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:

Under Blaze Fire Running Gray Phoenix Armour Gemini 3 Speedform Glacier Men's Orange Shoe alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Gemini Phoenix 3 Blaze Fire Under Men's Running Glacier Speedform Orange Shoe Armour Gray 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's Boot Lite X SCARPA Prime Natural w6q78z

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