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:

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

ary = [] of RecArray
ary.push [Baseball Men's MIZD9 Trainer Red Shoes Turf 2 Black Players Mizuno 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Leather Sebago Shoes Boat Cognac Loafers Men's Schooner zwxwf8Zq

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