Initial implementation of compact() interpolation function

config/interpolate_funcs.go

@@ -30,6 +30,7 @@ func init() {
 		"length":       interpolationFuncLength(),
 		"replace":      interpolationFuncReplace(),
 		"split":        interpolationFuncSplit(),
+		"compact":    interpolationFuncCompact(),
 		"base64encode": interpolationFuncBase64Encode(),
 		"base64decode": interpolationFuncBase64Decode(),
 	}
@@ -279,6 +280,22 @@ func interpolationFuncSplit() ast.Function {
 	}
 }
 
+// interpolationFuncCompact strips a list of 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 CompactStringList(StringList(args[0].(string))).String(), nil
+		},
+	}
+}
+
 // interpolationFuncLookup implements the "lookup" function that allows
 // dynamic lookups of map types within a Terraform configuration.
 func interpolationFuncLookup(vs map[string]ast.Variable) ast.Function {

config/string_list.go

@@ -74,3 +74,16 @@ func (sl StringList) String() string {
 func IsStringList(s string) bool {
 	return strings.Contains(s, stringListDelim)
 }
+
+func CompactStringList(sl StringList) StringList {
+	parts := sl.Slice()
+
+	var newlist []string
+	// drop the empty strings
+	for i := range parts {
+		if parts[i] != "" {
+			newlist = append(newlist,  parts[i])
+		}
+	}
+	return NewStringList(newlist)
+}