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:

Kids GS White Air Shoe Big Black Leather 9 Red Max Nike Kid's University alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [White Big Nike Kid's Leather Max 9 Black University Kids GS Red Shoe Air 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]NIKE Cool Boot Men's Leather Hiking Manoa Grey Black HpXrH

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