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:

Tan Whitford amp; fBuaLpzCZX Bass G Oxford H Men's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Bass Tan G amp; Whitford H Men's fBuaLpzCZX Oxford 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Olive Yoga 2 Dark Sling Sanuk Women’s fqzAXX

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