Exceptions/Catch an exception thrown in a nested call: Difference between revisions

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@@ Line 1,039: / Line 1,039: @@
     trace("complete")
 }</lang>
+{{out}}
-Output:
 <pre>
 main: start
@@ Line 1,074: / Line 1,074: @@
 main.main()
         t.go:61 +0x31
+</pre>
+A simpler example, closer to the task description:
+<lang go>package main
+import "fmt"
+type U0 struct {
+	error
+	s string
+}
+type U1 int
+func foo2() {
+	defer func() {
+		// We can't just "catch" U0 and ignore U1 directly but ...
+		if e := recover(); e != nil {
+			// e can be of any type, check for type U0
+			if x, ok := e.(*U0); ok {
+				// we can only execute code here,
+				// not return to the body of foo2
+				fmt.Println("Recovered U0:", x.s)
+				// We could cheat and call bar the second time
+				// from here, if it paniced again (even with U0)
+				// it wouldn't get recovered.
+				// Instead we've split foo into two calls to foo2.
+			} else {
+				// ... if we don't want to handle it we can
+				// pass it along.
+				fmt.Println("passing on:", e)
+				panic(e) // like a "re-throw"
+			}
+		}
+	}()
+	bar()
+}
+func foo() {
+	// Call bar twice via foo2
+	foo2()
+	foo2()
+	fmt.Println("not reached")
+}
+func bar() int {
+	return baz()
+}
+var done bool
+func baz() int {
+	if !done {
+		done = true
+		panic(&U0{nil, "a message"})
+	}
+	panic(U1(42))
+}
+func main() {
+	foo()
+	fmt.Println("No panic")
+}</lang>
+[http://play.golang.org/p/X2pa8zE1Ce Run in Go Playground].
+{{out}}
+<pre>Recovered U0: a message
+passing on: 42
+panic: (main.U1) (0xfc140,0x2a) [recovered]
+	panic: (main.U1) (0xfc140,0x2a)
+[... go-routine and stack trace omitted ...]
 </pre>