Quid Pro Quo

A contract programming library for Common Lisp in the style of Eiffel?s Design by Contract??.

What is it all about?

One main goals of every program is reliability, that is, correctness and robustness. A program is correct if it performs according to its specification, it is robust if it handles situations that were not covered in the specification in a graceful manner. One way to prove the correctness of a program with respect to a (formal) specification is the Hoare calculus. Based on this formal method Bertrand Meyer developed a method of software engineering called Design by Contract??.

The principal idea of contract programming is that a class and its clients have a contract with each other: The client must guarantee certain conditions before calling a method specialized on the class (the preconditions), the class guarantees certain properties after the call (the postconditions). If the pre- and postconditions are included in a form that the compiler can check, then any violation of the contract between caller and class can be detected immediately.

Support for Contract Programming in Programming Languages

The language that offers the best support for contract programming is Eiffel, designed by Bertrand Meyer. It is rather difficult to add support for contract programming to most other languages, but not so for Common Lisp: I [Matthias H?lzl] have written a package for Common Lisp that provides support for contract programming. It is still very new and not too well tested so you should expect some rough edges and changes in its future design. There is no larger program depending on this package available, only some silly test cases. Since I intend to use the Quid Pro Quo package for my own programs this should change in the not so distant future.

Contract Programming in Common Lisp.

One of the outstanding features of the Eiffel language is that it supports a concept called contract programming. A comprehensive description is given in the following books

Object Oriented Software Construction, 2nd ed. Bertrand Meyer Prentice Hall PTR, 1997 ISBN 0-13-629155-4

Eiffel: The Language, 2nd ed. Bertrand Meyer Prentice Hall PTR, 1992 ISBN ???

but the key point of contract programming is that the relationship between a class and its clients is specified by a contract: There are certain conditions that the caller of a method specialized on a class has to fulfill so that the method can do its job (the preconditions) and the method guarantees certain things after its completion (the postconditions). Furthermore a class may have certain properties that are always true about that class; these properties are called invariants.

This file contains an implementation of contract programming for CLOS. Pre- and postconditions as well as invariants are specified by qualified methods of type contract; the usual before, after and around method combinations are available for these methods as well.

Implementation Support

  • ABCL ? YES as of 1.1.1
  • Allegro ? YES (both ANSI & modern)
  • CLISP ? YES but creation invariants are ignored (33 pass, 4 fail)
  • Clozure ? YES
  • Corman ? NO, it?s not supported by Closer-MOP
  • LispWorks ? currently fails tests (10 pass, 25 fail)
  • SBCL ? YES
  • Scineer ? ? (would be happy to find someone to test it)


Preconditions (defrequirement) and postconditions (defguarantee) are added to functions. This works for both generic and non-generic functions (but contracts on non-generic functions may do nothing on certain lisp implementations).

(defrequirement put (item (stack stack))
  "the stack is not full"
  (declare (ignore item))
  (not (full stack)))

(defguarantee put (item (stack stack))
  (and (not (empty stack))
       (eq (top-item stack) item)
       (= (count stack) (1+ (old (count stack))))))

(defguarantee pop-stack ((stack stack))
  (and (not (full stack))
       (eq (results) (old (top-item stack)))
       (= (count stack) (1- (old (count stack)))))

This simple example illustrates a few things:

  • the docstring is included in any contract failure messages,
  • the macro old is available in postconditions so that state from before the call can be compared to the state after the call, and
  • the function results is available in postconditions which returns the same values returned by the primary method.

These contracts can also be created similarly to :before and :after methods, primarily for defgeneric convenience. In this case, the optional description of what is being required or guaranteed can be included between the method qualifier and the lambda list (this is because the docstring is not necessarily accessible).

(defgeneric put (item stack)
  (:method :require "the stack is not full" (item (stack stack))
    (declare (ignore item))
    (not (full stack)))
  (:method :guarantee (item (stack stack))
    (and (not (empty stack))
         (eq (top-item stack) item)
         (= (count stack) (1+ (old (count stack))))))

(defmethod pop-stack :guarantee ((stack stack))
  (and (not (full stack))
       (eq (results) (old (top-item stack)))
       (= (count stack) (1- (old (count stack)))))

Invariants can be placed on classes.

(defclass stack ()
  ((capacity :initarg :capacity :reader capacity :type integer)
   (count :initform 0 :reader count :type integer)
   (top-item :reader top-item))
  (:metaclass contracted-class)
  (:invariants (lambda (instance)
                 "the count must be between 0 and the capacity"
                 (<= 0 (count instance) (capacity instance)))
               (lambda (instance)
                 "there is no top-item if the stack is empty"
                 (implies (empty instance)
                          (not (slot-boundp instance 'top-item))))))

In order to have invariants on a class, the metaclass must be specified as contracted-class.

Invariants are added to classes explicitly with the :invariants option, which allows you to specify any number of predicates that take the instance as their only argument. When available (depending on the Lisp implementation), the documentation string for the function is used. If no documentation is available, we fall back to the body (in the case of a lambda) or the function name and its documentation as the description.

This also illustrates another macro that is useful in contracts ? implies. With implies, the second argument is only tested if the first argument is true.

Types are also checked as invariants. Most implementations check slot types little enough that it's possible for a bad value to end up there in some cases.

The description is included in the report if a contract-violation-error is raised. The description is also added to the documentation for the class, function, or primary method as appropriate. Slot type declarations are also added to the class documentation.

Matthias Hoelzl <tc@gauss.muc.de>
Greg Pfeil <greg@technomadic.org>
Public Domain
design by contract, language extension, public domain