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:

Slim True Low Authentic White Sneakers Vans Adults' Unisex Smoke Top Jersey alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [True Authentic Low Smoke Adults' Jersey Vans Unisex Sneakers Top White Slim 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Color Orange Men's Options Multiple 11 Molded Berzerk Navy Sizes Cleats Boombah ASwqvXX

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