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:

Warm Grey Warm Toggle Sandal Grey Metallic ECCO Jab Women's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Grey Warm Toggle Jab Metallic ECCO Warm Women's Grey Sandal 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Boot Chukka CLARKS Desert Bronze Men's q4qgwE7tx

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