go/ssa/interp/value.go - tools - Git at Google

 // Copyright 2013 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.

 // +build go1.5

 package interp

 // Values
 //
 // All interpreter values are "boxed" in the empty interface, value.
 // The range of possible dynamic types within value are:
 //
 // - bool
 // - numbers (all built-in int/float/complex types are distinguished)
 // - string
 // - map[value]value --- maps for which  usesBuiltinMap(keyType)
 //   *hashmap        --- maps for which !usesBuiltinMap(keyType)
 // - chan value
 // - []value --- slices
 // - iface --- interfaces.
 // - structure --- structs.  Fields are ordered and accessed by numeric indices.
 // - array --- arrays.
 // - *value --- pointers.  Careful: *value is a distinct type from *array etc.
 // - *ssa.Function \
 //   *ssa.Builtin   } --- functions.  A nil 'func' is always of type *ssa.Function.
 //   *closure      /
 // - tuple --- as returned by Return, Next, "value,ok" modes, etc.
 // - iter --- iterators from 'range' over map or string.
 // - bad --- a poison pill for locals that have gone out of scope.
 // - rtype -- the interpreter's concrete implementation of reflect.Type
 //
 // Note that nil is not on this list.
 //
 // Pay close attention to whether or not the dynamic type is a pointer.
 // The compiler cannot help you since value is an empty interface.

 import (
 	"bytes"
 	"fmt"
 	"go/types"
 	"io"
 	"reflect"
 	"strings"
 	"sync"
 	"unsafe"

 	"golang.org/x/tools/go/ssa"
 	"golang.org/x/tools/go/types/typeutil"
 )

 type value interface{}

 type tuple []value

 type array []value

 type iface struct {
 	t types.Type // never an "untyped" type
 	v value
 }

 type structure []value

 // For map, array, *array, slice, string or channel.
 type iter interface {
 	// next returns a Tuple (key, value, ok).
 	// key and value are unaliased, e.g. copies of the sequence element.
 	next() tuple
 }

 type closure struct {
 	Fn  *ssa.Function
 	Env []value
 }

 type bad struct{}

 type rtype struct {
 	t types.Type
 }

 // Hash functions and equivalence relation:

 // hashString computes the FNV hash of s.
 func hashString(s string) int {
 	var h uint32
 	for i := 0; i < len(s); i++ {
 		h ^= uint32(s[i])
 		h *= 16777619
 	}
 	return int(h)
 }

 var (
 	mu     sync.Mutex
 	hasher = typeutil.MakeHasher()
 )

 // hashType returns a hash for t such that
 // types.Identical(x, y) => hashType(x) == hashType(y).
 func hashType(t types.Type) int {
 	mu.Lock()
 	h := int(hasher.Hash(t))
 	mu.Unlock()
 	return h
 }

 // usesBuiltinMap returns true if the built-in hash function and
 // equivalence relation for type t are consistent with those of the
 // interpreter's representation of type t.  Such types are: all basic
 // types (bool, numbers, string), pointers and channels.
 //
 // usesBuiltinMap returns false for types that require a custom map
 // implementation: interfaces, arrays and structs.
 //
 // Panic ensues if t is an invalid map key type: function, map or slice.
 func usesBuiltinMap(t types.Type) bool {
 	switch t := t.(type) {
 	case *types.Basic, *types.Chan, *types.Pointer:
 		return true
 	case *types.Named:
 		return usesBuiltinMap(t.Underlying())
 	case *types.Interface, *types.Array, *types.Struct:
 		return false
 	}
 	panic(fmt.Sprintf("invalid map key type: %T", t))
 }

 func (x array) eq(t types.Type, _y interface{}) bool {
 	y := _y.(array)
 	tElt := t.Underlying().(*types.Array).Elem()
 	for i, xi := range x {
 		if !equals(tElt, xi, y[i]) {
 			return false
 		}
 	}
 	return true
 }

 func (x array) hash(t types.Type) int {
 	h := 0
 	tElt := t.Underlying().(*types.Array).Elem()
 	for _, xi := range x {
 		h += hash(tElt, xi)
 	}
 	return h
 }

 func (x structure) eq(t types.Type, _y interface{}) bool {
 	y := _y.(structure)
 	tStruct := t.Underlying().(*types.Struct)
 	for i, n := 0, tStruct.NumFields(); i < n; i++ {
 		if f := tStruct.Field(i); !f.Anonymous() {
 			if !equals(f.Type(), x[i], y[i]) {
 				return false
 			}
 		}
 	}
 	return true
 }

 func (x structure) hash(t types.Type) int {
 	tStruct := t.Underlying().(*types.Struct)
 	h := 0
 	for i, n := 0, tStruct.NumFields(); i < n; i++ {
 		if f := tStruct.Field(i); !f.Anonymous() {
 			h += hash(f.Type(), x[i])
 		}
 	}
 	return h
 }

 // nil-tolerant variant of types.Identical.
 func sameType(x, y types.Type) bool {
 	if x == nil {
 		return y == nil
 	}
 	return y != nil && types.Identical(x, y)
 }

 func (x iface) eq(t types.Type, _y interface{}) bool {
 	y := _y.(iface)
 	return sameType(x.t, y.t) && (x.t == nil || equals(x.t, x.v, y.v))
 }

 func (x iface) hash(_ types.Type) int {
 	return hashType(x.t)*8581 + hash(x.t, x.v)
 }

 func (x rtype) hash(_ types.Type) int {
 	return hashType(x.t)
 }

 func (x rtype) eq(_ types.Type, y interface{}) bool {
 	return types.Identical(x.t, y.(rtype).t)
 }

 // equals returns true iff x and y are equal according to Go's
 // linguistic equivalence relation for type t.
 // In a well-typed program, the dynamic types of x and y are
 // guaranteed equal.
 func equals(t types.Type, x, y value) bool {
 	switch x := x.(type) {
 	case bool:
 		return x == y.(bool)
 	case int:
 		return x == y.(int)
 	case int8:
 		return x == y.(int8)
 	case int16:
 		return x == y.(int16)
 	case int32:
 		return x == y.(int32)
 	case int64:
 		return x == y.(int64)
 	case uint:
 		return x == y.(uint)
 	case uint8:
 		return x == y.(uint8)
 	case uint16:
 		return x == y.(uint16)
 	case uint32:
 		return x == y.(uint32)
 	case uint64:
 		return x == y.(uint64)
 	case uintptr:
 		return x == y.(uintptr)
 	case float32:
 		return x == y.(float32)
 	case float64:
 		return x == y.(float64)
 	case complex64:
 		return x == y.(complex64)
 	case complex128:
 		return x == y.(complex128)
 	case string:
 		return x == y.(string)
 	case *value:
 		return x == y.(*value)
 	case chan value:
 		return x == y.(chan value)
 	case structure:
 		return x.eq(t, y)
 	case array:
 		return x.eq(t, y)
 	case iface:
 		return x.eq(t, y)
 	case rtype:
 		return x.eq(t, y)
 	}

 	// Since map, func and slice don't support comparison, this
 	// case is only reachable if one of x or y is literally nil
 	// (handled in eqnil) or via interface{} values.
 	panic(fmt.Sprintf("comparing uncomparable type %s", t))
 }

 // Returns an integer hash of x such that equals(x, y) => hash(x) == hash(y).
 func hash(t types.Type, x value) int {
 	switch x := x.(type) {
 	case bool:
 		if x {
 			return 1
 		}
 		return 0
 	case int:
 		return x
 	case int8:
 		return int(x)
 	case int16:
 		return int(x)
 	case int32:
 		return int(x)
 	case int64:
 		return int(x)
 	case uint:
 		return int(x)
 	case uint8:
 		return int(x)
 	case uint16:
 		return int(x)
 	case uint32:
 		return int(x)
 	case uint64:
 		return int(x)
 	case uintptr:
 		return int(x)
 	case float32:
 		return int(x)
 	case float64:
 		return int(x)
 	case complex64:
 		return int(real(x))
 	case complex128:
 		return int(real(x))
 	case string:
 		return hashString(x)
 	case *value:
 		return int(uintptr(unsafe.Pointer(x)))
 	case chan value:
 		return int(uintptr(reflect.ValueOf(x).Pointer()))
 	case structure:
 		return x.hash(t)
 	case array:
 		return x.hash(t)
 	case iface:
 		return x.hash(t)
 	case rtype:
 		return x.hash(t)
 	}
 	panic(fmt.Sprintf("%T is unhashable", x))
 }

 // reflect.Value struct values don't have a fixed shape, since the
 // payload can be a scalar or an aggregate depending on the instance.
 // So store (and load) can't simply use recursion over the shape of the
 // rhs value, or the lhs, to copy the value; we need the static type
 // information.  (We can't make reflect.Value a new basic data type
 // because its "structness" is exposed to Go programs.)

 // load returns the value of type T in *addr.
 func load(T types.Type, addr *value) value {
 	switch T := T.Underlying().(type) {
 	case *types.Struct:
 		v := (*addr).(structure)
 		a := make(structure, len(v))
 		for i := range a {
 			a[i] = load(T.Field(i).Type(), &v[i])
 		}
 		return a
 	case *types.Array:
 		v := (*addr).(array)
 		a := make(array, len(v))
 		for i := range a {
 			a[i] = load(T.Elem(), &v[i])
 		}
 		return a
 	default:
 		return *addr
 	}
 }

 // store stores value v of type T into *addr.
 func store(T types.Type, addr *value, v value) {
 	switch T := T.Underlying().(type) {
 	case *types.Struct:
 		lhs := (*addr).(structure)
 		rhs := v.(structure)
 		for i := range lhs {
 			store(T.Field(i).Type(), &lhs[i], rhs[i])
 		}
 	case *types.Array:
 		lhs := (*addr).(array)
 		rhs := v.(array)
 		for i := range lhs {
 			store(T.Elem(), &lhs[i], rhs[i])
 		}
 	default:
 		*addr = v
 	}
 }

 // Prints in the style of built-in println.
 // (More or less; in gc println is actually a compiler intrinsic and
 // can distinguish println(1) from println(interface{}(1)).)
 func writeValue(buf *bytes.Buffer, v value) {
 	switch v := v.(type) {
 	case nil, bool, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr, float32, float64, complex64, complex128, string:
 		fmt.Fprintf(buf, "%v", v)

 	case map[value]value:
 		buf.WriteString("map[")
 		sep := ""
 		for k, e := range v {
 			buf.WriteString(sep)
 			sep = " "
 			writeValue(buf, k)
 			buf.WriteString(":")
 			writeValue(buf, e)
 		}
 		buf.WriteString("]")

 	case *hashmap:
 		buf.WriteString("map[")
 		sep := " "
 		for _, e := range v.entries() {
 			for e != nil {
 				buf.WriteString(sep)
 				sep = " "
 				writeValue(buf, e.key)
 				buf.WriteString(":")
 				writeValue(buf, e.value)
 				e = e.next
 			}
 		}
 		buf.WriteString("]")

 	case chan value:
 		fmt.Fprintf(buf, "%v", v) // (an address)

 	case *value:
 		if v == nil {
 			buf.WriteString("<nil>")
 		} else {
 			fmt.Fprintf(buf, "%p", v)
 		}

 	case iface:
 		fmt.Fprintf(buf, "(%s, ", v.t)
 		writeValue(buf, v.v)
 		buf.WriteString(")")

 	case structure:
 		buf.WriteString("{")
 		for i, e := range v {
 			if i > 0 {
 				buf.WriteString(" ")
 			}
 			writeValue(buf, e)
 		}
 		buf.WriteString("}")

 	case array:
 		buf.WriteString("[")
 		for i, e := range v {
 			if i > 0 {
 				buf.WriteString(" ")
 			}
 			writeValue(buf, e)
 		}
 		buf.WriteString("]")

 	case []value:
 		buf.WriteString("[")
 		for i, e := range v {
 			if i > 0 {
 				buf.WriteString(" ")
 			}
 			writeValue(buf, e)
 		}
 		buf.WriteString("]")

 	case *ssa.Function, *ssa.Builtin, *closure:
 		fmt.Fprintf(buf, "%p", v) // (an address)

 	case rtype:
 		buf.WriteString(v.t.String())

 	case tuple:
 		// Unreachable in well-formed Go programs
 		buf.WriteString("(")
 		for i, e := range v {
 			if i > 0 {
 				buf.WriteString(", ")
 			}
 			writeValue(buf, e)
 		}
 		buf.WriteString(")")

 	default:
 		fmt.Fprintf(buf, "<%T>", v)
 	}
 }

 // Implements printing of Go values in the style of built-in println.
 func toString(v value) string {
 	var b bytes.Buffer
 	writeValue(&b, v)
 	return b.String()
 }

 // ------------------------------------------------------------------------
 // Iterators

 type stringIter struct {
 	*strings.Reader
 	i int
 }

 func (it *stringIter) next() tuple {
 	okv := make(tuple, 3)
 	ch, n, err := it.ReadRune()
 	ok := err != io.EOF
 	okv[0] = ok
 	if ok {
 		okv[1] = it.i
 		okv[2] = ch
 	}
 	it.i += n
 	return okv
 }

 type mapIter chan [2]value

 func (it mapIter) next() tuple {
 	kv, ok := <-it
 	return tuple{ok, kv[0], kv[1]}
 }
	// Copyright 2013 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.

	// +build go1.5

	package interp

	// Values
	//
	// All interpreter values are "boxed" in the empty interface, value.
	// The range of possible dynamic types within value are:
	//
	// - bool
	// - numbers (all built-in int/float/complex types are distinguished)
	// - string
	// - map[value]value --- maps for which usesBuiltinMap(keyType)
	// *hashmap --- maps for which !usesBuiltinMap(keyType)
	// - chan value
	// - []value --- slices
	// - iface --- interfaces.
	// - structure --- structs. Fields are ordered and accessed by numeric indices.
	// - array --- arrays.
	// - value --- pointers. Careful: value is a distinct type from *array etc.
	// - *ssa.Function \
	// ssa.Builtin } --- functions. A nil 'func' is always of type ssa.Function.
	// *closure /
	// - tuple --- as returned by Return, Next, "value,ok" modes, etc.
	// - iter --- iterators from 'range' over map or string.
	// - bad --- a poison pill for locals that have gone out of scope.
	// - rtype -- the interpreter's concrete implementation of reflect.Type
	//
	// Note that nil is not on this list.
	//
	// Pay close attention to whether or not the dynamic type is a pointer.
	// The compiler cannot help you since value is an empty interface.

	import (
	"bytes"
	"fmt"
	"go/types"
	"io"
	"reflect"
	"strings"
	"sync"
	"unsafe"

	"golang.org/x/tools/go/ssa"
	"golang.org/x/tools/go/types/typeutil"
	)

	type value interface{}

	type tuple []value

	type array []value

	type iface struct {
	t types.Type // never an "untyped" type
	v value
	}

	type structure []value

	// For map, array, *array, slice, string or channel.
	type iter interface {
	// next returns a Tuple (key, value, ok).
	// key and value are unaliased, e.g. copies of the sequence element.
	next() tuple
	}

	type closure struct {
	Fn *ssa.Function
	Env []value
	}

	type bad struct{}

	type rtype struct {
	t types.Type
	}

	// Hash functions and equivalence relation:

	// hashString computes the FNV hash of s.
	func hashString(s string) int {
	var h uint32
	for i := 0; i < len(s); i++ {
	h ^= uint32(s[i])
	h *= 16777619
	}
	return int(h)
	}

	var (
	mu sync.Mutex
	hasher = typeutil.MakeHasher()
	)

	// hashType returns a hash for t such that
	// types.Identical(x, y) => hashType(x) == hashType(y).
	func hashType(t types.Type) int {
	mu.Lock()
	h := int(hasher.Hash(t))
	mu.Unlock()
	return h
	}

	// usesBuiltinMap returns true if the built-in hash function and
	// equivalence relation for type t are consistent with those of the
	// interpreter's representation of type t. Such types are: all basic
	// types (bool, numbers, string), pointers and channels.
	//
	// usesBuiltinMap returns false for types that require a custom map
	// implementation: interfaces, arrays and structs.
	//
	// Panic ensues if t is an invalid map key type: function, map or slice.
	func usesBuiltinMap(t types.Type) bool {
	switch t := t.(type) {
	case types.Basic, types.Chan, *types.Pointer:
	return true
	case *types.Named:
	return usesBuiltinMap(t.Underlying())
	case types.Interface, types.Array, *types.Struct:
	return false
	}
	panic(fmt.Sprintf("invalid map key type: %T", t))
	}

	func (x array) eq(t types.Type, _y interface{}) bool {
	y := _y.(array)
	tElt := t.Underlying().(*types.Array).Elem()
	for i, xi := range x {
	if !equals(tElt, xi, y[i]) {
	return false
	}
	}
	return true
	}

	func (x array) hash(t types.Type) int {
	h := 0
	tElt := t.Underlying().(*types.Array).Elem()
	for _, xi := range x {
	h += hash(tElt, xi)
	}
	return h
	}

	func (x structure) eq(t types.Type, _y interface{}) bool {
	y := _y.(structure)
	tStruct := t.Underlying().(*types.Struct)
	for i, n := 0, tStruct.NumFields(); i < n; i++ {
	if f := tStruct.Field(i); !f.Anonymous() {
	if !equals(f.Type(), x[i], y[i]) {
	return false
	}
	}
	}
	return true
	}

	func (x structure) hash(t types.Type) int {
	tStruct := t.Underlying().(*types.Struct)
	h := 0
	for i, n := 0, tStruct.NumFields(); i < n; i++ {
	if f := tStruct.Field(i); !f.Anonymous() {
	h += hash(f.Type(), x[i])
	}
	}
	return h
	}

	// nil-tolerant variant of types.Identical.
	func sameType(x, y types.Type) bool {
	if x == nil {
	return y == nil
	}
	return y != nil && types.Identical(x, y)
	}

	func (x iface) eq(t types.Type, _y interface{}) bool {
	y := _y.(iface)
	return sameType(x.t, y.t) && (x.t == nil \|\| equals(x.t, x.v, y.v))
	}

	func (x iface) hash(_ types.Type) int {
	return hashType(x.t)*8581 + hash(x.t, x.v)
	}

	func (x rtype) hash(_ types.Type) int {
	return hashType(x.t)
	}

	func (x rtype) eq(_ types.Type, y interface{}) bool {
	return types.Identical(x.t, y.(rtype).t)
	}

	// equals returns true iff x and y are equal according to Go's
	// linguistic equivalence relation for type t.
	// In a well-typed program, the dynamic types of x and y are
	// guaranteed equal.
	func equals(t types.Type, x, y value) bool {
	switch x := x.(type) {
	case bool:
	return x == y.(bool)
	case int:
	return x == y.(int)
	case int8:
	return x == y.(int8)
	case int16:
	return x == y.(int16)
	case int32:
	return x == y.(int32)
	case int64:
	return x == y.(int64)
	case uint:
	return x == y.(uint)
	case uint8:
	return x == y.(uint8)
	case uint16:
	return x == y.(uint16)
	case uint32:
	return x == y.(uint32)
	case uint64:
	return x == y.(uint64)
	case uintptr:
	return x == y.(uintptr)
	case float32:
	return x == y.(float32)
	case float64:
	return x == y.(float64)
	case complex64:
	return x == y.(complex64)
	case complex128:
	return x == y.(complex128)
	case string:
	return x == y.(string)
	case *value:
	return x == y.(*value)
	case chan value:
	return x == y.(chan value)
	case structure:
	return x.eq(t, y)
	case array:
	return x.eq(t, y)
	case iface:
	return x.eq(t, y)
	case rtype:
	return x.eq(t, y)
	}

	// Since map, func and slice don't support comparison, this
	// case is only reachable if one of x or y is literally nil
	// (handled in eqnil) or via interface{} values.
	panic(fmt.Sprintf("comparing uncomparable type %s", t))
	}

	// Returns an integer hash of x such that equals(x, y) => hash(x) == hash(y).
	func hash(t types.Type, x value) int {
	switch x := x.(type) {
	case bool:
	if x {
	return 1
	}
	return 0
	case int:
	return x
	case int8:
	return int(x)
	case int16:
	return int(x)
	case int32:
	return int(x)
	case int64:
	return int(x)
	case uint:
	return int(x)
	case uint8:
	return int(x)
	case uint16:
	return int(x)
	case uint32:
	return int(x)
	case uint64:
	return int(x)
	case uintptr:
	return int(x)
	case float32:
	return int(x)
	case float64:
	return int(x)
	case complex64:
	return int(real(x))
	case complex128:
	return int(real(x))
	case string:
	return hashString(x)
	case *value:
	return int(uintptr(unsafe.Pointer(x)))
	case chan value:
	return int(uintptr(reflect.ValueOf(x).Pointer()))
	case structure:
	return x.hash(t)
	case array:
	return x.hash(t)
	case iface:
	return x.hash(t)
	case rtype:
	return x.hash(t)
	}
	panic(fmt.Sprintf("%T is unhashable", x))
	}

	// reflect.Value struct values don't have a fixed shape, since the
	// payload can be a scalar or an aggregate depending on the instance.
	// So store (and load) can't simply use recursion over the shape of the
	// rhs value, or the lhs, to copy the value; we need the static type
	// information. (We can't make reflect.Value a new basic data type
	// because its "structness" is exposed to Go programs.)

	// load returns the value of type T in *addr.
	func load(T types.Type, addr *value) value {
	switch T := T.Underlying().(type) {
	case *types.Struct:
	v := (*addr).(structure)
	a := make(structure, len(v))
	for i := range a {
	a[i] = load(T.Field(i).Type(), &v[i])
	}
	return a
	case *types.Array:
	v := (*addr).(array)
	a := make(array, len(v))
	for i := range a {
	a[i] = load(T.Elem(), &v[i])
	}
	return a
	default:
	return *addr
	}
	}

	// store stores value v of type T into *addr.
	func store(T types.Type, addr *value, v value) {
	switch T := T.Underlying().(type) {
	case *types.Struct:
	lhs := (*addr).(structure)
	rhs := v.(structure)
	for i := range lhs {
	store(T.Field(i).Type(), &lhs[i], rhs[i])
	}
	case *types.Array:
	lhs := (*addr).(array)
	rhs := v.(array)
	for i := range lhs {
	store(T.Elem(), &lhs[i], rhs[i])
	}
	default:
	*addr = v
	}
	}

	// Prints in the style of built-in println.
	// (More or less; in gc println is actually a compiler intrinsic and
	// can distinguish println(1) from println(interface{}(1)).)
	func writeValue(buf *bytes.Buffer, v value) {
	switch v := v.(type) {
	case nil, bool, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64, uintptr, float32, float64, complex64, complex128, string:
	fmt.Fprintf(buf, "%v", v)

	case map[value]value:
	buf.WriteString("map[")
	sep := ""
	for k, e := range v {
	buf.WriteString(sep)
	sep = " "
	writeValue(buf, k)
	buf.WriteString(":")
	writeValue(buf, e)
	}
	buf.WriteString("]")

	case *hashmap:
	buf.WriteString("map[")
	sep := " "
	for _, e := range v.entries() {
	for e != nil {
	buf.WriteString(sep)
	sep = " "
	writeValue(buf, e.key)
	buf.WriteString(":")
	writeValue(buf, e.value)
	e = e.next
	}
	}
	buf.WriteString("]")

	case chan value:
	fmt.Fprintf(buf, "%v", v) // (an address)

	case *value:
	if v == nil {
	buf.WriteString("<nil>")
	} else {
	fmt.Fprintf(buf, "%p", v)
	}

	case iface:
	fmt.Fprintf(buf, "(%s, ", v.t)
	writeValue(buf, v.v)
	buf.WriteString(")")

	case structure:
	buf.WriteString("{")
	for i, e := range v {
	if i > 0 {
	buf.WriteString(" ")
	}
	writeValue(buf, e)
	}
	buf.WriteString("}")

	case array:
	buf.WriteString("[")
	for i, e := range v {
	if i > 0 {
	buf.WriteString(" ")
	}
	writeValue(buf, e)
	}
	buf.WriteString("]")

	case []value:
	buf.WriteString("[")
	for i, e := range v {
	if i > 0 {
	buf.WriteString(" ")
	}
	writeValue(buf, e)
	}
	buf.WriteString("]")

	case ssa.Function, ssa.Builtin, *closure:
	fmt.Fprintf(buf, "%p", v) // (an address)

	case rtype:
	buf.WriteString(v.t.String())

	case tuple:
	// Unreachable in well-formed Go programs
	buf.WriteString("(")
	for i, e := range v {
	if i > 0 {
	buf.WriteString(", ")
	}
	writeValue(buf, e)
	}
	buf.WriteString(")")

	default:
	fmt.Fprintf(buf, "<%T>", v)
	}
	}

	// Implements printing of Go values in the style of built-in println.
	func toString(v value) string {
	var b bytes.Buffer
	writeValue(&b, v)
	return b.String()
	}

	// ------------------------------------------------------------------------
	// Iterators

	type stringIter struct {
	*strings.Reader
	i int
	}

	func (it *stringIter) next() tuple {
	okv := make(tuple, 3)
	ch, n, err := it.ReadRune()
	ok := err != io.EOF
	okv[0] = ok
	if ok {
	okv[1] = it.i
	okv[2] = ch
	}
	it.i += n
	return okv
	}

	type mapIter chan [2]value

	func (it mapIter) next() tuple {
	kv, ok := <-it
	return tuple{ok, kv[0], kv[1]}
	}