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:

Chunky Bamboo Glitter Single with Heel Ankle Sandal Band Strap Blue Women's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Heel Ankle Single Sandal Women's with Strap Glitter Band Bamboo Blue Chunky 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]White puma Men's Puma Soccer Sala White PUMA Astro Shoe X0xn0aO

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