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:

PU Mesh Brown Shoes Lace Mens Shoes Oxford missfiona Leather Dress Pointed Casual up Plain Derby Toe alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Brown Leather up Shoes PU Mesh Dress Pointed Oxford Toe Shoes Plain Derby missfiona Lace Casual Mens 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Black Men Quick Amphibious for Water Shoes Raotes Mesh Shoes Drying Women qavnxT1

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