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:

Blue Green Etnies Sneaker Etnies Sneaker Etnies Scout Blue Scout Green alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Blue Sneaker Blue Etnies Etnies Etnies Green Scout Scout Sneaker Green 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Shoes Summer Womens Flat Sandals Holiday Shoes on Black Shoes Espadrilles Slip For Fashion Sale nbsp; Clearance Women,Farjing 8YZvw8

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