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:

As Women's Slipper Haflinger Natural Classic alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Women's Haflinger As Classic Slipper Natural 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sepia White Athletic Do Summit Stone Just Sepia NIKE It Stone Benassi Men's Sandal 1Aqn0S

A real-world example of a recursive type is json:

module Json
  alias Type = Nil |
               Bool |
               Int64 |
               Float64 |
               String |
               Array(Type) |
               Hash(String, Type)