config: concat function supports lists (combines more lists into one)

config/interpolate_funcs.go

@@ -18,6 +18,7 @@ var Funcs map[string]ast.Function
 
 func init() {
 	Funcs = map[string]ast.Function{
+		"concat":     interpolationFuncConcat(),
 		"element":    interpolationFuncElement(),
 		"file":       interpolationFuncFile(),
 		"format":     interpolationFuncFormat(),
@@ -26,10 +27,6 @@ func init() {
 		"length":     interpolationFuncLength(),
 		"replace":    interpolationFuncReplace(),
 		"split":      interpolationFuncSplit(),
-
-		// Concat is a little useless now since we supported embeddded
-		// interpolations but we keep it around for backwards compat reasons.
-		"concat": interpolationFuncConcat(),
 	}
 }
 
@@ -45,11 +42,32 @@ func interpolationFuncConcat() ast.Function {
 		VariadicType: ast.TypeString,
 		Callback: func(args []interface{}) (interface{}, error) {
 			var b bytes.Buffer
-			for _, v := range args {
-				b.WriteString(v.(string))
+			var finalList []string
+
+			var isDeprecated = true
+
+			for _, arg := range args {
+				argument := arg.(string)
+
+				if len(argument) == 0 {
+					continue
+				}
+
+				if strings.Contains(argument, InterpSplitDelim) {
+					isDeprecated = false
+				}
+
+				finalList = append(finalList, argument)
+
+				// Deprecated concat behaviour
+				b.WriteString(argument)
+			}
+
+			if isDeprecated {
+				return b.String(), nil
 			}
 
-			return b.String(), nil
+			return strings.Join(finalList, InterpSplitDelim), nil
 		},
 	}
 }

config/interpolate_funcs_test.go

@@ -5,13 +5,14 @@ import (
 	"io/ioutil"
 	"os"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/hashicorp/terraform/config/lang"
 	"github.com/hashicorp/terraform/config/lang/ast"
 )
 
-func TestInterpolateFuncConcat(t *testing.T) {
+func TestInterpolateFuncDeprecatedConcat(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{
 			{
@@ -35,6 +36,86 @@ func TestInterpolateFuncConcat(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncConcat(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			// String + list
+			{
+				`${concat("a", split(",", "b,c"))}`,
+				fmt.Sprintf(
+					"%s%s%s%s%s",
+					"a",
+					InterpSplitDelim,
+					"b",
+					InterpSplitDelim,
+					"c"),
+				false,
+			},
+
+			// List + string
+			{
+				`${concat(split(",", "a,b"), "c")}`,
+				fmt.Sprintf(
+					"%s%s%s%s%s",
+					"a",
+					InterpSplitDelim,
+					"b",
+					InterpSplitDelim,
+					"c"),
+				false,
+			},
+
+			// Single list
+			{
+				`${concat(split(",", ",foo,"))}`,
+				fmt.Sprintf(
+					"%s%s%s",
+					InterpSplitDelim,
+					"foo",
+					InterpSplitDelim),
+				false,
+			},
+			{
+				`${concat(split(",", "a,b,c"))}`,
+				fmt.Sprintf(
+					"%s%s%s%s%s",
+					"a",
+					InterpSplitDelim,
+					"b",
+					InterpSplitDelim,
+					"c"),
+				false,
+			},
+
+			// Two lists
+			{
+				`${concat(split(",", "a,b,c"), split(",", "d,e"))}`,
+				strings.Join([]string{
+					"a", "b", "c", "d", "e",
+				}, InterpSplitDelim),
+				false,
+			},
+			// Two lists with different separators
+			{
+				`${concat(split(",", "a,b,c"), split(" ", "d e"))}`,
+				strings.Join([]string{
+					"a", "b", "c", "d", "e",
+				}, InterpSplitDelim),
+				false,
+			},
+
+			// More lists
+			{
+				`${concat(split(",", "a,b"), split(",", "c,d"), split(",", "e,f"), split(",", "0,1"))}`,
+				strings.Join([]string{
+					"a", "b", "c", "d", "e", "f", "0", "1",
+				}, InterpSplitDelim),
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncFile(t *testing.T) {
 	tf, err := ioutil.TempFile("", "tf")
 	if err != nil {

website/source/docs/configuration/interpolation.html.md

@@ -72,8 +72,8 @@ are documented below.
 
 The supported built-in functions are:
 
-  * `concat(args...)` - Concatenates the values of multiple arguments into
-      a single string.
+  * `concat(list1, list2)` - Combines two or more lists into a single list.
+     Example: `combine(aws_instance.db.*.tags.Name, aws_instance.web.*.tags.Name)`
 
   * `element(list, index)` - Returns a single element from a list
       at the given index. If the index is greater than the number of