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 Vs Sneaker Blue Pace Originals alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Sneaker Vs Men's Blue adidas Originals Pace 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Womens Slip Pink Bull Shoes Shoes for 02 Cold House Rfw Weather Indoor Mules Mens Happy Slippers On pUYq4qw

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