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:

Blue Black NIKE Free Hydrogen Shoe Running RN tough Red Men's alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Red NIKE Black tough Hydrogen Men's Free Running RN Shoe Blue 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Men's Brown Insulated Waterproof Lace Up 6" Boot 25945 Chippewa dqv8OTtwd

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