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:

Men's Slip Magnanni Reva Loafer on Cognac alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [on Loafer Reva Cognac Men's Magnanni Slip 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Suede Black Steve Madden Chukka Hacksaw Men's Boot XgXYvw

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