ch12¶
display/display.go¶
/gopl.io/ch12/display/display.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 333.
// Package display provides a means to display structured data.
package display
import (
"fmt"
"reflect"
"strconv"
)
//!+Display
func Display(name string, x interface{}) {
fmt.Printf("Display %s (%T):\n", name, x)
display(name, reflect.ValueOf(x))
}
//!-Display
// formatAtom formats a value without inspecting its internal structure.
// It is a copy of the the function in gopl.io/ch11/format.
func formatAtom(v reflect.Value) string {
switch v.Kind() {
case reflect.Invalid:
return "invalid"
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
return strconv.FormatInt(v.Int(), 10)
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
return strconv.FormatUint(v.Uint(), 10)
// ...floating-point and complex cases omitted for brevity...
case reflect.Bool:
if v.Bool() {
return "true"
}
return "false"
case reflect.String:
return strconv.Quote(v.String())
case reflect.Chan, reflect.Func, reflect.Ptr,
reflect.Slice, reflect.Map:
return v.Type().String() + " 0x" +
strconv.FormatUint(uint64(v.Pointer()), 16)
default: // reflect.Array, reflect.Struct, reflect.Interface
return v.Type().String() + " value"
}
}
//!+display
func display(path string, v reflect.Value) {
switch v.Kind() {
case reflect.Invalid:
fmt.Printf("%s = invalid\n", path)
case reflect.Slice, reflect.Array:
for i := 0; i < v.Len(); i++ {
display(fmt.Sprintf("%s[%d]", path, i), v.Index(i))
}
case reflect.Struct:
for i := 0; i < v.NumField(); i++ {
fieldPath := fmt.Sprintf("%s.%s", path, v.Type().Field(i).Name)
display(fieldPath, v.Field(i))
}
case reflect.Map:
for _, key := range v.MapKeys() {
display(fmt.Sprintf("%s[%s]", path,
formatAtom(key)), v.MapIndex(key))
}
case reflect.Ptr:
if v.IsNil() {
fmt.Printf("%s = nil\n", path)
} else {
display(fmt.Sprintf("(*%s)", path), v.Elem())
}
case reflect.Interface:
if v.IsNil() {
fmt.Printf("%s = nil\n", path)
} else {
fmt.Printf("%s.type = %s\n", path, v.Elem().Type())
display(path+".value", v.Elem())
}
default: // basic types, channels, funcs
fmt.Printf("%s = %s\n", path, formatAtom(v))
}
}
//!-display
display/display_test.go¶
/gopl.io/ch12/display/display_test.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package display
import (
"io"
"net"
"os"
"reflect"
"sync"
"testing"
"gopl.io/ch7/eval"
)
// NOTE: we can't use !+..!- comments to excerpt these tests
// into the book because it defeats the Example mechanism,
// which requires the // Output comment to be at the end
// of the function.
func Example_expr() {
e, _ := eval.Parse("sqrt(A / pi)")
Display("e", e)
// Output:
// Display e (eval.call):
// e.fn = "sqrt"
// e.args[0].type = eval.binary
// e.args[0].value.op = 47
// e.args[0].value.x.type = eval.Var
// e.args[0].value.x.value = "A"
// e.args[0].value.y.type = eval.Var
// e.args[0].value.y.value = "pi"
}
func Example_slice() {
Display("slice", []*int{new(int), nil})
// Output:
// Display slice ([]*int):
// (*slice[0]) = 0
// slice[1] = nil
}
func Example_nilInterface() {
var w io.Writer
Display("w", w)
// Output:
// Display w (<nil>):
// w = invalid
}
func Example_ptrToInterface() {
var w io.Writer
Display("&w", &w)
// Output:
// Display &w (*io.Writer):
// (*&w) = nil
}
func Example_struct() {
Display("x", struct{ x interface{} }{3})
// Output:
// Display x (struct { x interface {} }):
// x.x.type = int
// x.x.value = 3
}
func Example_interface() {
var i interface{} = 3
Display("i", i)
// Output:
// Display i (int):
// i = 3
}
func Example_ptrToInterface2() {
var i interface{} = 3
Display("&i", &i)
// Output:
// Display &i (*interface {}):
// (*&i).type = int
// (*&i).value = 3
}
func Example_array() {
Display("x", [1]interface{}{3})
// Output:
// Display x ([1]interface {}):
// x[0].type = int
// x[0].value = 3
}
func Example_movie() {
//!+movie
type Movie struct {
Title, Subtitle string
Year int
Color bool
Actor map[string]string
Oscars []string
Sequel *string
}
//!-movie
//!+strangelove
strangelove := Movie{
Title: "Dr. Strangelove",
Subtitle: "How I Learned to Stop Worrying and Love the Bomb",
Year: 1964,
Color: false,
Actor: map[string]string{
"Dr. Strangelove": "Peter Sellers",
"Grp. Capt. Lionel Mandrake": "Peter Sellers",
"Pres. Merkin Muffley": "Peter Sellers",
"Gen. Buck Turgidson": "George C. Scott",
"Brig. Gen. Jack D. Ripper": "Sterling Hayden",
`Maj. T.J. "King" Kong`: "Slim Pickens",
},
Oscars: []string{
"Best Actor (Nomin.)",
"Best Adapted Screenplay (Nomin.)",
"Best Director (Nomin.)",
"Best Picture (Nomin.)",
},
}
//!-strangelove
Display("strangelove", strangelove)
// We don't use an Output: comment since displaying
// a map is nondeterministic.
/*
//!+output
Display strangelove (display.Movie):
strangelove.Title = "Dr. Strangelove"
strangelove.Subtitle = "How I Learned to Stop Worrying and Love the Bomb"
strangelove.Year = 1964
strangelove.Color = false
strangelove.Actor["Gen. Buck Turgidson"] = "George C. Scott"
strangelove.Actor["Brig. Gen. Jack D. Ripper"] = "Sterling Hayden"
strangelove.Actor["Maj. T.J. \"King\" Kong"] = "Slim Pickens"
strangelove.Actor["Dr. Strangelove"] = "Peter Sellers"
strangelove.Actor["Grp. Capt. Lionel Mandrake"] = "Peter Sellers"
strangelove.Actor["Pres. Merkin Muffley"] = "Peter Sellers"
strangelove.Oscars[0] = "Best Actor (Nomin.)"
strangelove.Oscars[1] = "Best Adapted Screenplay (Nomin.)"
strangelove.Oscars[2] = "Best Director (Nomin.)"
strangelove.Oscars[3] = "Best Picture (Nomin.)"
strangelove.Sequel = nil
//!-output
*/
}
// This test ensures that the program terminates without crashing.
func Test(t *testing.T) {
// Some other values (YMMV)
Display("os.Stderr", os.Stderr)
// Output:
// Display os.Stderr (*os.File):
// (*(*os.Stderr).file).fd = 2
// (*(*os.Stderr).file).name = "/dev/stderr"
// (*(*os.Stderr).file).nepipe = 0
var w io.Writer = os.Stderr
Display("&w", &w)
// Output:
// Display &w (*io.Writer):
// (*&w).type = *os.File
// (*(*(*&w).value).file).fd = 2
// (*(*(*&w).value).file).name = "/dev/stderr"
// (*(*(*&w).value).file).nepipe = 0
var locker sync.Locker = new(sync.Mutex)
Display("(&locker)", &locker)
// Output:
// Display (&locker) (*sync.Locker):
// (*(&locker)).type = *sync.Mutex
// (*(*(&locker)).value).state = 0
// (*(*(&locker)).value).sema = 0
Display("locker", locker)
// Output:
// Display locker (*sync.Mutex):
// (*locker).state = 0
// (*locker).sema = 0
// (*(&locker)) = nil
locker = nil
Display("(&locker)", &locker)
// Output:
// Display (&locker) (*sync.Locker):
// (*(&locker)) = nil
ips, _ := net.LookupHost("golang.org")
Display("ips", ips)
// Output:
// Display ips ([]string):
// ips[0] = "173.194.68.141"
// ips[1] = "2607:f8b0:400d:c06::8d"
// Even metarecursion! (YMMV)
Display("rV", reflect.ValueOf(os.Stderr))
// Output:
// Display rV (reflect.Value):
// (*rV.typ).size = 8
// (*rV.typ).ptrdata = 8
// (*rV.typ).hash = 871609668
// (*rV.typ)._ = 0
// ...
// a pointer that points to itself
type P *P
var p P
p = &p
if false {
Display("p", p)
// Output:
// Display p (display.P):
// ...stuck, no output...
}
// a map that contains itself
type M map[string]M
m := make(M)
m[""] = m
if false {
Display("m", m)
// Output:
// Display m (display.M):
// ...stuck, no output...
}
// a slice that contains itself
type S []S
s := make(S, 1)
s[0] = s
if false {
Display("s", s)
// Output:
// Display s (display.S):
// ...stuck, no output...
}
// a linked list that eats its own tail
type Cycle struct {
Value int
Tail *Cycle
}
var c Cycle
c = Cycle{42, &c}
if false {
Display("c", c)
// Output:
// Display c (display.Cycle):
// c.Value = 42
// (*c.Tail).Value = 42
// (*(*c.Tail).Tail).Value = 42
// ...ad infinitum...
}
}
format/format.go¶
/gopl.io/ch12/format/format.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 332.
// Package format provides an Any function that can format any value.
//!+
package format
import (
"reflect"
"strconv"
)
// Any formats any value as a string.
func Any(value interface{}) string {
return formatAtom(reflect.ValueOf(value))
}
// formatAtom formats a value without inspecting its internal structure.
func formatAtom(v reflect.Value) string {
switch v.Kind() {
case reflect.Invalid:
return "invalid"
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
return strconv.FormatInt(v.Int(), 10)
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
return strconv.FormatUint(v.Uint(), 10)
// ...floating-point and complex cases omitted for brevity...
case reflect.Bool:
return strconv.FormatBool(v.Bool())
case reflect.String:
return strconv.Quote(v.String())
case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Slice, reflect.Map:
return v.Type().String() + " 0x" +
strconv.FormatUint(uint64(v.Pointer()), 16)
default: // reflect.Array, reflect.Struct, reflect.Interface
return v.Type().String() + " value"
}
}
//!-
format/format_test.go¶
/gopl.io/ch12/format/format_test.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package format_test
import (
"fmt"
"testing"
"time"
"gopl.io/ch12/format"
)
func Test(t *testing.T) {
// The pointer values are just examples, and may vary from run to run.
//!+time
var x int64 = 1
var d time.Duration = 1 * time.Nanosecond
fmt.Println(format.Any(x)) // "1"
fmt.Println(format.Any(d)) // "1"
fmt.Println(format.Any([]int64{x})) // "[]int64 0x8202b87b0"
fmt.Println(format.Any([]time.Duration{d})) // "[]time.Duration 0x8202b87e0"
//!-time
}
methods/methods.go¶
/gopl.io/ch12/methods/methods.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 351.
// Package methods provides a function to print the methods of any value.
package methods
import (
"fmt"
"reflect"
"strings"
)
//!+print
// Print prints the method set of the value x.
func Print(x interface{}) {
v := reflect.ValueOf(x)
t := v.Type()
fmt.Printf("type %s\n", t)
for i := 0; i < v.NumMethod(); i++ {
methType := v.Method(i).Type()
fmt.Printf("func (%s) %s%s\n", t, t.Method(i).Name,
strings.TrimPrefix(methType.String(), "func"))
}
}
//!-print
methods/methods_test.go¶
/gopl.io/ch12/methods/methods_test.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package methods_test
import (
"strings"
"time"
"gopl.io/ch12/methods"
)
func ExamplePrintDuration() {
methods.Print(time.Hour)
// Output:
// type time.Duration
// func (time.Duration) Hours() float64
// func (time.Duration) Minutes() float64
// func (time.Duration) Nanoseconds() int64
// func (time.Duration) Seconds() float64
// func (time.Duration) String() string
}
func ExamplePrintReplacer() {
methods.Print(new(strings.Replacer))
// Output:
// type *strings.Replacer
// func (*strings.Replacer) Replace(string) string
// func (*strings.Replacer) WriteString(io.Writer, string) (int, error)
}
/*
//!+output
methods.Print(time.Hour)
// Output:
// type time.Duration
// func (time.Duration) Hours() float64
// func (time.Duration) Minutes() float64
// func (time.Duration) Nanoseconds() int64
// func (time.Duration) Seconds() float64
// func (time.Duration) String() string
methods.Print(new(strings.Replacer))
// Output:
// type *strings.Replacer
// func (*strings.Replacer) Replace(string) string
// func (*strings.Replacer) WriteString(io.Writer, string) (int, error)
//!-output
*/
params/params.go¶
/gopl.io/ch12/params/params.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 349.
// Package params provides a reflection-based parser for URL parameters.
package params
import (
"fmt"
"net/http"
"reflect"
"strconv"
"strings"
)
//!+Unpack
// Unpack populates the fields of the struct pointed to by ptr
// from the HTTP request parameters in req.
func Unpack(req *http.Request, ptr interface{}) error {
if err := req.ParseForm(); err != nil {
return err
}
// Build map of fields keyed by effective name.
fields := make(map[string]reflect.Value)
v := reflect.ValueOf(ptr).Elem() // the struct variable
for i := 0; i < v.NumField(); i++ {
fieldInfo := v.Type().Field(i) // a reflect.StructField
tag := fieldInfo.Tag // a reflect.StructTag
name := tag.Get("http")
if name == "" {
name = strings.ToLower(fieldInfo.Name)
}
fields[name] = v.Field(i)
}
// Update struct field for each parameter in the request.
for name, values := range req.Form {
f := fields[name]
if !f.IsValid() {
continue // ignore unrecognized HTTP parameters
}
for _, value := range values {
if f.Kind() == reflect.Slice {
elem := reflect.New(f.Type().Elem()).Elem()
if err := populate(elem, value); err != nil {
return fmt.Errorf("%s: %v", name, err)
}
f.Set(reflect.Append(f, elem))
} else {
if err := populate(f, value); err != nil {
return fmt.Errorf("%s: %v", name, err)
}
}
}
}
return nil
}
//!-Unpack
//!+populate
func populate(v reflect.Value, value string) error {
switch v.Kind() {
case reflect.String:
v.SetString(value)
case reflect.Int:
i, err := strconv.ParseInt(value, 10, 64)
if err != nil {
return err
}
v.SetInt(i)
case reflect.Bool:
b, err := strconv.ParseBool(value)
if err != nil {
return err
}
v.SetBool(b)
default:
return fmt.Errorf("unsupported kind %s", v.Type())
}
return nil
}
//!-populate
search/main.go¶
/gopl.io/ch12/search/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 348.
// Search is a demo of the params.Unpack function.
package main
import (
"fmt"
"log"
"net/http"
)
//!+
import "gopl.io/ch12/params"
// search implements the /search URL endpoint.
func search(resp http.ResponseWriter, req *http.Request) {
var data struct {
Labels []string `http:"l"`
MaxResults int `http:"max"`
Exact bool `http:"x"`
}
data.MaxResults = 10 // set default
if err := params.Unpack(req, &data); err != nil {
http.Error(resp, err.Error(), http.StatusBadRequest) // 400
return
}
// ...rest of handler...
fmt.Fprintf(resp, "Search: %+v\n", data)
}
//!-
func main() {
http.HandleFunc("/search", search)
log.Fatal(http.ListenAndServe(":12345", nil))
}
/*
//!+output
$ go build gopl.io/ch12/search
$ ./search &
$ ./fetch 'http://localhost:12345/search'
Search: {Labels:[] MaxResults:10 Exact:false}
$ ./fetch 'http://localhost:12345/search?l=golang&l=programming'
Search: {Labels:[golang programming] MaxResults:10 Exact:false}
$ ./fetch 'http://localhost:12345/search?l=golang&l=programming&max=100'
Search: {Labels:[golang programming] MaxResults:100 Exact:false}
$ ./fetch 'http://localhost:12345/search?x=true&l=golang&l=programming'
Search: {Labels:[golang programming] MaxResults:10 Exact:true}
$ ./fetch 'http://localhost:12345/search?q=hello&x=123'
x: strconv.ParseBool: parsing "123": invalid syntax
$ ./fetch 'http://localhost:12345/search?q=hello&max=lots'
max: strconv.ParseInt: parsing "lots": invalid syntax
//!-output
*/
sexpr/decode.go¶
/gopl.io/ch12/sexpr/decode.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 344.
// Package sexpr provides a means for converting Go objects to and
// from S-expressions.
package sexpr
import (
"bytes"
"fmt"
"reflect"
"strconv"
"text/scanner"
)
//!+Unmarshal
// Unmarshal parses S-expression data and populates the variable
// whose address is in the non-nil pointer out.
func Unmarshal(data []byte, out interface{}) (err error) {
lex := &lexer{scan: scanner.Scanner{Mode: scanner.GoTokens}}
lex.scan.Init(bytes.NewReader(data))
lex.next() // get the first token
defer func() {
// NOTE: this is not an example of ideal error handling.
if x := recover(); x != nil {
err = fmt.Errorf("error at %s: %v", lex.scan.Position, x)
}
}()
read(lex, reflect.ValueOf(out).Elem())
return nil
}
//!-Unmarshal
//!+lexer
type lexer struct {
scan scanner.Scanner
token rune // the current token
}
func (lex *lexer) next() { lex.token = lex.scan.Scan() }
func (lex *lexer) text() string { return lex.scan.TokenText() }
func (lex *lexer) consume(want rune) {
if lex.token != want { // NOTE: Not an example of good error handling.
panic(fmt.Sprintf("got %q, want %q", lex.text(), want))
}
lex.next()
}
//!-lexer
// The read function is a decoder for a small subset of well-formed
// S-expressions. For brevity of our example, it takes many dubious
// shortcuts.
//
// The parser assumes
// - that the S-expression input is well-formed; it does no error checking.
// - that the S-expression input corresponds to the type of the variable.
// - that all numbers in the input are non-negative decimal integers.
// - that all keys in ((key value) ...) struct syntax are unquoted symbols.
// - that the input does not contain dotted lists such as (1 2 . 3).
// - that the input does not contain Lisp reader macros such 'x and #'x.
//
// The reflection logic assumes
// - that v is always a variable of the appropriate type for the
// S-expression value. For example, v must not be a boolean,
// interface, channel, or function, and if v is an array, the input
// must have the correct number of elements.
// - that v in the top-level call to read has the zero value of its
// type and doesn't need clearing.
// - that if v is a numeric variable, it is a signed integer.
//!+read
func read(lex *lexer, v reflect.Value) {
switch lex.token {
case scanner.Ident:
// The only valid identifiers are
// "nil" and struct field names.
if lex.text() == "nil" {
v.Set(reflect.Zero(v.Type()))
lex.next()
return
}
case scanner.String:
s, _ := strconv.Unquote(lex.text()) // NOTE: ignoring errors
v.SetString(s)
lex.next()
return
case scanner.Int:
i, _ := strconv.Atoi(lex.text()) // NOTE: ignoring errors
v.SetInt(int64(i))
lex.next()
return
case '(':
lex.next()
readList(lex, v)
lex.next() // consume ')'
return
}
panic(fmt.Sprintf("unexpected token %q", lex.text()))
}
//!-read
//!+readlist
func readList(lex *lexer, v reflect.Value) {
switch v.Kind() {
case reflect.Array: // (item ...)
for i := 0; !endList(lex); i++ {
read(lex, v.Index(i))
}
case reflect.Slice: // (item ...)
for !endList(lex) {
item := reflect.New(v.Type().Elem()).Elem()
read(lex, item)
v.Set(reflect.Append(v, item))
}
case reflect.Struct: // ((name value) ...)
for !endList(lex) {
lex.consume('(')
if lex.token != scanner.Ident {
panic(fmt.Sprintf("got token %q, want field name", lex.text()))
}
name := lex.text()
lex.next()
read(lex, v.FieldByName(name))
lex.consume(')')
}
case reflect.Map: // ((key value) ...)
v.Set(reflect.MakeMap(v.Type()))
for !endList(lex) {
lex.consume('(')
key := reflect.New(v.Type().Key()).Elem()
read(lex, key)
value := reflect.New(v.Type().Elem()).Elem()
read(lex, value)
v.SetMapIndex(key, value)
lex.consume(')')
}
default:
panic(fmt.Sprintf("cannot decode list into %v", v.Type()))
}
}
func endList(lex *lexer) bool {
switch lex.token {
case scanner.EOF:
panic("end of file")
case ')':
return true
}
return false
}
//!-readlist
sexpr/encode.go¶
/gopl.io/ch12/sexpr/encode.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 339.
package sexpr
import (
"bytes"
"fmt"
"reflect"
)
//!+Marshal
// Marshal encodes a Go value in S-expression form.
func Marshal(v interface{}) ([]byte, error) {
var buf bytes.Buffer
if err := encode(&buf, reflect.ValueOf(v)); err != nil {
return nil, err
}
return buf.Bytes(), nil
}
//!-Marshal
// encode writes to buf an S-expression representation of v.
//!+encode
func encode(buf *bytes.Buffer, v reflect.Value) error {
switch v.Kind() {
case reflect.Invalid:
buf.WriteString("nil")
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
fmt.Fprintf(buf, "%d", v.Int())
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
fmt.Fprintf(buf, "%d", v.Uint())
case reflect.String:
fmt.Fprintf(buf, "%q", v.String())
case reflect.Ptr:
return encode(buf, v.Elem())
case reflect.Array, reflect.Slice: // (value ...)
buf.WriteByte('(')
for i := 0; i < v.Len(); i++ {
if i > 0 {
buf.WriteByte(' ')
}
if err := encode(buf, v.Index(i)); err != nil {
return err
}
}
buf.WriteByte(')')
case reflect.Struct: // ((name value) ...)
buf.WriteByte('(')
for i := 0; i < v.NumField(); i++ {
if i > 0 {
buf.WriteByte(' ')
}
fmt.Fprintf(buf, "(%s ", v.Type().Field(i).Name)
if err := encode(buf, v.Field(i)); err != nil {
return err
}
buf.WriteByte(')')
}
buf.WriteByte(')')
case reflect.Map: // ((key value) ...)
buf.WriteByte('(')
for i, key := range v.MapKeys() {
if i > 0 {
buf.WriteByte(' ')
}
buf.WriteByte('(')
if err := encode(buf, key); err != nil {
return err
}
buf.WriteByte(' ')
if err := encode(buf, v.MapIndex(key)); err != nil {
return err
}
buf.WriteByte(')')
}
buf.WriteByte(')')
default: // float, complex, bool, chan, func, interface
return fmt.Errorf("unsupported type: %s", v.Type())
}
return nil
}
//!-encode
sexpr/pretty.go¶
/gopl.io/ch12/sexpr/pretty.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package sexpr
// This file implements the algorithm described in Derek C. Oppen's
// 1979 Stanford technical report, "Pretty Printing".
import (
"bytes"
"fmt"
"reflect"
)
func MarshalIndent(v interface{}) ([]byte, error) {
p := printer{width: margin}
if err := pretty(&p, reflect.ValueOf(v)); err != nil {
return nil, err
}
return p.Bytes(), nil
}
const margin = 80
type token struct {
kind rune // one of "s ()" (string, blank, start, end)
str string
size int
}
type printer struct {
tokens []*token // FIFO buffer
stack []*token // stack of open ' ' and '(' tokens
rtotal int // total number of spaces needed to print stream
bytes.Buffer
indents []int
width int // remaining space
}
func (p *printer) string(str string) {
tok := &token{kind: 's', str: str, size: len(str)}
if len(p.stack) == 0 {
p.print(tok)
} else {
p.tokens = append(p.tokens, tok)
p.rtotal += len(str)
}
}
func (p *printer) pop() (top *token) {
last := len(p.stack) - 1
top, p.stack = p.stack[last], p.stack[:last]
return
}
func (p *printer) begin() {
if len(p.stack) == 0 {
p.rtotal = 1
}
t := &token{kind: '(', size: -p.rtotal}
p.tokens = append(p.tokens, t)
p.stack = append(p.stack, t) // push
p.string("(")
}
func (p *printer) end() {
p.string(")")
p.tokens = append(p.tokens, &token{kind: ')'})
x := p.pop()
x.size += p.rtotal
if x.kind == ' ' {
p.pop().size += p.rtotal
}
if len(p.stack) == 0 {
for _, tok := range p.tokens {
p.print(tok)
}
p.tokens = nil
}
}
func (p *printer) space() {
last := len(p.stack) - 1
x := p.stack[last]
if x.kind == ' ' {
x.size += p.rtotal
p.stack = p.stack[:last] // pop
}
t := &token{kind: ' ', size: -p.rtotal}
p.tokens = append(p.tokens, t)
p.stack = append(p.stack, t)
p.rtotal++
}
func (p *printer) print(t *token) {
switch t.kind {
case 's':
p.WriteString(t.str)
p.width -= len(t.str)
case '(':
p.indents = append(p.indents, p.width)
case ')':
p.indents = p.indents[:len(p.indents)-1] // pop
case ' ':
if t.size > p.width {
p.width = p.indents[len(p.indents)-1] - 1
fmt.Fprintf(&p.Buffer, "\n%*s", margin-p.width, "")
} else {
p.WriteByte(' ')
p.width--
}
}
}
func (p *printer) stringf(format string, args ...interface{}) {
p.string(fmt.Sprintf(format, args...))
}
func pretty(p *printer, v reflect.Value) error {
switch v.Kind() {
case reflect.Invalid:
p.string("nil")
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
p.stringf("%d", v.Int())
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
p.stringf("%d", v.Uint())
case reflect.String:
p.stringf("%q", v.String())
case reflect.Array, reflect.Slice: // (value ...)
p.begin()
for i := 0; i < v.Len(); i++ {
if i > 0 {
p.space()
}
if err := pretty(p, v.Index(i)); err != nil {
return err
}
}
p.end()
case reflect.Struct: // ((name value ...)
p.begin()
for i := 0; i < v.NumField(); i++ {
if i > 0 {
p.space()
}
p.begin()
p.string(v.Type().Field(i).Name)
p.space()
if err := pretty(p, v.Field(i)); err != nil {
return err
}
p.end()
}
p.end()
case reflect.Map: // ((key value ...)
p.begin()
for i, key := range v.MapKeys() {
if i > 0 {
p.space()
}
p.begin()
if err := pretty(p, key); err != nil {
return err
}
p.space()
if err := pretty(p, v.MapIndex(key)); err != nil {
return err
}
p.end()
}
p.end()
case reflect.Ptr:
return pretty(p, v.Elem())
default: // float, complex, bool, chan, func, interface
return fmt.Errorf("unsupported type: %s", v.Type())
}
return nil
}
sexpr/sexpr_test.go¶
/gopl.io/ch12/sexpr/sexpr_test.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package sexpr
import (
"reflect"
"testing"
)
// Test verifies that encoding and decoding a complex data value
// produces an equal result.
//
// The test does not make direct assertions about the encoded output
// because the output depends on map iteration order, which is
// nondeterministic. The output of the t.Log statements can be
// inspected by running the test with the -v flag:
//
// $ go test -v gopl.io/ch12/sexpr
//
func Test(t *testing.T) {
type Movie struct {
Title, Subtitle string
Year int
Actor map[string]string
Oscars []string
Sequel *string
}
strangelove := Movie{
Title: "Dr. Strangelove",
Subtitle: "How I Learned to Stop Worrying and Love the Bomb",
Year: 1964,
Actor: map[string]string{
"Dr. Strangelove": "Peter Sellers",
"Grp. Capt. Lionel Mandrake": "Peter Sellers",
"Pres. Merkin Muffley": "Peter Sellers",
"Gen. Buck Turgidson": "George C. Scott",
"Brig. Gen. Jack D. Ripper": "Sterling Hayden",
`Maj. T.J. "King" Kong`: "Slim Pickens",
},
Oscars: []string{
"Best Actor (Nomin.)",
"Best Adapted Screenplay (Nomin.)",
"Best Director (Nomin.)",
"Best Picture (Nomin.)",
},
}
// Encode it
data, err := Marshal(strangelove)
if err != nil {
t.Fatalf("Marshal failed: %v", err)
}
t.Logf("Marshal() = %s\n", data)
// Decode it
var movie Movie
if err := Unmarshal(data, &movie); err != nil {
t.Fatalf("Unmarshal failed: %v", err)
}
t.Logf("Unmarshal() = %+v\n", movie)
// Check equality.
if !reflect.DeepEqual(movie, strangelove) {
t.Fatal("not equal")
}
// Pretty-print it:
data, err = MarshalIndent(strangelove)
if err != nil {
t.Fatal(err)
}
t.Logf("MarshalIdent() = %s\n", data)
}