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:

Cozy Colors Hot Different Fleece Slippers House Bear TrendsBlue Pink Baby Fabric amp; alias RecArray = Array(Int32) | Array(RecArray)

ary = [] of RecArray
ary.push [Bear TrendsBlue Colors Baby amp; Different Slippers House Hot Cozy Fleece Pink Fabric 1, 2, 3]
ary.push ary
ary #=> [[1, 2, 3], [...]]Shower Kawa Slide Grey Sandals NIKE Men's Black Dark wfPqnH

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