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:

Flip Sandals Heel Stylish Platform Wedge Summer Fashion Flops High Blue Women's Beach Slippers Hotmarzz alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [High Sandals Beach Women's Heel Platform Fashion Summer Flip Blue Hotmarzz Stylish Slippers Wedge Flops 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Se Slipper Gold Green Daniel Black Women's Dormie Slide q4vw6Up

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