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:

Nice Assous Denim André Women's Flat Sandal alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Nice Sandal Denim Women's André Flat Assous 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Classic Unisex white Black Checker Trainers Slip Adults’ Vans On wanxP1qCTT

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