exp/template: tweak behavior of booleans.

Russ suggested this technique, making the "and" and "or" functions handier.
But it's hacky, and I can be talked out of it.

R=dsymonds, rsc
CC=golang-dev
https://golang.org/cl/4698044

src/pkg/exp/template/doc.go

@@ -199,7 +199,10 @@ the set but the Funcs methods can be used to add them.
 Predefined global functions are named as follows.
 
 	and
-		Returns the boolean AND of its arguments.
+		Returns the boolean AND of its arguments by returning the
+		first empty argument or the last argument, that is,
+		"and x y" behaves as "if x then y else x". All the
+		arguments are evaluated.
 	html
 		Returns the escaped HTML equivalent of the textual
 		representation of its arguments.
@@ -213,7 +216,10 @@ Predefined global functions are named as follows.
 	not
 		Returns the boolean negation of its single argument.
 	or
-		Returns the boolean OR of its arguments.
+		Returns the boolean OR of its arguments by returning the
+		first non-empty argument or the last argument, that is,
+		"or x y" behaves as "if x then x else y". All the
+		arguments are evaluated.
 	print
 		An alias for fmt.Sprint
 	printf

src/pkg/exp/template/exec.go

@@ -289,7 +289,7 @@ func (s *state) evalCommand(dot reflect.Value, cmd *commandNode, final reflect.V
 	case *stringNode:
 		return reflect.ValueOf(word.text)
 	}
-	s.errorf("can't handle command %q", firstWord)
+	s.errorf("can't evaluate command %q", firstWord)
 	panic("not reached")
 }
 

src/pkg/exp/template/exec_test.go

@@ -280,8 +280,8 @@ var execTests = []execTest{
 
 	// Booleans
 	{"not", "{{not true}} {{not false}}", "false true", nil, true},
-	{"and", "{{and 0 0}} {{and 1 0}} {{and 0 1}} {{and 1 1}}", "false false false true", nil, true},
-	{"or", "{{or 0 0}} {{or 1 0}} {{or 0 1}} {{or 1 1}}", "false true true true", nil, true},
+	{"and", "{{and false 0}} {{and 1 0}} {{and 0 true}} {{and 1 1}}", "false 0 0 1", nil, true},
+	{"or", "{{or 0 0}} {{or 1 0}} {{or 0 true}} {{or 1 1}}", "0 1 true 1", nil, true},
 	{"boolean if", "{{if and true 1 `hi`}}TRUE{{else}}FALSE{{end}}", "TRUE", tVal, true},
 	{"boolean if not", "{{if and true 1 `hi` | not}}TRUE{{else}}FALSE{{end}}", "FALSE", nil, true},
 
@@ -326,6 +326,10 @@ var execTests = []execTest{
 	{"range empty map else", "{{range .MSIEmpty}}-{{.}}-{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
 	{"range empty interface", "{{range .Empty3}}-{{.}}-{{else}}EMPTY{{end}}", "-7--8-", tVal, true},
 
+	// Cute examples.
+	{"or as if true", `{{or .SI "slice is empty"}}`, "[3 4 5]", tVal, true},
+	{"or as if false", `{{or .SIEmpty "slice is empty"}}`, "slice is empty", tVal, true},
+
 	// Error handling.
 	{"error method, error", "{{.EPERM true}}", "", tVal, false},
 	{"error method, no error", "{{.EPERM false}}", "false", tVal, true},

src/pkg/exp/template/funcs.go

@@ -122,22 +122,39 @@ func index(item interface{}, indices ...interface{}) (interface{}, os.Error) {
 
 // Boolean logic.
 
-// and returns the Boolean AND of its arguments.
-func and(arg0 interface{}, args ...interface{}) (truth bool) {
-	truth, _ = isTrue(reflect.ValueOf(arg0))
-	for i := 0; truth && i < len(args); i++ {
-		truth, _ = isTrue(reflect.ValueOf(args[i]))
+func truth(a interface{}) bool {
+	t, _ := isTrue(reflect.ValueOf(a))
+	return t
+}
+
+// and computes the Boolean AND of its arguments, returning
+// the first false argument it encounters, or the last argument.
+func and(arg0 interface{}, args ...interface{}) interface{} {
+	if !truth(arg0) {
+		return arg0
+	}
+	for i := range args {
+		arg0 = args[i]
+		if !truth(arg0) {
+			break
+		}
 	}
-	return
+	return arg0
 }
 
-// or returns the Boolean OR of its arguments.
-func or(arg0 interface{}, args ...interface{}) (truth bool) {
-	truth, _ = isTrue(reflect.ValueOf(arg0))
-	for i := 0; !truth && i < len(args); i++ {
-		truth, _ = isTrue(reflect.ValueOf(args[i]))
+// or computes the Boolean OR of its arguments, returning
+// the first true argument it encounters, or the last argument.
+func or(arg0 interface{}, args ...interface{}) interface{} {
+	if truth(arg0) {
+		return arg0
+	}
+	for i := range args {
+		arg0 = args[i]
+		if truth(arg0) {
+			break
+		}
 	}
-	return
+	return arg0
 }
 
 // not returns the Boolean negation of its argument.