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:

Sports Traveling Fishing Breathable Shoes Running Yoga Sneakers Blue4 Hiking Jogging Oumanke Men alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Yoga Men Blue4 Breathable Hiking Fishing Oumanke Traveling Jogging Shoes Sports Sneakers Running 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Fish PUMA Ultra 4 Shoe peacoat Evopower Men's orange Soccer TT Vigor Clown Yellow 7qxw7Arg

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

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