Support for basic try-catch style exception handling.

Meant as illustration of the Go pattern that is using
goroutines and channels to handle exceptional situations.

Note: There is no need for "Finally" since the
"try block" (the function f supplied to Try)
cannot do a Smalltalk-style non-local return
and terminate the function surrounding Try.

Replaces CL 157083.

R=r, rsc
https://golang.org/cl/157087

src/pkg/Makefile

@@ -50,6 +50,7 @@ DIRS=\
 	exec\
 	exp/datafmt\
 	exp/eval\
+	exp/exception\
 	exp/iterable\
 	expvar\
 	flag\

src/pkg/exp/exception/Makefile

+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+include $(GOROOT)/src/Make.$(GOARCH)
+
+TARG=exp/exception
+GOFILES=\
+	exception.go\
+
+include $(GOROOT)/src/Make.pkg

src/pkg/exp/exception/exception.go

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This package illustrates how basic try-catch exception handling
+// can be emulated using goroutines, channels, and closures.
+//
+// This package is *not* intended as a general exception handler
+// library.
+//
+package exception
+
+import (
+	"fmt";
+	"runtime";
+)
+
+// A Handler function handles an arbitrary exception value x.
+type Handler func(x interface{})
+
+// An Exception carries an exception value.
+type Exception struct {
+	Value interface{};	// Value may be the nil exception
+}
+
+// Try invokes a function f with a Handler to throw exceptions.
+// The function f may terminate abnormally with an arbitrary
+// exception x by calling throw(x) within f. If an exception is
+// thrown, Try returns an *Exception; otherwise it returns nil.
+//
+// Usage pattern:
+//
+//	if x := exception.Try(func(throw exception.Handler) {
+//		...
+//		throw(42);  // terminate f by throwing exception 42
+//		...
+//	}); x != nil {
+//		// catch exception, e.g. print it
+//		fmt.Println(x.Value);
+//	}
+//
+// Alternative:
+//
+//	exception.Try(func(throw exception.Handler) {
+//		...
+//		throw(42);  // terminate f by throwing exception 42
+//		...
+//	}).Catch(func (x interface{}) {
+//		// catch exception, e.g. print it
+//		fmt.Println(x);
+//	})
+//
+func Try(f func(throw Handler)) *Exception {
+	h := make(chan *Exception);
+
+	// execute try block
+	go func() {
+		f(func(x interface{}) {
+			h <- &Exception{x};
+			runtime.Goexit();
+		});
+		h <- nil;	// clean termination
+	}();
+
+	return <-h;
+}
+
+
+// If x != nil, Catch invokes f with the exception value x.Value.
+// See Try for usage patterns.
+func (x *Exception) Catch(f Handler) {
+	if x != nil {
+		f(x.Value)
+	}
+}
+
+
+func (x *Exception) String() string {
+	if x != nil {
+		return fmt.Sprintf("exception: %v", x.Value)
+	}
+	return "";
+}

src/pkg/exp/exception/exception_test.go

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package exception
+
+import "testing"
+
+func TestNoException(t *testing.T) {
+	e := Try(func(throw Handler) {});
+	if e != nil {
+		t.Fatalf("no exception expected, found: %v", e)
+	}
+}
+
+
+func TestNilException(t *testing.T) {
+	e := Try(func(throw Handler) { throw(nil) });
+	if e == nil {
+		t.Fatalf("exception expected", e)
+	}
+	if e.Value != nil {
+		t.Fatalf("nil exception expected, found: %v", e)
+	}
+}
+
+
+func TestTry(t *testing.T) {
+	s := 0;
+	for i := 1; i <= 10; i++ {
+		e := Try(func(throw Handler) {
+			if i%3 == 0 {
+				throw(i);
+				panic("throw returned");
+			}
+		});
+		if e != nil {
+			s += e.Value.(int)
+		}
+	}
+	result := 3 + 6 + 9;
+	if s != result {
+		t.Fatalf("expected: %d, found: %d", result, s)
+	}
+}
+
+
+func TestCatch(t *testing.T) {
+	s := 0;
+	for i := 1; i <= 10; i++ {
+		Try(func(throw Handler) {
+			if i%3 == 0 {
+				throw(i)
+			}
+		}).Catch(func(x interface{}) { s += x.(int) })
+	}
+	result := 3 + 6 + 9;
+	if s != result {
+		t.Fatalf("expected: %d, found: %d", result, s)
+	}
+}