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:

Top Unisex Vans Authentic White Sneakers Smoke Low Adults' Slim Jersey True alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [White Authentic Vans Jersey True Slim Low Top Unisex Smoke Sneakers Adults' 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]NS Martens 7B10 Slip Dr Eye Resistant Aztec Boots 7 Unisex Leather qgXxRwp

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