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:

Perforated Microfiber Chestnut Dearfoams Slipper Suede Men's Clog alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men's Microfiber Suede Dearfoams Perforated Clog Slipper Chestnut 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slippers Tassel Suede Brown Moccasin Mens Loafers Prom Shoes Dress Driving TM Shoes Men For HAPPYSHOP Loafers Leather Ezg88w

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)