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:

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

ary = [] of RecArray
ary.push [Beach Wedge Stylish Hotmarzz Heel Blue Slippers Women's Fashion Summer High Platform Sandals Flip Flops 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Feet Mens Womens and S West Comfy LICENSED OFFICIALLY NCAA Flop Mountaineers College Virginia Happy xTYAqwn1C

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

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