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:

Snow blue RUN Men's Boots Outdoor L Short Boots Winter 6811 Fur Sneaker alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Fur Winter L Men's Short 6811 Outdoor Sneaker Boots Boots RUN Snow blue 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]brown dk brn Dk Shoes Men's Pitts Bruno Marc Oxfords 16 nOYq01xfCw

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