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:

Oumanke Blue4 Running Shoes Men Sneakers Traveling Jogging Hiking Yoga Sports Breathable Fishing alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Men Yoga Blue4 Fishing Breathable Shoes Oumanke Sneakers Running Hiking Jogging Traveling Sports 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Green 8 Black Leather ARTISTS KRAZY Men's Jungle Combat Boot Inch SHOE Tactical n0TRCxqI8w

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