good good study, day day up!

vendor/gopl.io/ch9/bank1/bank.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 261.
+//!+
+
+// Package bank provides a concurrency-safe bank with one account.
+package bank
+
+var deposits = make(chan int) // send amount to deposit
+var balances = make(chan int) // receive balance
+
+func Deposit(amount int) { deposits <- amount }
+func Balance() int       { return <-balances }
+
+func teller() {
+	var balance int // balance is confined to teller goroutine
+	for {
+		select {
+		case amount := <-deposits:
+			balance += amount
+		case balances <- balance:
+		}
+	}
+}
+
+func init() {
+	go teller() // start the monitor goroutine
+}
+
+//!-

vendor/gopl.io/ch9/bank1/bank_test.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package bank_test
+
+import (
+	"fmt"
+	"testing"
+
+	"gopl.io/ch9/bank1"
+)
+
+func TestBank(t *testing.T) {
+	done := make(chan struct{})
+
+	// Alice
+	go func() {
+		bank.Deposit(200)
+		fmt.Println("=", bank.Balance())
+		done <- struct{}{}
+	}()
+
+	// Bob
+	go func() {
+		bank.Deposit(100)
+		done <- struct{}{}
+	}()
+
+	// Wait for both transactions.
+	<-done
+	<-done
+
+	if got, want := bank.Balance(), 300; got != want {
+		t.Errorf("Balance = %d, want %d", got, want)
+	}
+}