The Alien types form a subsystem of the CMU Common Lisp type system. Analien type specifier provides a way to use any Alien type as a Lisp type specifier. For example
Note that the Alien type system overlaps with normal Lisp type specifiers in some cases. For example, the type specifier(alien single-float) is identical to single-float, since Alien floats are automatically converted to Lisp floats. Whentype-of is called on an Alien value that is not automatically converted to a Lisp value, then it will return an alien type specifier.