exp/template: allow niladic methods inside chained field references.

Also really fix the bug about dot vs. receivers.

R=rsc, r
CC=golang-dev
https://golang.org/cl/4705047

src/pkg/exp/template/doc.go

@@ -105,11 +105,11 @@ An argument is a simple value, denoted by one of the following.
 	  any type) or two return values, the second of which is an os.Error.
 	  If it has two and the returned error is non-nil, execution terminates
 	  and an error is returned to the caller as the value of Execute.
-	  Method invocations may be chained, but only the last element of
-	  the chain may be a method; others must be struct fields:
-	    .Field1.Field2.Method
+	  Method invocations may be chained and combined with fields
+	  to any depth:
+	    .Field1.Method1.Field2.Method2
 	  Methods can also be evaluated on variables, including chaining:
-	    $x.Field1.Method
+	    $x.Method1.Field
 	- The name of a niladic function, such as
 		fun
 	  The result is the value of invoking the function, fun(). The return
@@ -125,7 +125,10 @@ value (argument) or a function or method call, possibly with multiple arguments:
 	Argument
 		The result is the value of evaluating the argument.
 	.Method [Argument...]
-		The result is the value of calling the method with the arguments:
+		The method can be alone or the last element of a chain but,
+		unlike methods in the middle of a chain, it can take arguments.
+		The result is the value of calling the method with the
+		arguments:
 			dot.Method(Argument1, etc.)
 	functionName [Argument...]
 		The result is the value of calling the function associated

src/pkg/exp/template/exec.go

@@ -331,13 +331,15 @@ func (s *state) evalVariableNode(dot reflect.Value, v *variableNode, args []node
 	return s.evalFieldChain(dot, value, v.ident[1:], args, final)
 }
 
+// evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
+// dot is the environment in which to evaluate arguments, while
+// receiver is the value being walked along the chain.
 func (s *state) evalFieldChain(dot, receiver reflect.Value, ident []string, args []node, final reflect.Value) reflect.Value {
-	// Up to the last entry, it must be a field.
 	n := len(ident)
 	for i := 0; i < n-1; i++ {
-		dot = s.evalField(dot, ident[i], nil, zero, zero)
+		receiver = s.evalField(dot, ident[i], args[:1], zero, receiver)
 	}
-	// Now it can be a field or method and if a method, gets arguments.
+	// Now if it's a method, it gets the arguments.
 	return s.evalField(dot, ident[n-1], args, final, receiver)
 }
 
@@ -358,12 +360,11 @@ func isExported(name string) bool {
 // evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
 // The 'final' argument represents the return value from the preceding
 // value of the pipeline, if any.
-// If we're in a chain, such as (.X.Y.Z), .X and .Y cannot be methods;
-// canBeMethod will be true only for the last element of such chains (here .Z).
-func (s *state) evalField(dot reflect.Value, fieldName string, args []node, final reflect.Value,
-receiver reflect.Value) reflect.Value {
-	typ := dot.Type()
-	if receiver.IsValid() {
+func (s *state) evalField(dot reflect.Value, fieldName string, args []node, final, receiver reflect.Value) reflect.Value {
+	if !receiver.IsValid() {
+		return zero
+	}
+	typ := receiver.Type()
 	receiver, _ = indirect(receiver)
 	// Need to get to a value of type *T to guarantee we see all
 	// methods of T and *T.
@@ -374,11 +375,10 @@ receiver reflect.Value) reflect.Value {
 	if method, ok := methodByName(ptr.Type(), fieldName); ok {
 		return s.evalCall(dot, ptr, method.Func, fieldName, args, final)
 	}
-	}
 	// It's not a method; is it a field of a struct?
-	dot, isNil := indirect(dot)
-	if dot.Kind() == reflect.Struct {
-		field := dot.FieldByName(fieldName)
+	receiver, isNil := indirect(receiver)
+	if receiver.Kind() == reflect.Struct {
+		field := receiver.FieldByName(fieldName)
 		if field.IsValid() {
 			if len(args) > 1 || final.IsValid() {
 				s.errorf("%s is not a method but has arguments", fieldName)
@@ -391,7 +391,7 @@ receiver reflect.Value) reflect.Value {
 	if isNil {
 		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
 	}
-	s.errorf("can't handle evaluation of field %s in type %s", fieldName, typ)
+	s.errorf("can't evaluate field %s in type %s", fieldName, typ)
 	panic("not reached")
 }
 

src/pkg/exp/template/exec_test.go

@@ -17,6 +17,7 @@ import (
 // T has lots of interesting pieces to use to test execution.
 type T struct {
 	// Basics
+	True        bool
 	I           int
 	U16         uint16
 	X           string
@@ -52,6 +53,7 @@ type U struct {
 }
 
 var tVal = &T{
+	True:   true,
 	I:      17,
 	U16:    16,
 	X:      "x",
@@ -128,6 +130,18 @@ func (t *T) EPERM(error bool) (bool, os.Error) {
 	return false, nil
 }
 
+// A few methods to test chaining.
+func (t *T) GetU() *U {
+	return t.U
+}
+
+func (u *U) TrueFalse(b bool) string {
+	if b {
+		return "true"
+	}
+	return ""
+}
+
 func typeOf(arg interface{}) string {
 	return fmt.Sprintf("%T", arg)
 }
@@ -211,6 +225,15 @@ var execTests = []execTest{
 	{".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},
 	{"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}}",
+		"true", tVal, true},
+	{"chained method",
+		"{{range .MSIone}}{{if $.GetU.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
+		"true", tVal, true},
+	{"chained method on variable",
+		"{{with $x := .}}{{with .SI}}{{$.GetU.TrueFalse $.True}}{{end}}{{end}}",
+		"true", tVal, true},
 
 	// Pipelines.
 	{"pipeline", "-{{.Method0 | .Method2 .U16}}-", "-Method2: 16 M0-", tVal, true},
@@ -309,7 +332,7 @@ var execTests = []execTest{
 
 	// Fixed bugs.
 	// Must separate dot and receiver; otherwise args are evaluated with dot set to variable.
-	{"problem", "{{range .MSIone}}-{{if $.Method1 .}}X{{end}}{{end}}-", "-X-", tVal, true},
+	{"bug0", "{{range .MSIone}}{{if $.Method1 .}}X{{end}}{{end}}", "X", tVal, true},
 }
 
 func zeroArgs() string {