With 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:

Two Gray Shoe Gray Originals Running Three EQT 17 Men's adidas Support Two 93 Gray alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [17 Originals EQT adidas Two Gray Two Men's Running Shoe Three Gray Gray 93 Support 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]NEOS Explorer Waterproof EXPG 11" Overshoes Winter PPqw7xr

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