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:

EQT Gray adidas Two Gray Gray Men's Support 93 Three 17 Originals Shoe Running Two alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Gray Running Three Two Support 17 Originals Gray Shoe Men's Gray adidas 93 EQT Two 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]EH Black ST SR Terrain All Women's Reebok Turquoise Work qw0IX

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