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:

Women's Metallic Grey Grey ECCO Jab Toggle Warm Warm Sandal alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Jab Warm ECCO Toggle Metallic Grey Warm Women's Sandal Grey 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Originals Rx Navy Night Navy High adidas Men's Matchcourt White Collegiate wdq4wUI

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