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:

MIZD9 Mizuno Red Black Men's Trainer Baseball Turf 2 Players Shoes alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Red Players 2 Trainer MIZD9 Black Mizuno Baseball Shoes Men's Turf 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Red Running Leather TX Round Red V Shoe Men ASICS Lyte G Gel Toe Zwaqxp4z

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