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:

Sandal André Flat Women's Assous Denim Nice alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Sandal Flat Women's Nice Denim André Assous 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Water Quick Swim Shoes Heeta e Barefoot Black Dry Men Shoes Socks Sports for Aqua Swim Women Beach RwqdU

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