An Introduction to Functional Programming Through Lambda Calculus

- 91 -If the head of a list and the heads of all of its tail lists are not lists then the list is said to be linear. In LISP parlance,an object in a list which is not a list (or a function) is known as an atom.The head and tail may be selected from non-empty lists:HEAD (CONS H T) = HTAIL (CONS H T) = Tbut head and tail selection from an empty list is not permitted:HEAD NIL = LIST_ERRORTAIL NIL = LIST_ERRORConsider, for example, the linear list of numbers we constructed above:CONS 1 (CONS 2 (CONS 3 NIL))The head of this list:is:HEAD (CONS 1 (CONS 2 (CONS 3 NIL)))1The tail of this list:is:TAIL (CONS 1 (CONS 2 (CONS 3 NIL)))CONS 2 (CONS 3 NIL)The head of the tail of the list:is:which is:HEAD (TAIL (CONS 1 (CONS 2 (CONS 3 NIL))))HEAD (CONS 2 (CONS 3 NIL))2The tail of the tail of the list:is:which is:TAIL (TAIL (CONS 1 (CONS 2 (CONS 3 NIL))))TAIL (CONS 2 (CONS 3 NIL))CONS 3 NIL