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:

Classic Old Green Skool Shoes Vans Skate Unisex alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Skool Skate Shoes Unisex Old Green Classic Vans 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Boot Black Stacy Santee Adams Men's aqwwYUA

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