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:

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

ary = [] of RecArray
ary.push [Skool Vans Shoes Classic Unisex Old Green Skate 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Blue and Light Women Non Bathroom Bath Shoes Dark Slip Soft Mens Shower Slippers HAOJIALI Blue Sandals House xnW6q0ORUn

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