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:

5 WXDZ Swim Pool Shoes Barefoot Quick Men Running Sports Dry Walking Women Shoes Water Aqua Beach blue Socks alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Shoes Water Aqua Running Walking Socks Dry Beach Women Pool Swim Quick Barefoot blue 5 WXDZ Men Shoes Sports 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Gargoyle Mens Pressure Chico Shoe Suede Footwear SC Castlerock Skate Dvs x15gYqwpn

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

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