 	// Check for newline if required.
 	if !s.nlIsSpace {
 		for {
-			rune, err := s.GetRune()
-			if err != nil {
-				if err == os.EOF {
-					break
-				}
-				s.err = err
-				break
-			}
-			if rune == '\n' {
+			rune := s.getRune()
+			if rune == '\n' || rune == EOF {
 				break
 			}
 			if !unicode.IsSpace(rune) {
-				s.err = os.ErrorString("Scan: expected newline")
+				s.errorString("Scan: expected newline")
 				break
 			}
 		}
 	}
-	return len(a)
+	return
 }
 
 // doScanf does the real work when scanning with a format string.
 //  At the moment, it handles only pointers to basic types.
-func (s *ss) doScanf(format string, a []interface{}) int {
+func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err os.Error) {
+	defer errorHandler(&err)
 	end := len(format) - 1
-	fieldnum := 0 // we process one item per non-trivial format
+	// We process one item per non-trivial format
 	for i := 0; i <= end; {
 		c, w := utf8.DecodeRuneInString(format[i:])
 		if c != '%' || i == end {
@@ -563,17 +746,14 @@ func (s *ss) doScanf(format string, a []interface{}) int {
 			continue
 		}
 
-		if fieldnum >= len(a) { // out of operands
-			s.err = os.ErrorString("too few operands for format %" + format[i-w:])
+		if numProcessed >= len(a) { // out of operands
+			s.errorString("too few operands for format %" + format[i-w:])
 			break
 		}
-		field := a[fieldnum]
+		field := a[numProcessed]
 
 		s.scanOne(c, field)
-		if s.err != nil {
-			break
-		}
-		fieldnum++
+		numProcessed++
 	}
-	return fieldnum
+	return
 }
diff --git a/src/pkg/fmt/scan_test.go b/src/pkg/fmt/scan_test.go
index 55808e964..353aa7e74 100644
--- a/src/pkg/fmt/scan_test.go
+++ b/src/pkg/fmt/scan_test.go
@@ -27,49 +27,65 @@ type ScanfTest struct {
 }
 
 type (
-	renamedInt     int
-	renamedInt8    int8
-	renamedInt16   int16
-	renamedInt32   int32
-	renamedInt64   int64
-	renamedUint    uint
-	renamedUint8   uint8
-	renamedUint16  uint16
-	renamedUint32  uint32
-	renamedUint64  uint64
-	renamedUintptr uintptr
+	renamedBool       bool
+	renamedInt        int
+	renamedInt8       int8
+	renamedInt16      int16
+	renamedInt32      int32
+	renamedInt64      int64
+	renamedUint       uint
+	renamedUint8      uint8
+	renamedUint16     uint16
+	renamedUint32     uint32
+	renamedUint64     uint64
+	renamedUintptr    uintptr
+	renamedString     string
+	renamedFloat      float
+	renamedFloat32    float32
+	renamedFloat64    float64
+	renamedComplex    complex
+	renamedComplex64  complex64
+	renamedComplex128 complex128
 )
 
 var (
-	boolVal           bool
-	intVal            int
-	int8Val           int8
-	int16Val          int16
-	int32Val          int32
-	int64Val          int64
-	uintVal           uint
-	uint8Val          uint8
-	uint16Val         uint16
-	uint32Val         uint32
-	uint64Val         uint64
-	floatVal          float
-	float32Val        float32
-	float64Val        float64
-	stringVal         string
-	complexVal        complex
-	complex64Val      complex64
-	complex128Val     complex128
-	renamedIntVal     renamedInt
-	renamedInt8Val    renamedInt8
-	renamedInt16Val   renamedInt16
-	renamedInt32Val   renamedInt32
-	renamedInt64Val   renamedInt64
-	renamedUintVal    renamedUint
-	renamedUint8Val   renamedUint8
-	renamedUint16Val  renamedUint16
-	renamedUint32Val  renamedUint32
-	renamedUint64Val  renamedUint64
-	renamedUintptrVal renamedUintptr
+	boolVal              bool
+	intVal               int
+	int8Val              int8
+	int16Val             int16
+	int32Val             int32
+	int64Val             int64
+	uintVal              uint
+	uint8Val             uint8
+	uint16Val            uint16
+	uint32Val            uint32
+	uint64Val            uint64
+	floatVal             float
+	float32Val           float32
+	float64Val           float64
+	stringVal            string
+	complexVal           complex
+	complex64Val         complex64
+	complex128Val        complex128
+	renamedBoolVal       renamedBool
+	renamedIntVal        renamedInt
+	renamedInt8Val       renamedInt8
+	renamedInt16Val      renamedInt16
+	renamedInt32Val      renamedInt32
+	renamedInt64Val      renamedInt64
+	renamedUintVal       renamedUint
+	renamedUint8Val      renamedUint8
+	renamedUint16Val     renamedUint16
+	renamedUint32Val     renamedUint32
+	renamedUint64Val     renamedUint64
+	renamedUintptrVal    renamedUintptr
+	renamedStringVal     renamedString
+	renamedFloatVal      renamedFloat
+	renamedFloat32Val    renamedFloat32
+	renamedFloat64Val    renamedFloat64
+	renamedComplexVal    renamedComplex
+	renamedComplex64Val  renamedComplex64
+	renamedComplex128Val renamedComplex128
 )
 
 // Xs accepts any non-empty run of x's.
@@ -92,7 +108,9 @@ func (x *Xs) Scan(state ScanState) os.Error {
 var xVal Xs
 
 var scanTests = []ScanTest{
-	ScanTest{"T\n", &boolVal, true},
+	// Numbers
+	ScanTest{"T\n", &boolVal, true},  // boolean test vals toggle to be sure they are written
+	ScanTest{"F\n", &boolVal, false}, // restored to zero value
 	ScanTest{"21\n", &intVal, 21},
 	ScanTest{"22\n", &int8Val, int8(22)},
 	ScanTest{"23\n", &int16Val, int16(23)},
@@ -125,8 +143,11 @@ var scanTests = []ScanTest{
 	ScanTest{"(3.4e1-2i)\n", &complexVal, 3.4e1 - 2i},
 	ScanTest{"-3.45e1-3i\n", &complex64Val, complex64(-3.45e1 - 3i)},
 	ScanTest{"-.45e1-1e2i\n", &complex128Val, complex128(-.45e1 - 100i)},
+	ScanTest{"hello\n", &stringVal, "hello"},
 
 	// Renamed types
+	ScanTest{"true\n", &renamedBoolVal, renamedBool(true)},
+	ScanTest{"F\n", &renamedBoolVal, renamedBool(false)},
 	ScanTest{"101\n", &renamedIntVal, renamedInt(101)},
 	ScanTest{"102\n", &renamedIntVal, renamedInt(102)},
 	ScanTest{"103\n", &renamedUintVal, renamedUint(103)},
@@ -140,14 +161,15 @@ var scanTests = []ScanTest{
 	ScanTest{"111\n", &renamedUint32Val, renamedUint32(111)},
 	ScanTest{"112\n", &renamedUint64Val, renamedUint64(112)},
 	ScanTest{"113\n", &renamedUintptrVal, renamedUintptr(113)},
+	ScanTest{"114\n", &renamedStringVal, renamedString("114")},
 
 	// Custom scanner.
 	ScanTest{"  xxx ", &xVal, Xs("xxx")},
 }
 
 var scanfTests = []ScanfTest{
-	ScanfTest{"%v", "FALSE\n", &boolVal, false},
-	ScanfTest{"%t", "true\n", &boolVal, true},
+	ScanfTest{"%v", "TRUE\n", &boolVal, true},
+	ScanfTest{"%t", "false\n", &boolVal, false},
 	ScanfTest{"%v", "-71\n", &intVal, -71},
 	ScanfTest{"%d", "72\n", &intVal, 72},
 	ScanfTest{"%d", "73\n", &int8Val, int8(73)},
@@ -168,7 +190,15 @@ var scanfTests = []ScanfTest{
 	ScanfTest{"%x", "a75\n", &uintVal, uint(0xa75)},
 	ScanfTest{"%x", "A75\n", &uintVal, uint(0xa75)},
 
+	// Strings
+	ScanfTest{"%s", "using-%s\n", &stringVal, "using-%s"},
+	ScanfTest{"%x", "7573696e672d2578\n", &stringVal, "using-%x"},
+	ScanfTest{"%q", `"quoted\twith\\do\u0075bl\x65s"` + "\n", &stringVal, "quoted\twith\\doubles"},
+	ScanfTest{"%q", "`quoted with backs`\n", &stringVal, "quoted with backs"},
+
 	// Renamed types
+	ScanfTest{"%v\n", "true\n", &renamedBoolVal, renamedBool(true)},
+	ScanfTest{"%t\n", "F\n", &renamedBoolVal, renamedBool(false)},
 	ScanfTest{"%v", "101\n", &renamedIntVal, renamedInt(101)},
 	ScanfTest{"%o", "0146\n", &renamedIntVal, renamedInt(102)},
 	ScanfTest{"%v", "103\n", &renamedUintVal, renamedUint(103)},
@@ -182,6 +212,13 @@ var scanfTests = []ScanfTest{
 	ScanfTest{"%d", "111\n", &renamedUint32Val, renamedUint32(111)},
 	ScanfTest{"%d", "112\n", &renamedUint64Val, renamedUint64(112)},
 	ScanfTest{"%d", "113\n", &renamedUintptrVal, renamedUintptr(113)},
+	ScanfTest{"%s", "114\n", &renamedStringVal, renamedString("114")},
+	ScanfTest{"%g", "115.1\n", &renamedFloatVal, renamedFloat(115.1)},
+	ScanfTest{"%g", "116e1\n", &renamedFloat32Val, renamedFloat32(116e1)},
+	ScanfTest{"%g", "-11.7e+1", &renamedFloat64Val, renamedFloat64(-11.7e+1)},
+	ScanfTest{"%g", "11+5.1i\n", &renamedComplexVal, renamedComplex(11 + 5.1i)},
+	ScanfTest{"%g", "11+6e1i\n", &renamedComplex64Val, renamedComplex64(11 + 6e1i)},
+	ScanfTest{"%g", "-11.+7e+1i", &renamedComplex128Val, renamedComplex128(-11. + 7e+1i)},
 
 	ScanfTest{"%x", "FFFFFFFF\n", &uint32Val, uint32(0xFFFFFFFF)},
 }