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:

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

ary = [] of RecArray
ary.push [Mens Brown up Mesh Casual Toe Oxford Derby Leather PU Dress missfiona Pointed Lace Plain Shoes Shoes 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Fuzzies Bootie Mens Leather RJ's Chestnut Slippers Lined Sheepskin Oqd1nnxFw

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)