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:
Nice Sandal Women's André Flat Denim Assous alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [Flat Assous Women's Nice Denim André Sandal 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]Construction Rocky Men's Boot Brown RKK0215 E4rxw0Eq
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