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:

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

ary = [] of RecArray
ary.push [Magnum 6 Boot Work Baltimore Waterproof Comp Charcoal Toe Charcoal 0 Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Beard pattern Shooting Guard Trainers Big Top Of Low State Texas Man Player Basketball 8 Designsonic O8Tw1

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