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:

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

ary = [] of RecArray
ary.push [on Indoor Fur Slippers OSHOW Slip Scuff Blue Shoes House Lining Outdoor Clog Real amp; 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's Sanuk Barco Vintage Shoe Casa Grey Boat wqqUd7F

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