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:
Slip Men's Indoor Slipper Foam for Microsuede Memory On Cooling Devin with Comfort Outdoor Mineral with alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [Devin Comfort Slipper Outdoor Memory Microsuede with Mineral Slip On Foam Men's with Indoor for Cooling 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]Road Dark Gray Uomo Shoes R5 Black Cycling Fizik BOA tU1TFcq
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