Constructor `cons'

The constructor consists of a synthesis of a head-item and a tail-item.

(define cons 
   (lambda (hd tl)
      (lambda (sel)
         (sel hd tl))))

The abstraction `cons' takes two arguments, the head and the tail of the pair to be built and returns a function taking one argument which represents the new list.

To be precise we should have said the new `pair'. But because a pair has a tail pointing normally to another pair, we effectively have a new list as well. This is true however only if the convention is followed that the last pair in the list has a tail-value of `nil'.

`nil' is a special abstraction required at the end of a list to be able to determine the length of the list which depends on being able to recognize its end. The abstraction `nil', also called the `empty list', is defined as follows:

(define nil 
   (lambda (f)
      true))

Georg P. Loczewski 2004-03-05

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