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:

Kicks Women's Skechers Bandana Meditation Studio Navy alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Skechers Kicks Navy Studio Bandana Women's Meditation 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Lightweight Sneakers Shoes Flyknit Men's Red 66 2 Town No Running CtSq6

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