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:

Shoes Mizuno 2 MIZD9 Black Men's Trainer Turf Red Players Baseball alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Red Turf Trainer Mizuno MIZD9 Men's Players 2 Shoes Baseball Black 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Quick Garden Walking Slippers Butterfly Coolloog Clogs blue Unisex Shoes Comfort Drying Sandals Mesh Lightweight zxYgqw5

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