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 André Denim Flat Women's Assous Sandal alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [Nice Flat Assous Denim André Women's Sandal 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]Sneaker Walker Stability Black Grey Medicare Hcpcs Diabetic A5500 Code Shoe Propét Men's qwCzEBA
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