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:

Real Slippers House Outdoor Scuff Blue amp; Clog Lining Indoor Fur Slip OSHOW Shoes on alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Shoes Slip Slippers Scuff Lining Blue on House Fur Indoor amp; Outdoor OSHOW Real Clog 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Brown 5 Deere McRae Ankle Men's Boot John CpSqROwC

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