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:

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

ary = [] of RecArray
ary.push [André Denim Flat Assous Nice Sandal Women's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Messi Soccer 2 Solar 16 Ground Copper Shoe adidas Black Green Cleats Men's Firm Metallic H0qR51nwxg

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