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:

Arch Sandals Pink DAWGS Soft Z Womens Support alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Womens Soft Arch Support Pink Z DAWGS Sandals 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Toe Rubber Work with Fleece 12" 6147 Ranger Steel Heavy Men's Duty Insulated Boots qPWFB

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

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