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:

Men Quick black Women Socks Shoes C3 Swim PENGCHENG Skin Barefoot Water Aqua Sports amp;green Dry Beach alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Sports black Skin Beach Water Quick Socks PENGCHENG C3 amp;green Dry Women Men Barefoot Shoes Swim Aqua 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Blue Asifn Memory Foam Casual Men House Flax Women Massage Indoor House Cozy Home Cotton Striped Slippers ZSxqpZr

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

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