exp/template: support field syntax on maps

While using exp/template in practice, the syntax for
indexing values using the "index" action was found to be
very inconvenient for frequent use when handling dynamic
data from maps such as the ones used with json and yaml,
that use a type like map[string]interface{}.

For these kinds of maps, the default handling of fields as
{{.Field}} makes the task of handling the several references
significantly more pleasant and elegant, and is equivalent
to what's currently done in the "template" package and in
other external packages (e.g. mustache).

Even with this change, the index action is still relevant
as it allows indexing maps in other scenarios where keys
wouldn't be valid field names.

R=golang-dev, r, gustavo
CC=golang-dev
https://golang.org/cl/4898043

Author: niemeyer

src/pkg/exp/template/doc.go

@@ -101,6 +101,17 @@ An argument is a simple value, denoted by one of the following.
 	    .Field1.Field2
 	  Fields can also be evaluated on variables, including chaining:
 	    $x.Field1.Field2
+	- The name of a key of the data, which must be a map, preceded
+	  by a period, such as
+		.Key
+	  The result is the map element value indexed by the key.
+	  Key invocations may be chained and combined with fields to any
+	  depth:
+	    .Field1.Key1.Field2.Key2
+	  Although the key must be an alphanumeric identifier, unlike with
+	  field names they do not need to start with an upper case letter.
+	  Keys can also be evaluated on variables, including chaining:
+	    $x.key1.key2
 	- The name of a niladic method of the data, preceded by a period,
 	  such as
 		.Method
@@ -109,9 +120,9 @@ 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 and combined with fields
+	  Method invocations may be chained and combined with fields and keys
 	  to any depth:
-	    .Field1.Method1.Field2.Method2
+	    .Field1.Key1.Method1.Field2.Key2.Method2
 	  Methods can also be evaluated on variables, including chaining:
 	    $x.Method1.Field
 	- The name of a niladic function, such as

src/pkg/exp/template/exec.go

@@ -394,20 +394,31 @@ func (s *state) evalField(dot reflect.Value, fieldName string, args []parse.Node
 	if method, ok := methodByName(ptr, fieldName); ok {
 		return s.evalCall(dot, method, fieldName, args, final)
 	}
+	hasArgs := len(args) > 1 || final.IsValid()
 	// It's not a method; is it a field of a struct?
 	receiver, isNil := indirect(receiver)
 	if receiver.Kind() == reflect.Struct {
 		tField, ok := receiver.Type().FieldByName(fieldName)
 		if ok {
 			field := receiver.FieldByIndex(tField.Index)
-			if len(args) > 1 || final.IsValid() {
+			if hasArgs {
 				s.errorf("%s is not a method but has arguments", fieldName)
 			}
 			if tField.PkgPath == "" { // field is exported
 				return field
 			}
 		}
 	}
+	// If it's a map, attempt to use the field name as a key.
+	if receiver.Kind() == reflect.Map {
+		nameVal := reflect.ValueOf(fieldName)
+		if nameVal.Type().AssignableTo(receiver.Type().Key()) {
+			if hasArgs {
+				s.errorf("%s is not a method but has arguments", fieldName)
+			}
+			return receiver.MapIndex(nameVal)
+		}
+	}
 	if isNil {
 		s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
 	}

src/pkg/exp/template/exec_test.go

@@ -39,6 +39,8 @@ type T struct {
 	MSI      map[string]int
 	MSIone   map[string]int // one element, for deterministic output
 	MSIEmpty map[string]int
+	MXI      map[interface{}]int
+	MII      map[int]int
 	SMSI     []map[string]int
 	// Empty interfaces; used to see if we can dig inside one.
 	Empty0 interface{} // nil
@@ -85,6 +87,8 @@ var tVal = &T{
 	SB:     []bool{true, false},
 	MSI:    map[string]int{"one": 1, "two": 2, "three": 3},
 	MSIone: map[string]int{"one": 1},
+	MXI:    map[interface{}]int{"one": 1},
+	MII:    map[int]int{1: 1},
 	SMSI: []map[string]int{
 		{"one": 1, "two": 2},
 		{"eleven": 11, "twelve": 12},
@@ -211,6 +215,14 @@ var execTests = []execTest{
 	{".X", "-{{.X}}-", "-x-", tVal, true},
 	{".U.V", "-{{.U.V}}-", "-v-", tVal, true},
 
+	// Fields on maps.
+	{"map .one", "{{.MSI.one}}", "1", tVal, true},
+	{"map .two", "{{.MSI.two}}", "2", tVal, true},
+	{"map .NO", "{{.MSI.NO}}", "<no value>", tVal, true},
+	{"map .one interface", "{{.MXI.one}}", "1", tVal, true},
+	{"map .WRONG args", "{{.MSI.one 1}}", "", tVal, false},
+	{"map .WRONG type", "{{.MII.one}}", "", tVal, false},
+
 	// Dots of all kinds to test basic evaluation.
 	{"dot int", "<{{.}}>", "<13>", 13, true},
 	{"dot uint", "<{{.}}>", "<14>", uint(14), true},