exp/template: A template can be in one set only.

This simplifies the API and makes it easier to make the template
invocation statically secure, at the cost of some minor flexibility.

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/4794045

src/pkg/exp/template/doc.go

@@ -252,11 +252,12 @@ Template sets
 Each template is named by a string specified when it is created.  A template may
 use a template invocation to instantiate another template directly or by its
 name; see the explanation of the template action above. The name is looked up
-in the template set active during the invocation.
+in the template set associated with the template.
 
 If no template invocation actions occur in the template, the issue of template
-sets can be ignored. If it does contain invocations, though, a set must be
-defined in which to look up the names.
+sets can be ignored.  If it does contain invocations, though, the template
+containing the invocations must be part of a template set in which to look up
+the names.
 
 There are two ways to construct template sets.
 
@@ -274,22 +275,19 @@ constant. Here is a simple example of input to Set.Parse:
 This defines two templates, T1 and T2, and a third T3 that invokes the other two
 when it is executed.
 
-The second way to build a template set is to use Set's Add method to add
-a template to a set. A template may be bound to multiple sets.
+The second way to build a template set is to use Set's Add method to add a
+parsed template to a set.  A template may be bound at most one set.  If it's
+necessary to have a template in multiple sets, the template definition must be
+parsed multiple times to create distinct *Template values.
 
 Set.Parse may be called multiple times on different inputs to construct the set.
 Two sets may therefore be constructed with a common base set of templates plus,
 through a second Parse call each, specializations for some elements.
 
-When a template is executed via Template.Execute, no set is defined and so no
-template invocations are possible. The method Template.ExecuteInSet provides a
-way to specify a template set when executing a template directly.
+A template may be executed directly or through Set.Execute, which executes a
+named template from the set.  To invoke our example above, we might write,
 
-A more direct technique is to use Set.Execute, which executes a named template
-from the set and provides the context for looking up templates in template
-invocations. To invoke our example above, we might write,
-
-	err := set.Execute("T3", os.Stdout, "no data needed")
+	err := set.Execute(os.Stdout, "T3", "no data needed")
 	if err != nil {
 		log.Fatalf("execution failed: %s", err)
 	}

src/pkg/exp/template/exec.go

@@ -20,7 +20,6 @@ import (
 type state struct {
 	tmpl *Template
 	wr   io.Writer
-	set  *Set
 	line int        // line number for errors
 	vars []variable // push-down stack of variable values.
 }
@@ -77,20 +76,12 @@ func (s *state) error(err os.Error) {
 
 // Execute applies a parsed template to the specified data object,
 // writing the output to wr.
-func (t *Template) Execute(wr io.Writer, data interface{}) os.Error {
-	return t.ExecuteInSet(wr, data, nil)
-}
-
-// ExecuteInSet applies a parsed template to the specified data object,
-// writing the output to wr. Nested template invocations will be resolved
-// from the specified set.
-func (t *Template) ExecuteInSet(wr io.Writer, data interface{}, set *Set) (err os.Error) {
+func (t *Template) Execute(wr io.Writer, data interface{}) (err os.Error) {
 	defer t.recover(&err)
 	value := reflect.ValueOf(data)
 	state := &state{
 		tmpl: t,
 		wr:   wr,
-		set:  set,
 		line: 1,
 		vars: []variable{{"$", value}},
 	}
@@ -225,10 +216,11 @@ func (s *state) walkRange(dot reflect.Value, r *rangeNode) {
 }
 
 func (s *state) walkTemplate(dot reflect.Value, t *templateNode) {
-	if s.set == nil {
+	set := s.tmpl.set
+	if set == nil {
 		s.errorf("no set defined in which to invoke template named %q", t.name)
 	}
-	tmpl := s.set.tmpl[t.name]
+	tmpl := set.tmpl[t.name]
 	if tmpl == nil {
 		s.errorf("template %q not in set", t.name)
 	}
@@ -349,7 +341,7 @@ func (s *state) evalFieldChain(dot, receiver reflect.Value, ident []string, args
 }
 
 func (s *state) evalFunction(dot reflect.Value, name string, args []node, final reflect.Value) reflect.Value {
-	function, ok := findFunction(name, s.tmpl, s.set)
+	function, ok := findFunction(name, s.tmpl, s.tmpl.set)
 	if !ok {
 		s.errorf("%q is not a defined function", name)
 	}

src/pkg/exp/template/exec_test.go

@@ -358,13 +358,13 @@ func testExecute(execTests []execTest, set *Set, t *testing.T) {
 	funcs := FuncMap{"zeroArgs": zeroArgs, "oneArg": oneArg, "typeOf": typeOf}
 	for _, test := range execTests {
 		tmpl := New(test.name).Funcs(funcs)
-		err := tmpl.Parse(test.input)
+		err := tmpl.ParseInSet(test.input, set)
 		if err != nil {
 			t.Errorf("%s: parse error: %s", test.name, err)
 			continue
 		}
 		b.Reset()
-		err = tmpl.ExecuteInSet(b, test.data, set)
+		err = tmpl.Execute(b, test.data)
 		switch {
 		case !test.ok && err == nil:
 			t.Errorf("%s: expected error; got none", test.name)

src/pkg/exp/template/parse.go

@@ -20,12 +20,13 @@ type Template struct {
 	name  string
 	root  *listNode
 	funcs map[string]reflect.Value
+	set   *Set // can be nil.
 	// Parsing only; cleared after parse.
-	set       *Set
+	parseSet  *Set // for function lookup during parse.
 	lex       *lexer
-	token     [2]item // two-token lookahead for parser
+	token     [2]item // two-token lookahead for parser.
 	peekCount int
-	vars      []string // variables defined at the moment
+	vars      []string // variables defined at the moment.
 }
 
 // Name returns the name of the template.
@@ -574,15 +575,16 @@ func (t *Template) recover(errp *os.Error) {
 // startParse starts the template parsing from the lexer.
 func (t *Template) startParse(set *Set, lex *lexer) {
 	t.root = nil
-	t.set = set
 	t.lex = lex
 	t.vars = []string{"$"}
+	t.parseSet = set
 }
 
 // stopParse terminates parsing.
 func (t *Template) stopParse() {
-	t.set, t.lex = nil, nil
+	t.lex = nil
 	t.vars = nil
+	t.parseSet = nil
 }
 
 // atEOF returns true if, possibly after spaces, we're at EOF.
@@ -609,25 +611,33 @@ func (t *Template) atEOF() bool {
 // Parse parses the template definition string to construct an internal
 // representation of the template for execution.
 func (t *Template) Parse(s string) (err os.Error) {
-	t.startParse(nil, lex(t.name, s))
 	defer t.recover(&err)
+	t.startParse(t.set, lex(t.name, s))
 	t.parse(true)
 	t.stopParse()
 	return
 }
 
 // ParseInSet parses the template definition string to construct an internal
-// representation of the template for execution.
+// representation of the template for execution. It also adds the template
+// to the set.
 // Function bindings are checked against those in the set.
 func (t *Template) ParseInSet(s string, set *Set) (err os.Error) {
-	t.startParse(set, lex(t.name, s))
 	defer t.recover(&err)
+	t.startParse(set, lex(t.name, s))
 	t.parse(true)
-	if len(t.vars) != 1 { // $ should still be defined
-		t.errorf("internal error: vars not popped")
-	}
 	t.stopParse()
-	return
+	t.addToSet(set)
+	return nil
+}
+
+// addToSet adds the template to the set, verifying it's not being double-assigned.
+func (t *Template) addToSet(set *Set) {
+	if set == nil || t.set == set {
+		return
+	}
+	// If double-assigned, Add will panic and we will turn that into an error.
+	set.Add(t)
 }
 
 // parse is the helper for Parse.
@@ -846,7 +856,7 @@ Loop:
 		case itemError:
 			t.errorf("%s", token.val)
 		case itemIdentifier:
-			if _, ok := findFunction(token.val, t, t.set); !ok {
+			if _, ok := findFunction(token.val, t, t.parseSet); !ok {
 				t.errorf("function %q not defined", token.val)
 			}
 			cmd.append(newIdentifier(token.val))

src/pkg/exp/template/set.go

@@ -39,41 +39,20 @@ func (s *Set) Funcs(funcMap FuncMap) *Set {
 }
 
 // Add adds the argument templates to the set. It panics if two templates
-// with the same name are added.
+// with the same name are added or if a template is already a member of
+// a set.
 // The return value is the set, so calls can be chained.
 func (s *Set) Add(templates ...*Template) *Set {
 	s.init()
 	for _, t := range templates {
-		if _, ok := s.tmpl[t.name]; ok {
-			panic(fmt.Errorf("template: %q already defined in set", t.name))
+		if t.set != nil {
+			panic(fmt.Errorf("template: %q already in a set", t.name))
 		}
-		s.tmpl[t.name] = t
-	}
-	return s
-}
-
-// AddSet adds the templates from the provided set to the to the receiver.
-// It panics if the call attempts to reuse a name defined in the set.
-// The return value is the set, so calls can be chained.
-func (s *Set) AddSet(set *Set) *Set {
-	s.init()
-	for _, t := range set.tmpl {
 		if _, ok := s.tmpl[t.name]; ok {
 			panic(fmt.Errorf("template: %q already defined in set", t.name))
 		}
 		s.tmpl[t.name] = t
-	}
-	return s
-}
-
-// Union adds the templates from the provided set to the to the receiver.
-// Unlike AddSet, it does not panic if a name is reused; instead the old
-// template is replaced.
-// The return value is the set, so calls can be chained.
-func (s *Set) Union(set *Set) *Set {
-	s.init()
-	for _, t := range set.tmpl {
-		s.tmpl[t.name] = t
+		t.set = s
 	}
 	return s
 }
@@ -85,13 +64,13 @@ func (s *Set) Template(name string) *Template {
 }
 
 // Execute applies the named template to the specified data object, writing
-// the output to wr.  Nested template invocations will be resolved from the set.
+// the output to wr.
 func (s *Set) Execute(wr io.Writer, name string, data interface{}) os.Error {
 	tmpl := s.tmpl[name]
 	if tmpl == nil {
 		return fmt.Errorf("template: no template %q in set", name)
 	}
-	return tmpl.ExecuteInSet(wr, data, s)
+	return tmpl.Execute(wr, data)
 }
 
 // recover is the handler that turns panics into returns from the top
@@ -140,6 +119,7 @@ func (s *Set) Parse(text string) (err os.Error) {
 			t.errorf("unexpected %s in %s", end, context)
 		}
 		t.stopParse()
+		t.addToSet(s)
 		s.tmpl[t.name] = t
 	}
 	return nil