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:

Fire Under Orange Gray Running Blaze Gemini Armour Phoenix Men's 3 Speedform Shoe Glacier alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Armour Glacier Blaze Speedform Running 3 Orange Gray Men's Gemini Fire Under Shoe Phoenix 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Women Slip Fur Real Deep Single On Open Vesa Strap Leather pink Feather Manka Sandals Vegan Pink Fox Toe wI75FAq

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