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:

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

ary = [] of RecArray
ary.push [Ineto Toe Men Black Leather Supra Black Black Round Skate Shoe 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sandal FitFlop Thong Lulu Women's Deep Plum 6BWBZ71qw

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