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:

Stone Boot Stone Arhus Arhus Duckfeet Duckfeet Boot alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Duckfeet Stone Stone Boot Arhus Duckfeet Arhus Boot 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]up Makani OLUKAI Blue Mustard Mens Lace Channel xFApqwp67t

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