good good study, day day up!

vendor/gopl.io/ch5/trace/main.go

@@ -0,0 +1,40 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 146.
+
+// The trace program uses defer to add entry/exit diagnostics to a function.
+package main
+
+import (
+	"log"
+	"time"
+)
+
+//!+main
+func bigSlowOperation() {
+	defer trace("bigSlowOperation")() // don't forget the extra parentheses
+	// ...lots of work...
+	time.Sleep(10 * time.Second) // simulate slow operation by sleeping
+}
+
+func trace(msg string) func() {
+	start := time.Now()
+	log.Printf("enter %s", msg)
+	return func() { log.Printf("exit %s (%s)", msg, time.Since(start)) }
+}
+
+//!-main
+
+func main() {
+	bigSlowOperation()
+}
+
+/*
+!+output
+$ go build gopl.io/ch5/trace
+$ ./trace
+2015/11/18 09:53:26 enter bigSlowOperation
+2015/11/18 09:53:36 exit bigSlowOperation (10.000589217s)
+!-output
+*/