good good study, day day up!

vendor/gopl.io/ch6/urlvalues/main.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 160.
+
+// The urlvalues command demonstrates a map type with methods.
+package main
+
+/*
+//!+values
+package url
+
+// Values maps a string key to a list of values.
+type Values map[string][]string
+
+// Get returns the first value associated with the given key,
+// or "" if there are none.
+func (v Values) Get(key string) string {
+	if vs := v[key]; len(vs) > 0 {
+		return vs[0]
+	}
+	return ""
+}
+
+// Add adds the value to key.
+// It appends to any existing values associated with key.
+func (v Values) Add(key, value string) {
+	v[key] = append(v[key], value)
+}
+//!-values
+*/
+
+import (
+	"fmt"
+	"net/url"
+)
+
+func main() {
+	//!+main
+	m := url.Values{"lang": {"en"}} // direct construction
+	m.Add("item", "1")
+	m.Add("item", "2")
+
+	fmt.Println(m.Get("lang")) // "en"
+	fmt.Println(m.Get("q"))    // ""
+	fmt.Println(m.Get("item")) // "1"      (first value)
+	fmt.Println(m["item"])     // "[1 2]"  (direct map access)
+
+	m = nil
+	fmt.Println(m.Get("item")) // ""
+	m.Add("item", "3")         // panic: assignment to entry in nil map
+	//!-main
+}