text/template: add 'nil' as a keyword in the language

The keyword reprents an untyped nil and is useful for
passing nil values to methods and functions. The
nil will be promoted to the appropriate type when
used; if a type cannot be assigned, an error results.

R=rsc, dsymonds
CC=golang-dev
https://golang.org/cl/6459056

src/pkg/text/template/doc.go

@@ -100,6 +100,7 @@ An argument is a simple value, denoted by one of the following.
 	- A boolean, string, character, integer, floating-point, imaginary
 	  or complex constant in Go syntax. These behave like Go's untyped
 	  constants, although raw strings may not span newlines.
+	- The keyword nil, representing an untyped Go nil.
 	- The character '.' (period):
 		.
 	  The result is the value of dot.

src/pkg/text/template/exec.go

@@ -327,6 +327,8 @@ func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final ref
 		return reflect.ValueOf(word.True)
 	case *parse.DotNode:
 		return dot
+	case *parse.NilNode:
+		s.errorf("nil is not a command")
 	case *parse.NumberNode:
 		return s.idealConstant(word)
 	case *parse.StringNode:
@@ -507,17 +509,23 @@ func (s *state) evalCall(dot, fun reflect.Value, name string, args []parse.Node,
 	return result[0]
 }
 
+// canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
+func canBeNil(typ reflect.Type) bool {
+	switch typ.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+		return true
+	}
+	return false
+}
+
 // 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() {
-		switch typ.Kind() {
-		case reflect.Interface, reflect.Ptr, reflect.Chan, reflect.Map, reflect.Slice, reflect.Func:
+		if canBeNil(typ) {
 			// An untyped nil interface{}. Accept as a proper nil value.
-			// TODO: Can we delete the other types in this list? Should we?
-			value = reflect.Zero(typ)
-		default:
-			s.errorf("invalid value; expected %s", typ)
+			return reflect.Zero(typ)
 		}
+		s.errorf("invalid value; expected %s", typ)
 	}
 	if !value.Type().AssignableTo(typ) {
 		if value.Kind() == reflect.Interface && !value.IsNil() {
@@ -547,6 +555,11 @@ func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) refle
 	switch arg := n.(type) {
 	case *parse.DotNode:
 		return s.validateType(dot, typ)
+	case *parse.NilNode:
+		if canBeNil(typ) {
+			return reflect.Zero(typ)
+		}
+		s.errorf("cannot assign nil to %s", typ)
 	case *parse.FieldNode:
 		return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, zero), typ)
 	case *parse.VariableNode:
@@ -644,6 +657,9 @@ func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Valu
 		return s.evalFieldNode(dot, n, nil, zero)
 	case *parse.IdentifierNode:
 		return s.evalFunction(dot, n.Ident, nil, zero)
+	case *parse.NilNode:
+		// NilNode is handled in evalArg, the only place that calls here.
+		s.errorf("evalEmptyInterface: nil (can't happen)")
 	case *parse.NumberNode:
 		return s.idealConstant(n)
 	case *parse.StringNode:

src/pkg/text/template/exec_test.go

@@ -63,6 +63,7 @@ type T struct {
 	BinaryFunc      func(string, string) string
 	VariadicFunc    func(...string) string
 	VariadicFuncInt func(int, ...string) string
+	NilOKFunc       func(*int) bool
 	// Template to test evaluation of templates.
 	Tmpl *Template
 	// Unexported field; cannot be accessed by template.
@@ -127,6 +128,7 @@ var tVal = &T{
 	BinaryFunc:        func(a, b string) string { return fmt.Sprintf("[%s=%s]", a, b) },
 	VariadicFunc:      func(s ...string) string { return fmt.Sprint("<", strings.Join(s, "+"), ">") },
 	VariadicFuncInt:   func(a int, s ...string) string { return fmt.Sprint(a, "=<", strings.Join(s, "+"), ">") },
+	NilOKFunc:         func(s *int) bool { return s == nil },
 	Tmpl:              Must(New("x").Parse("test template")), // "x" is the value of .X
 }
 
@@ -222,6 +224,7 @@ var execTests = []execTest{
 	// Trivial cases.
 	{"empty", "", "", nil, true},
 	{"text", "some text", "some text", nil, true},
+	{"nil action", "{{nil}}", "", nil, false},
 
 	// Ideal constants.
 	{"ideal int", "{{typeOf 3}}", "int", 0, true},
@@ -230,6 +233,7 @@ var execTests = []execTest{
 	{"ideal complex", "{{typeOf 1i}}", "complex128", 0, true},
 	{"ideal int", "{{typeOf " + bigInt + "}}", "int", 0, true},
 	{"ideal too big", "{{typeOf " + bigUint + "}}", "", 0, false},
+	{"ideal nil without type", "{{nil}}", "", 0, false},
 
 	// Fields of structs.
 	{".X", "-{{.X}}-", "-x-", tVal, true},
@@ -295,7 +299,8 @@ var execTests = []execTest{
 	{".Method2(3, .X)", "-{{.Method2 3 .X}}-", "-Method2: 3 x-", tVal, true},
 	{".Method2(.U16, `str`)", "-{{.Method2 .U16 `str`}}-", "-Method2: 16 str-", tVal, true},
 	{".Method2(.U16, $x)", "{{if $x := .X}}-{{.Method2 .U16 $x}}{{end}}-", "-Method2: 16 x-", tVal, true},
-	{".Method3(nil)", "-{{.Method3 .MXI.unset}}-", "-Method3: <nil>-", tVal, true},
+	{".Method3(nil constant)", "-{{.Method3 nil}}-", "-Method3: <nil>-", tVal, true},
+	{".Method3(nil value)", "-{{.Method3 .MXI.unset}}-", "-Method3: <nil>-", tVal, true},
 	{"method on var", "{{if $x := .}}-{{$x.Method2 .U16 $x.X}}{{end}}-", "-Method2: 16 x-", tVal, true},
 	{"method on chained var",
 		"{{range .MSIone}}{{if $.U.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
@@ -306,6 +311,8 @@ var execTests = []execTest{
 	{"chained method on variable",
 		"{{with $x := .}}{{with .SI}}{{$.GetU.TrueFalse $.True}}{{end}}{{end}}",
 		"true", tVal, true},
+	{".NilOKFunc not nil", "{{call .NilOKFunc .PI}}", "false", tVal, true},
+	{".NilOKFunc nil", "{{call .NilOKFunc nil}}", "true", tVal, true},
 
 	// Function call builtin.
 	{".BinaryFunc", "{{call .BinaryFunc `1` `2`}}", "[1=2]", tVal, true},
@@ -324,6 +331,7 @@ var execTests = []execTest{
 	{".VariadicFuncBad0", "{{call .VariadicFunc 3}}", "", tVal, false},
 	{".VariadicFuncIntBad0", "{{call .VariadicFuncInt}}", "", tVal, false},
 	{".VariadicFuncIntBad`", "{{call .VariadicFuncInt `x`}}", "", tVal, false},
+	{".VariadicFuncNilBad", "{{call .VariadicFunc nil}}", "", tVal, false},
 
 	// Pipelines.
 	{"pipeline", "-{{.Method0 | .Method2 .U16}}-", "-Method2: 16 M0-", tVal, true},
@@ -332,6 +340,7 @@ var execTests = []execTest{
 	// If.
 	{"if true", "{{if true}}TRUE{{end}}", "TRUE", tVal, true},
 	{"if false", "{{if false}}TRUE{{else}}FALSE{{end}}", "FALSE", tVal, true},
+	{"if nil", "{{if nil}}TRUE{{end}}", "", tVal, false},
 	{"if 1", "{{if 1}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
 	{"if 0", "{{if 0}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
 	{"if 1.5", "{{if 1.5}}NON-ZERO{{else}}ZERO{{end}}", "NON-ZERO", tVal, true},
@@ -351,7 +360,8 @@ var execTests = []execTest{
 
 	// Print etc.
 	{"print", `{{print "hello, print"}}`, "hello, print", tVal, true},
-	{"print", `{{print 1 2 3}}`, "1 2 3", tVal, true},
+	{"print 123", `{{print 1 2 3}}`, "1 2 3", tVal, true},
+	{"print nil", `{{print nil}}`, "<nil>", tVal, true},
 	{"println", `{{println 1 2 3}}`, "1 2 3\n", tVal, true},
 	{"printf int", `{{printf "%04x" 127}}`, "007f", tVal, true},
 	{"printf float", `{{printf "%g" 3.5}}`, "3.5", tVal, true},
@@ -391,6 +401,7 @@ var execTests = []execTest{
 	{"map[one]", "{{index .MSI `one`}}", "1", tVal, true},
 	{"map[two]", "{{index .MSI `two`}}", "2", tVal, true},
 	{"map[NO]", "{{index .MSI `XXX`}}", "0", tVal, true},
+	{"map[nil]", "{{index .MSI nil}}", "0", tVal, true},
 	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
 	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
 

src/pkg/text/template/funcs.go

@@ -122,6 +122,9 @@ func index(item interface{}, indices ...interface{}) (interface{}, error) {
 			}
 			v = v.Index(int(x))
 		case reflect.Map:
+			if !index.IsValid() {
+				index = reflect.Zero(v.Type().Key())
+			}
 			if !index.Type().AssignableTo(v.Type().Key()) {
 				return nil, fmt.Errorf("%s is not index type for %s", index.Type(), v.Type())
 			}
@@ -187,10 +190,13 @@ func call(fn interface{}, args ...interface{}) (interface{}, error) {
 		} else {
 			argType = dddType
 		}
+		if !value.IsValid() && canBeNil(argType) {
+			value = reflect.Zero(argType)
+		}
 		if !value.Type().AssignableTo(argType) {
 			return nil, fmt.Errorf("arg %d has type %s; should be %s", i, value.Type(), argType)
 		}
-		argv[i] = reflect.ValueOf(arg)
+		argv[i] = value
 	}
 	result := v.Call(argv)
 	if len(result) == 2 {

src/pkg/text/template/parse/lex.go

@@ -60,6 +60,7 @@ const (
 	itemElse     // else keyword
 	itemEnd      // end keyword
 	itemIf       // if keyword
+	itemNil      // the untyped nil constant, easiest to treat as a keyword
 	itemRange    // range keyword
 	itemTemplate // template keyword
 	itemWith     // with keyword
@@ -89,6 +90,7 @@ var itemName = map[itemType]string{
 	itemElse:     "else",
 	itemIf:       "if",
 	itemEnd:      "end",
+	itemNil:      "nil",
 	itemRange:    "range",
 	itemTemplate: "template",
 	itemWith:     "with",
@@ -109,6 +111,7 @@ var key = map[string]itemType{
 	"end":      itemEnd,
 	"if":       itemIf,
 	"range":    itemRange,
+	"nil":      itemNil,
 	"template": itemTemplate,
 	"with":     itemWith,
 }

src/pkg/text/template/parse/lex_test.go

@@ -85,6 +85,12 @@ var lexTests = []lexTest{
 		tRight,
 		tEOF,
 	}},
+	{"nil", "{{nil}}", []item{
+		tLeft,
+		{itemNil, 0, "nil"},
+		tRight,
+		tEOF,
+	}},
 	{"dots", "{{.x . .2 .x.y}}", []item{
 		tLeft,
 		{itemField, 0, ".x"},

src/pkg/text/template/parse/node.go

@@ -44,6 +44,7 @@ const (
 	NodeIdentifier                 // An identifier; always a function name.
 	NodeIf                         // An if action.
 	NodeList                       // A list of Nodes.
+	NodeNil                        // An untyped nil constant.
 	NodeNumber                     // A numerical constant.
 	NodePipe                       // A pipeline of commands.
 	NodeRange                      // A range action.
@@ -288,6 +289,26 @@ func (d *DotNode) Copy() Node {
 	return newDot()
 }
 
+// NilNode holds the special identifier 'nil' representing an untyped nil constant.
+// It is represented by a nil pointer.
+type NilNode bool
+
+func newNil() *NilNode {
+	return nil
+}
+
+func (d *NilNode) Type() NodeType {
+	return NodeNil
+}
+
+func (d *NilNode) String() string {
+	return "nil"
+}
+
+func (d *NilNode) Copy() Node {
+	return newNil()
+}
+
 // FieldNode holds a field (identifier starting with '.').
 // The names may be chained ('.x.y').
 // The period is dropped from each ident.

src/pkg/text/template/parse/parse.go

@@ -355,7 +355,7 @@ func (t *Tree) pipeline(context string) (pipe *PipeNode) {
 			}
 			return
 		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
-			itemVariable, itemNumber, itemRawString, itemString:
+			itemNumber, itemNil, itemRawString, itemString, itemVariable:
 			t.backup()
 			pipe.append(t.command())
 		default:
@@ -470,6 +470,8 @@ Loop:
 			cmd.append(NewIdentifier(token.val))
 		case itemDot:
 			cmd.append(newDot())
+		case itemNil:
+			cmd.append(newNil())
 		case itemVariable:
 			cmd.append(t.useVar(token.val))
 		case itemField:

src/pkg/text/template/parse/parse_test.go

@@ -205,8 +205,8 @@ var parseTests = []parseTest{
 		`{{range $x := .SI}}{{.}}{{end}}`},
 	{"range 2 vars", "{{range $x, $y := .SI}}{{.}}{{end}}", noError,
 		`{{range $x, $y := .SI}}{{.}}{{end}}`},
-	{"constants", "{{range .SI 1 -3.2i true false 'a'}}{{end}}", noError,
-		`{{range .SI 1 -3.2i true false 'a'}}{{end}}`},
+	{"constants", "{{range .SI 1 -3.2i true false 'a' nil}}{{end}}", noError,
+		`{{range .SI 1 -3.2i true false 'a' nil}}{{end}}`},
 	{"template", "{{template `x`}}", noError,
 		`{{template "x"}}`},
 	{"template with arg", "{{template `x` .Y}}", noError,