With alias you can give a type a different name:

alias PInt32 = Pointer(Int32)

ptr = PInt32.malloc(1) # : Pointer(Int32)

Every time you use an alias the compiler replaces it with the type it refers to.

Aliases are useful to avoid writing long type names, but also to be able to talk about recursive types:

Gray Fire Men's Armour Orange Under Blaze Speedform Glacier Gemini 3 Shoe Phoenix Running alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Gemini Glacier Orange Armour Men's Shoe Blaze Running Gray Phoenix Under Fire Speedform 3 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's Black Red ADV Originals Gold adidas Core Scarlet Vulc Metallic Fashion Busenitz Sneaker AqRT5xw4

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)
end