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:

Shoe Blue Running Black tough Free Men's Red Hydrogen NIKE RN alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Running tough RN Shoe NIKE Free Blue Hydrogen Men's Black Red 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Shoe Skateboarding Diadora Suede Spelt B Elite Beige 6qPwaCA

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