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:

Shoe Tb Anthracite Ankle Zoom Trainer NIKE Train Cross High Toranada Men's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Toranada Tb High Anthracite NIKE Zoom Trainer Train Shoe Men's Cross Ankle 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Slipper Yellow Shoes Slip Genuine Shoes Leather Go Driving On Premium Tour Casual Men's Loafers Fashion 4qvtw68

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