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:

Boot Toe Work 6 0 Charcoal Magnum Waterproof Baltimore Men's Charcoal Comp alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Magnum 0 Baltimore Waterproof Boot Work Men's Toe Comp Charcoal Charcoal 6 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]mastr Grey Madden Men's Canvas Sneaker M nawZW8C

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