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:

Black Supra Round Black Ineto Black Skate Leather Toe Shoe Men alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Round Toe Men Skate Supra Shoe Black Black Ineto Leather Black 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slip Gaorui Dress Leather Toe Business Flat Oxfords Formal Shoes On Loafers Brown Pointed Men n4PqrF4

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