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:

Sneaker Sneaker Blue Etnies Etnies Scout Etnies Blue Scout Green Green alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Blue Etnies Sneaker Scout Etnies Scout Blue Sneaker Etnies Green Green 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Forfoot Women's Skid Slipper Block Coral Sole Non Socks Unisex Soft Men's White Fleece rrwp5a6q

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