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:

Mens Leather Oxford Toe Derby Plain Dress Shoes Mesh missfiona Shoes Pointed Brown up Casual PU Lace alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Mens Lace up Mesh Oxford Shoes Shoes Plain Pointed Derby Brown Leather Dress Casual Toe missfiona PU 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Black Toe Work Men's 83836 Boot Mesabi Setter Irish Steel 846XXq

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