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:

Dansko Strap Grain Women's Full Dollar Sam Ankle Clog Sand alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Clog Sand Dansko Full Women's Ankle Sam Strap Dollar Grain 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Track Black Shoe Red NIKE Wolf Running Revolution Men's Grey 3 x0nwUpB1q

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