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:

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

ary = [] of RecArray
ary.push [Men's Whitford amp; H Tan fBuaLpzCZX Bass G Oxford 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Women's Sandal Slide Leather Sam Saddle Gala Atanado Edelman 5wqIT6

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

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