Literals: Hash

Hash

A Hash representing a mapping of keys of a type K to values of a type V. It is typically created with a hash literal:

{1 => 2, 3 => 4}     # Hash(Int32, Int32)
{1 => 2, 'a' => 3}   # Hash(Int32 | Char, Int32)

A Hash can have mixed types, both for the keys and values, meaning K/V will be union types. These types are determined when the hash is created, either by specifying K and V or by using a hash literal. In the latter case, K will be set to the union of the hash literal keys, and V will be set to the union of the hash literal values.

When creating an empty hash you must always specify K and V:

{} of Int32 => Int32 # same as Hash(Int32, Int32).new
{}                   # syntax error

Hash-like types

You can use a special hash literal syntax with other types too, as long as they define an argless new method and a []= method:

MyType{"foo" => "bar"}

If MyType is not generic, the above is equivalent to this:

tmp = MyType.new
tmp["foo"] = "bar"
tmp

If MyType is generic, the above is equivalent to this:

tmp = MyType(typeof("foo"), typeof("bar")).new
tmp["foo"] = "bar"
tmp

In the case of a generic type, the type arguments can be specified too:

MyType(String, String) {"foo" => "bar"}