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:
LTT Work Rocky Brown Iron Inch Clad Boot Men's Eight alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [Inch Men's Work Clad Rocky Eight LTT Brown Iron Boot 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]Cerulean Crocband Unisex Crocs Blue Navy Clog Zw7WRxpOq
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