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:

Ankle Clog Dansko Sam Dollar Full Grain Strap Sand Women's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Dollar Sam Ankle Strap Clog Sand Women's Dansko Grain Full 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Reebok Cushion Steel Work 3 N Men's White Sneaker 0 1qwH14Z

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