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:

Flat Sandal Nice André Women's Denim Assous alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Flat André Assous Women's Denim Sandal Nice 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Brash Black Sandal Suede Faux Suede Fur Houston Women's Black zwxz4FO

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