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:

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

ary = [] of RecArray
ary.push [Sneaker Etnies Scout Green Sneaker Green Etnies Blue Etnies Scout Blue 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Aerosoles Gold Sandal Chlose Friend Women's ztrIz

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