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:
Baseball Mizuno Shoes MIZD9 Men's Trainer Players Red Black Turf 2 alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [2 Red Trainer Turf Shoes Mizuno MIZD9 Players Black Men's Baseball 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]Brown Fox Casual Men's Wear Boots Stylish MPX808579 Or Polar Work Roy for RfqwpPxAP
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