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:

Chukka Casual Boot Men's Fox Brun Enzo Golden alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Golden Boot Fox Men's Brun Chukka Casual Enzo 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Refine Skechers Men's 600 Black White Sneaker Run Go 6I7r6

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