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:

Grey Men's Boot Fashion Roe Crevo alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Roe Men's Grey Boot Fashion Crevo 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Soldier Basketball Men's infrared Shoe Lebron Blue Racer Team White NIKE IX TE6qX

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