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:

Flip Summer Fashion Beach Platform Hotmarzz Flops Women's Blue Stylish Wedge Sandals Slippers Heel High alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [High Flip Slippers Stylish Fashion Wedge Heel Platform Sandals Summer Blue Hotmarzz Flops Beach Women's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sebago Brown Leather Waxy Eye Shoe Two Oiled Liteside Bottom Boat Men's 6Owr6

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)