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:

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

ary = [] of RecArray
ary.push [Running Blue Free NIKE Men's Black Red Hydrogen RN tough Shoe 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Balance Mw813 Men's New New Balance Brown E1ZqI

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