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:

Golden Fox Brun Boot Chukka Enzo Men's Casual alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men's Chukka Golden Enzo Casual Boot Brun Fox 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]toe western genuine leather Men cowboy boots skin Beige j eel OxFqCwAC

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