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:

Slippers Soft Cushioned Terry Closed Fidlansky House Men's Black Back alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Back Black Cushioned Terry Soft Men's Fidlansky Slippers Closed House 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Sole Marilyn Green Moccasin Flower Girls Socks Skid Non Slipper Leather nwRrwYTq

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