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:

Boot Charcoal Work 0 Baltimore Comp 6 Toe Magnum Charcoal Waterproof Men's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [6 Charcoal Work 0 Boot Toe Magnum Baltimore Waterproof Charcoal Comp Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]NIKE AF1 Black black Men's Black Basketball Mid SF Shoe qfRrq

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