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:

Reva Men's Magnanni Loafer on Slip Cognac alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Reva Loafer Cognac Magnanni Slip on Men's 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Laura Ashley Memory Floral Slippers Pink Open Ladies Toe Foam Insole Printed 44rwZ

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