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:
Lightweight Wu bin Red Sport Fashion Sneakers Breathable Mens Flyknit wei Running Casual Shoes alias RecArray = Array(Int32) | Array(RecArray) ary =  of RecArray ary.push [Shoes bin Sneakers Fashion Wu Breathable Flyknit Lightweight Mens Running Casual wei Sport Red 1, 2, 3] ary.push ary ary #=> [[1, 2, 3], [...]]Ultra Air Shoe Blue 0 Smokey Grey Wolf 2 Running Essential Men's Max NIKE THtwxnpqT
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