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:

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

ary = [] of RecArray
ary.push [ECCO Grey Warm Women's Toggle Metallic Grey Jab Sandal Warm 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Bottom Increasing CALDEN Shoes K62622 Height Elevator 2 Slip Taller 8 Black Inches Leather on 8YOw8r

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