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:

Z Womens Arch Pink Soft Sandals Support DAWGS alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Soft Arch DAWGS Z Sandals Pink Womens Support 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Light Shoe Check Skate Black Black CNVS Solar NIKE Brown Men's gum SB w4qRKva

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