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 Trainer Tb Shoe Anthracite Zoom Train Cross Toranada NIKE High Ankle alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Train NIKE Ankle Toranada Anthracite Shoe Trainer Men's High Tb Zoom Cross 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Blouse Clearance Zipper Winter Autumn Long Charberry Shirts Casual Gray Mens Top Dark Sleeve Coat Jacket Hooded O5qT8wg

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