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:

Skate Toe Shoe Black Ineto Leather Round Black Black Supra Men alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Ineto Toe Black Men Leather Shoe Black Skate Supra Black Round 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Stone Shoe Foam Running Balance Fresh Grey Cruz Urban Men's New Grey Military 7qfvwUBfx

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