Alias Types

Alias types are used to define the new type that is an alias of existing type.

Alias Declaration

Syntax:

define AliasTypeName ExistingType;

define GenericAliasTypeName<T1, T2, ...> ExistingType;

where

AliasTypeName or GenericAliasTypeName is the valid identifier of the alias type being declared
ExistingType is some existing type
T1, T2, etc. are optional generic type arguments

Alias Usage

There’re multiple reasons to use alias types. Some of them are described below with examples.

Self-documenting alias types

Alias types may be defined to give the reader better understanding what a certain type is used for.

Example:

define ItemTag string;

record Item
{
   string name;
   list<ItemTag> tags;
}

Generic type reduction

Alias types may be used to simplify the type name of a commonly used generic type.

In the following example the type ordered_map is introduced as an alternative to built-in generic dict type, which does not provide guarantees on order persistence.

Example:

record KeyValuePair<TKey, TValue>
{
    TKey key;
    TValue value;
}

define ordered_map<TKey, TValue> list<KeyValuePair<TKey, TValue>>;

record TestRecord
{
    ordered_map<int, string> values;
}

Providing attributes

Very common reason to use alias types is to provide attributes modifying various aspects of the type behaviour. It may be handy to provide custom serialization attributes, schema attributes, or target language-specific attributes.

Example:

[csharp alias="DateTime" struct json.serializer="DateTimeJsonSerializer.Instance" binary.serializer="DateTimeBinarySerializer.Instance"]
[erlang alias="calendar:datetime"]
[erlang binary.parser="igor_custom:datetime_from_binary" binary.packer="igor_custom:datetime_to_binary"]
[erlang json.parser="igor_custom:datetime_from_json" json.packer="igor_custom:datetime_to_json"]
[schema editor=datetime]
define DateTime string;

In this example we define DateTime as an alias of the string type, but we use attributes to adjust code generation aspects for C# and Erlang languages, and setup schema information.