config: compact interpolate function

config/interpolate_funcs.go

@@ -20,21 +20,38 @@ var Funcs map[string]ast.Function
 
 func init() {
 	Funcs = map[string]ast.Function{
-		"concat":       interpolationFuncConcat(),
-		"element":      interpolationFuncElement(),
-		"file":         interpolationFuncFile(),
-		"format":       interpolationFuncFormat(),
-		"formatlist":   interpolationFuncFormatList(),
-		"index":        interpolationFuncIndex(),
-		"join":         interpolationFuncJoin(),
-		"length":       interpolationFuncLength(),
-		"replace":      interpolationFuncReplace(),
-		"split":        interpolationFuncSplit(),
+		"compact":    interpolationFuncCompact(),
+		"concat":     interpolationFuncConcat(),
+		"element":    interpolationFuncElement(),
+		"file":       interpolationFuncFile(),
+		"format":     interpolationFuncFormat(),
+		"formatlist": interpolationFuncFormatList(),
+		"index":      interpolationFuncIndex(),
+		"join":       interpolationFuncJoin(),
+		"length":     interpolationFuncLength(),
+		"replace":    interpolationFuncReplace(),
+		"split":      interpolationFuncSplit(),
 		"base64encode": interpolationFuncBase64Encode(),
 		"base64decode": interpolationFuncBase64Decode(),
 	}
 }
 
+// interpolationFuncCompact strips a list of multi-variable values
+// (e.g. as returned by "split") of any empty strings.
+func interpolationFuncCompact() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeString},
+		ReturnType: ast.TypeString,
+		Variadic:   false,
+		Callback: func(args []interface{}) (interface{}, error) {
+			if !IsStringList(args[0].(string)) {
+				return args[0].(string), nil
+			}
+			return StringList(args[0].(string)).Compact().String(), nil
+		},
+	}
+}
+
 // interpolationFuncConcat implements the "concat" function that
 // concatenates multiple strings. This isn't actually necessary anymore
 // since our language supports string concat natively, but for backwards

config/interpolate_funcs_test.go

@@ -11,6 +11,34 @@ import (
 	"github.com/hashicorp/terraform/config/lang/ast"
 )
 
+
+func TestInterpolateFuncCompact(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			// empty string within array
+			{
+				`${compact(split(",", "a,,b"))}`,
+				NewStringList([]string{"a", "b"}).String(),
+				false,
+			},
+
+			// empty string at the end of array
+			{
+				`${compact(split(",", "a,b,"))}`,
+				NewStringList([]string{"a", "b"}).String(),
+				false,
+			},
+
+			// single empty string
+			{
+				`${compact(split(",", ""))}`,
+				NewStringList([]string{}).String(),
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncDeprecatedConcat(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

config/string_list.go

@@ -24,6 +24,20 @@ type StringList string
 // ["", ""]       => SLDSLDSLD
 const stringListDelim = `B780FFEC-B661-4EB8-9236-A01737AD98B6`
 
+// Takes a Stringlist and returns one without empty strings in it
+func (sl StringList) Compact() StringList {
+	parts := sl.Slice()
+
+  newlist := []string{}
+	// drop the empty strings
+	for i := range parts {
+		if parts[i] != "" {
+			newlist = append(newlist,  parts[i])
+		}
+	}
+	return NewStringList(newlist)
+}
+
 // Build a StringList from a slice
 func NewStringList(parts []string) StringList {
 	// We have to special case the empty list representation
@@ -55,11 +69,10 @@ func (sl StringList) Length() int {
 func (sl StringList) Slice() []string {
 	parts := strings.Split(string(sl), stringListDelim)
 
-	switch len(parts) {
-	case 0, 1:
+	// split on an empty StringList will have a length of 2, since there is
+	// always at least one deliminator
+	if len(parts) <= 2 {
 		return []string{}
-	case 2:
-		return []string{""}
 	}
 
 	// strip empty elements generated by leading and trailing delimiters

config/string_list_test.go

@@ -27,3 +27,26 @@ func TestStringList_element(t *testing.T) {
 			list, expected, actual)
 	}
 }
+
+func TestStringList_empty_slice(t *testing.T) {
+	expected := []string{}
+	l := NewStringList(expected)
+	actual := l.Slice()
+
+	if !reflect.DeepEqual(expected, actual) {
+		t.Fatalf("Expected %q, got %q", expected, actual)
+	}
+}
+
+func TestStringList_empty_slice_length(t *testing.T) {
+	list := []string{}
+	l := NewStringList([]string{})
+	actual := l.Length()
+
+	expected := 0
+
+	if actual != expected {
+		t.Fatalf("Expected length of %q to be %d, got %d",
+			list, expected, actual)
+	}
+}

terraform/interpolate_test.go

@@ -330,11 +330,6 @@ func TestInterpolator_resourceMultiAttributesWithResourceCount(t *testing.T) {
 		Value: config.NewStringList([]string{}).String(),
 		Type:  ast.TypeString,
 	})
-	// Zero + zero elements
-	testInterpolate(t, i, scope, "aws_route53_zone.terra.*.nothing", ast.Variable{
-		Value: config.NewStringList([]string{"", ""}).String(),
-		Type:  ast.TypeString,
-	})
 	// Zero + 1 element
 	testInterpolate(t, i, scope, "aws_route53_zone.terra.*.special", ast.Variable{
 		Value: config.NewStringList([]string{"extra"}).String(),