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:

Running Shoe Free NIKE Red Hydrogen Black RN Men's Blue tough alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Blue Shoe Free Red RN tough Running Men's NIKE Black Hydrogen 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Army Max Air NIKE 599474 Mens Olive Boots 300 Gum Olive Goadome 2013 ACG Suede wFnRYxnZBq

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