json: allow one to unmarshal a top-level JSON array.

Fixies issue 114.

R=rsc
CC=golang-dev
https://golang.org/cl/154121

src/pkg/json/struct.go

@@ -224,7 +224,7 @@ func (b *structBuilder) Key(k string) Builder {
 }
 
 // Unmarshal parses the JSON syntax string s and fills in
-// an arbitrary struct or array pointed at by val.
+// an arbitrary struct or slice pointed at by val.
 // It uses the reflect package to assign to fields
 // and arrays embedded in val.  Well-formed data that does not fit
 // into the struct is discarded.
@@ -279,11 +279,27 @@ func (b *structBuilder) Key(k string) Builder {
 // assign to upper case fields.  Unmarshal uses a case-insensitive
 // comparison to match JSON field names to struct field names.
 //
+// To unmarshal a top-level JSON array, pass in a pointer to an empty
+// slice of the correct type.
+//
 // On success, Unmarshal returns with ok set to true.
 // On a syntax error, it returns with ok set to false and errtok
 // set to the offending token.
 func Unmarshal(s string, val interface{}) (ok bool, errtok string) {
-	b := &structBuilder{val: reflect.NewValue(val)};
+	v := reflect.NewValue(val);
+	var b *structBuilder;
+
+	// If val is a pointer to a slice, we mutate the pointee.
+	if ptr, ok := v.(*reflect.PtrValue); ok {
+		if slice, ok := ptr.Elem().(*reflect.SliceValue); ok {
+			b = &structBuilder{val: slice}
+		}
+	}
+
+	if b == nil {
+		b = &structBuilder{val: v}
+	}
+
 	ok, _, errtok = Parse(s, b);
 	if !ok {
 		return false, errtok

src/pkg/json/struct_test.go

@@ -126,9 +126,34 @@ func TestIssue147(t *testing.T) {
 	var timeline Issue147;
 	Unmarshal(issue147Input, &timeline);
 
+	if len(timeline.Test) != 30 {
+		t.Errorf("wrong length: got %d want 30", len(timeline.Test))
+	}
+
 	for i, e := range timeline.Test {
 		if e.Text != strconv.Itoa(i) {
 			t.Errorf("index: %d got: %s want: %d", i, e.Text, i)
 		}
 	}
 }
+
+type Issue114 struct {
+	Text string;
+}
+
+const issue114Input = `[{"text" : "0"}, {"text" : "1"}, {"text" : "2"}, {"text" : "3"}]`
+
+func TestIssue114(t *testing.T) {
+	var items []Issue114;
+	Unmarshal(issue114Input, &items);
+
+	if len(items) != 4 {
+		t.Errorf("wrong length: got %d want 4", len(items))
+	}
+
+	for i, e := range items {
+		if e.Text != strconv.Itoa(i) {
+			t.Errorf("index: %d got: %s want: %d", i, e.Text, i)
+		}
+	}
+}