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:

Speedform Orange Gray Shoe Armour Under Men's Blaze Gemini Running Fire Phoenix 3 Glacier alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Orange Shoe Armour Phoenix Men's Glacier Under Fire Speedform Gray Blaze Gemini 3 Running 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Mizuno Running Men's White Paradox Wave Imperial Shoe Peacoat Blue 4 1H1pqPxnwr

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