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:

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ary = [] of RecArray
ary.push [Gray Men's Big Elastic with Shoes Loafers Half on Santcro Buckle Romeo Fashion Print Enzo Runs Size Crocodile Slip Dress 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Women’s Sling Sanuk 2 Yoga Black White FzcOv

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

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