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:

Men's Trail v1 Trail Foam Balance Running Shoe White New Fresh Kaymin Grey alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [v1 Foam New Fresh Balance Shoe Grey Trail Trail White Kaymin Running Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Quick Shoes and Mens Exercise Socks for Beach Aqua Swim Water Yoga Purple Barefoot Summer Womens Bohemian Shoes Dry for wxtS8

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