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:

adidas Men's Originals Pace Blue Sneaker Vs alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Blue Pace Vs Sneaker Men's Originals adidas 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's AF1 Orange Basketball Mid Laser SF Orange Shoe Laser NIKE 7qwEndBd

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