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:

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

ary = [] of RecArray
ary.push [MIZD9 Men's Red 2 Baseball Black Mizuno Turf Trainer Shoes Players 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Court Fashion Gum Low Lace Dark Up Cappuccino Mid Dx Top Mens Vans Sneakers 57qA8xan

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