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:

Grey ECCO Women's Grey Sandal Jab Warm Metallic Toggle Warm alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Toggle Sandal Women's Warm Grey Jab Grey Warm Metallic ECCO 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slipper Amali Slip Checker Burgundy Loafer On Smoking Style Mens Hemingway Shoe Velvet Printed Chinese Dress RrY0grwq

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