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:

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

ary = [] of RecArray
ary.push [Womens Soft DAWGS Z Support Pink Sandals Arch 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Chukka Artists Timber High Ankle US ARTISTS KRAZY Shoe 11D Man SHOE I8tx7

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