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

ary = [] of RecArray
ary.push [Support Sandals Pink Z Womens Soft DAWGS Arch 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Mens Charcoal On Skechers Dynamight Sneakers Slip Yq0SnZ

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