exp/template: better template invocation

1) Make the value optional ({{template "foo"}})
2) Allow the template identifier to be a thing of type *Template.
The second makes it easy to drop templates in to a set dynamically
during invocation.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4671056

src/pkg/exp/template/exec.go

@@ -220,13 +220,27 @@ func (s *state) walkRange(data reflect.Value, r *rangeNode) {
 }
 
 func (s *state) walkTemplate(data reflect.Value, t *templateNode) {
-	name := s.evalArg(data, reflect.TypeOf("string"), t.name).String()
-	if s.set == nil {
-		s.errorf("no set defined in which to invoke template named %q", name)
-	}
-	tmpl := s.set.tmpl[name]
-	if tmpl == nil {
-		s.errorf("template %q not in set", name)
+	// Can't use evalArg because there are two types we expect.
+	arg := s.evalEmptyInterface(data, t.name)
+	if !arg.IsValid() {
+		s.errorf("invalid value in template invocation; expected string or *Template")
+	}
+	var tmpl *Template
+	if arg.Type() == reflect.TypeOf((*Template)(nil)) {
+		tmpl = arg.Interface().(*Template)
+		if tmpl == nil {
+			s.errorf("nil template")
+		}
+	} else {
+		s.validateType(arg, reflect.TypeOf(""))
+		name := arg.String()
+		if s.set == nil {
+			s.errorf("no set defined in which to invoke template named %q", name)
+		}
+		tmpl = s.set.tmpl[name]
+		if tmpl == nil {
+			s.errorf("template %q not in set", name)
+		}
 	}
 	defer s.pop(s.mark())
 	data = s.evalPipeline(data, t.pipe)
@@ -245,8 +259,10 @@ func (s *state) walkTemplate(data reflect.Value, t *templateNode) {
 // pipeline has a variable declaration, the variable will be pushed on the
 // stack. Callers should therefore pop the stack after they are finished
 // executing commands depending on the pipeline value.
-func (s *state) evalPipeline(data reflect.Value, pipe *pipeNode) reflect.Value {
-	value := zero
+func (s *state) evalPipeline(data reflect.Value, pipe *pipeNode) (value reflect.Value) {
+	if pipe == nil {
+		return
+	}
 	for _, cmd := range pipe.cmds {
 		value = s.evalCommand(data, cmd, value) // previous value is this one's final arg.
 		// If the object has type interface{}, dig down one level to the thing inside.
@@ -434,8 +450,11 @@ func (s *state) evalCall(v, fun reflect.Value, name string, isMethod bool, args
 	return result[0]
 }
 
-// validateType guarantees that the value is assignable to the type.
+// validateType guarantees that the value is valid and assignable to the type.
 func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
+	if !value.IsValid() {
+		s.errorf("invalid value; expected %s", typ)
+	}
 	if !value.Type().AssignableTo(typ) {
 		s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
 	}
@@ -462,7 +481,7 @@ func (s *state) evalArg(data reflect.Value, typ reflect.Type, n node) reflect.Va
 		return s.evalInteger(typ, n)
 	case reflect.Interface:
 		if typ.NumMethod() == 0 {
-			return s.evalEmptyInterface(data, typ, n)
+			return s.evalEmptyInterface(data, n)
 		}
 	case reflect.String:
 		return s.evalString(typ, n)
@@ -533,7 +552,7 @@ func (s *state) evalComplex(typ reflect.Type, n node) reflect.Value {
 	panic("not reached")
 }
 
-func (s *state) evalEmptyInterface(data reflect.Value, typ reflect.Type, n node) reflect.Value {
+func (s *state) evalEmptyInterface(data reflect.Value, n node) reflect.Value {
 	switch n := n.(type) {
 	case *boolNode:
 		return reflect.ValueOf(n.true)

src/pkg/exp/template/exec_test.go

@@ -42,6 +42,8 @@ type T struct {
 	PI  *int
 	PSI *[]int
 	NIL *int
+	// Template to test evaluation of templates.
+	Tmpl *Template
 }
 
 type U struct {
@@ -67,6 +69,7 @@ var tVal = &T{
 	Empty4: &U{"v"},
 	PI:     newInt(23),
 	PSI:    newIntSlice(21, 22, 23),
+	Tmpl:   New("x").MustParse("test template"), // "x" is the value of .X
 }
 
 // Helpers for creation.

src/pkg/exp/template/parse.go

@@ -469,6 +469,9 @@ func newTemplate(line int, name node, pipe *pipeNode) *templateNode {
 }
 
 func (t *templateNode) String() string {
+	if t.pipe == nil {
+		return fmt.Sprintf("{{template %s}}", t.name)
+	}
 	return fmt.Sprintf("{{template %s %s}}", t.name, t.pipe)
 }
 
@@ -748,7 +751,6 @@ func (t *Template) withControl() node {
 	return newWith(t.parseControl("with"))
 }
 
-
 // End:
 //	{{end}}
 // End keyword is past.
@@ -790,7 +792,12 @@ func (t *Template) templateControl() node {
 	default:
 		t.unexpected(token, "template invocation")
 	}
-	return newTemplate(t.lex.lineNumber(), name, t.pipeline("template"))
+	var pipe *pipeNode
+	if t.next().typ != itemRightDelim {
+		t.backup()
+		pipe = t.pipeline("template")
+	}
+	return newTemplate(t.lex.lineNumber(), name, pipe)
 }
 
 // command:

src/pkg/exp/template/parse_test.go

@@ -174,6 +174,8 @@ var parseTests = []parseTest{
 		`[({{range [(command: [F=[SI]])]}} [(action: [(command: [{{<.>}}])])])]`},
 	{"constants", "{{range .SI 1 -3.2i true false }}{{end}}", noError,
 		`[({{range [(command: [F=[SI] N=1 N=-3.2i B=true B=false])]}} [])]`},
+	{"template", "{{template `x`}}", noError,
+		"[{{template S=`x`}}]"},
 	{"template", "{{template `x` .Y}}", noError,
 		"[{{template S=`x` [(command: [F=[Y]])]}}]"},
 	{"with", "{{with .X}}hello{{end}}", noError,

src/pkg/exp/template/set_test.go

@@ -76,11 +76,15 @@ func TestSetParse(t *testing.T) {
 var setExecTests = []execTest{
 	{"empty", "", "", nil, true},
 	{"text", "some text", "some text", nil, true},
-	{"invoke text", `{{template "text" .SI}}`, "TEXT", tVal, true},
+	{"invoke x", `{{template "x" .SI}}`, "TEXT", tVal, true},
+	{"invoke x no args", `{{template "x"}}`, "TEXT", tVal, true},
 	{"invoke dot int", `{{template "dot" .I}}`, "17", tVal, true},
 	{"invoke dot []int", `{{template "dot" .SI}}`, "[3 4 5]", tVal, true},
 	{"invoke dotV", `{{template "dotV" .U}}`, "v", tVal, true},
 	{"invoke nested int", `{{template "nested" .I}}`, "17", tVal, true},
+	{"invoke template by field", `{{template .X}}`, "TEXT", tVal, true},
+	{"invoke template by template", `{{template .Tmpl}}`, "test template", tVal, true},
+	{"invalid: invoke template by []int", `{{template .SI}}`, "", tVal, false},
 
 	// User-defined function: test argument evaluator.
 	{"testFunc literal", `{{oneArg "joe"}}`, "oneArg=joe", tVal, true},
@@ -88,7 +92,7 @@ var setExecTests = []execTest{
 }
 
 const setText = `
-	{{define "text"}}TEXT{{end}}
+	{{define "x"}}TEXT{{end}}
 	{{define "dotV"}}{{.V}}{{end}}
 	{{define "dot"}}{{.}}{{end}}
 	{{define "nested"}}{{template "dot" .}}{{end}}