config: add join() function

config/interpolate_funcs.go

@@ -14,6 +14,7 @@ func init() {
 	Funcs = map[string]InterpolationFunc{
 		"concat": interpolationFuncConcat,
 		"file":   interpolationFuncFile,
+		"join":   interpolationFuncJoin,
 		"lookup": interpolationFuncLookup,
 	}
 }
@@ -50,6 +51,23 @@ func interpolationFuncFile(
 	return string(data), nil
 }
 
+// interpolationFuncJoin implements the "join" function that allows
+// multi-variable values to be joined by some character.
+func interpolationFuncJoin(
+	vs map[string]string, args ...string) (string, error) {
+	if len(args) < 2 {
+		return "", fmt.Errorf("join expects 2 arguments")
+	}
+
+	var list []string
+	for _, arg := range args[1:] {
+		parts := strings.Split(arg, InterpSplitDelim)
+		list = append(list, parts...)
+	}
+
+	return strings.Join(list, args[0]), nil
+}
+
 // interpolationFuncLookup implements the "lookup" function that allows
 // dynamic lookups of map types within a Terraform configuration.
 func interpolationFuncLookup(

config/interpolate_funcs_test.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"testing"
@@ -91,6 +92,55 @@ func TestInterpolateFuncFile(t *testing.T) {
 	}
 }
 
+func TestInterpolateFuncJoin(t *testing.T) {
+	cases := []struct {
+		Args   []string
+		Result string
+		Error  bool
+	}{
+		{
+			[]string{","},
+			"",
+			true,
+		},
+
+		{
+			[]string{",", "foo"},
+			"foo",
+			false,
+		},
+
+		{
+			[]string{",", "foo", "bar"},
+			"foo,bar",
+			false,
+		},
+
+		{
+			[]string{
+				".",
+				fmt.Sprintf(
+					"foo%sbar%sbaz",
+					InterpSplitDelim,
+					InterpSplitDelim),
+			},
+			"foo.bar.baz",
+			false,
+		},
+	}
+
+	for i, tc := range cases {
+		actual, err := interpolationFuncJoin(nil, tc.Args...)
+		if (err != nil) != tc.Error {
+			t.Fatalf("%d: err: %s", i, err)
+		}
+
+		if actual != tc.Result {
+			t.Fatalf("%d: bad: %#v", i, actual)
+		}
+	}
+}
+
 func TestInterpolateFuncLookup(t *testing.T) {
 	cases := []struct {
 		M      map[string]string

config/interpolate_walk_test.go

@@ -103,6 +103,32 @@ func TestInterpolationWalker_detect(t *testing.T) {
 				},
 			},
 		},
+
+		{
+			Input: map[string]interface{}{
+				"foo": `${join(",", foo.bar.*.id)}`,
+			},
+			Result: []Interpolation{
+				&FunctionInterpolation{
+					Func: nil,
+					Args: []Interpolation{
+						&LiteralInterpolation{
+							Literal: ",",
+						},
+						&VariableInterpolation{
+							Variable: &ResourceVariable{
+								Type:  "foo",
+								Name:  "bar",
+								Field: "id",
+								Multi: true,
+								Index: -1,
+								key:   "foo.bar.*.id",
+							},
+						},
+					},
+				},
+			},
+		},
 	}
 
 	for i, tc := range cases {