An alias declaration inside a lib declares a C typedef:

lib X
  alias MyInt = Int32

Now Int32 and MyInt are interchangeable:

lib X
  alias MyInt = Int32

  fun some_fun(value : MyInt)

X.some_fun 1 # OK

An alias is most useful to avoid writing long types over and over, but also to declare a type based on compile-time flags:

lib C
  {% if flag?(:x86_64) %}
    alias SizeT = Int64
  {% else %}
    alias SizeT = Int32
  {% end %}

  fun memcmp(p1 : Void*, p2 : Void*, size : C::SizeT) : Int32

Refer to the type grammar for the notation used in alias types.

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