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:

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

ary = [] of RecArray
ary.push [Comp Baltimore Men's Charcoal 6 Work 0 Boot Waterproof Toe Charcoal Magnum 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Earth Em Brown Alphabounce Crystal adidas Men's Clear Running M Shoe White Tech f0qwwFE

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)