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:

Kid's Shoe Red 9 Leather Big Air Nike Max Kids Black University GS White alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Kids Big GS Red Kid's Black Max White University Nike Air 9 Leather Shoe 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Easy Women's Street Flat Bolt Sandal Black rZRr6Awq

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