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:
Iron Inch Boot Work Eight Clad Men's LTT Rocky Brown alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [Iron Men's Boot Clad Eight Inch LTT Rocky Brown Work 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]W3851 Slip White On Propet Women's amp; Wear Wash gXO5wq
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