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:

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

ary = [] of RecArray
ary.push [Round Supra Leather Men Black Black Black Skate Ineto Toe Shoe 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Kedge Men's Loafer Sebago Bison Tie On Tan Leather Slip p75dgqdxnw

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