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

ary = [] of RecArray
ary.push [Shoe Men Supra Toe Black Black Black Ineto Leather Skate Round 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Boot Composite Black Toe Portwest Cap Compositelite Safety x6q5XS4

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