Update to gobook@c6d7a22edd03e7738c38d5baa2174ff325d8d863

ch9/memo2/memo.go

@@ -0,0 +1,44 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 275.
+
+// Package memo provides a concurrency-safe memoization a function of
+// type Func.  Concurrent requests are serialized by a Mutex.
+package memo
+
+import "sync"
+
+// Func is the type of the function to memoize.
+type Func func(string) (interface{}, error)
+
+type result struct {
+	value interface{}
+	err   error
+}
+
+func New(f Func) *Memo {
+	return &Memo{f: f, cache: make(map[string]result)}
+}
+
+//!+
+
+type Memo struct {
+	f     Func
+	mu    sync.Mutex // guards cache
+	cache map[string]result
+}
+
+// Get is concurrency-safe.
+func (memo *Memo) Get(key string) (value interface{}, err error) {
+	memo.mu.Lock()
+	res, ok := memo.cache[key]
+	if !ok {
+		res.value, res.err = memo.f(key)
+		memo.cache[key] = res
+	}
+	memo.mu.Unlock()
+	return res.value, res.err
+}
+
+//!-

ch9/memo2/memo_test.go

@@ -0,0 +1,23 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package memo_test
+
+import (
+	"testing"
+
+	"gopl.io/ch9/memo2"
+	"gopl.io/ch9/memotest"
+)
+
+var httpGetBody = memotest.HTTPGetBody
+
+func Test(t *testing.T) {
+	m := memo.New(httpGetBody)
+	memotest.Sequential(t, m)
+}
+
+func TestConcurrent(t *testing.T) {
+	m := memo.New(httpGetBody)
+	memotest.Concurrent(t, m)
+}