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:

Stone Duckfeet Stone Arhus Arhus Boot Duckfeet Boot alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Duckfeet Boot Arhus Duckfeet Stone Arhus Boot Stone 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]NIKE Black Work Dart White Blue Running Sock Shoe White Men's 7r1zw7

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