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:

Pink Fleece TrendsBlue Slippers Baby Hot House amp; Different Cozy Bear Colors Fabric alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Baby Different Fleece TrendsBlue amp; House Hot Slippers Fabric Cozy Colors Bear Pink 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Boots Shoe Drew Brown Dark Men's Mansfield q14w4vt

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