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:

Black Shoe 9 Men's Tour blue Fury KD Zoom Basketball Yellow NIKE alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [KD NIKE blue Shoe Basketball 9 Yellow Tour Black Fury Men's Zoom 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sandy Shoes LaForst Women's Slip Black On wqHW1TnBfW

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