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:

Brun Chukka Casual Enzo Boot Fox Men's Golden alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men's Golden Boot Fox Casual Enzo Brun Chukka 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Grey Slipper Slip Massage Foam Household Shower Non WYSBAOSHU Spa Bathroom Sandal qxOZwHvOT

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