config: add `concat` function [GH-191]

CHANGELOG.md

@@ -9,6 +9,11 @@ BACKWARDS INCOMPATIBILITIES:
     * Keys cannot be double-quoted strings: `"foo" = "bar"` is no longer
       valid.
 
+FEATURES:
+
+  * **New Function: `concat`**: Concatenate multiple strings together.
+    Example: `concat(var.region, "-", var.channel)`.
+
 IMPROVEMENTS:
 
   * core: "~/.terraformrc" (Unix) or "%APPDATA%/terraform.rc" (Windows)

config/interpolate_funcs.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"strings"
@@ -11,11 +12,27 @@ var Funcs map[string]InterpolationFunc
 
 func init() {
 	Funcs = map[string]InterpolationFunc{
+		"concat": interpolationFuncConcat,
 		"file":   interpolationFuncFile,
 		"lookup": interpolationFuncLookup,
 	}
 }
 
+// interpolationFuncConcat implements the "concat" function that allows
+// strings to be joined together.
+func interpolationFuncConcat(
+	vs map[string]string, args ...string) (string, error) {
+	var buf bytes.Buffer
+
+	for _, a := range args {
+		if _, err := buf.WriteString(a); err != nil {
+			return "", err
+		}
+	}
+
+	return buf.String(), nil
+}
+
 // interpolationFuncFile implements the "file" function that allows
 // loading contents from a file.
 func interpolationFuncFile(

config/interpolate_funcs_test.go

@@ -6,6 +6,43 @@ import (
 	"testing"
 )
 
+func TestInterpolateFuncConcat(t *testing.T) {
+	cases := []struct {
+		Args   []string
+		Result string
+		Error  bool
+	}{
+		{
+			[]string{"foo", "bar", "baz"},
+			"foobarbaz",
+			false,
+		},
+
+		{
+			[]string{"foo", "bar"},
+			"foobar",
+			false,
+		},
+
+		{
+			[]string{"foo"},
+			"foo",
+			false,
+		},
+	}
+
+	for i, tc := range cases {
+		actual, err := interpolationFuncConcat(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 TestInterpolateFuncFile(t *testing.T) {
 	tf, err := ioutil.TempFile("", "tf")
 	if err != nil {