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:

Dry WXDZ Walking Pool Women Men Water 5 Quick Beach Shoes Running Swim Aqua Shoes blue Barefoot Socks Sports alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [5 Pool blue Quick Sports Running Shoes Barefoot Aqua Beach Socks Swim Dry Walking Women WXDZ Men Shoes Water 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Loafer on Black Slip Nubuck Mephisto Men's Baduard Calf wgOq7nzpax

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)