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Screamer is an extension of Common Lisp that adds support for nondeterministic
programming. Screamer consists of two levels. The basic nondeterministic level
adds support for backtracking and undoable side effects. On top of this
nondeterministic substrate, Screamer provides a comprehensive constraint
programming language in which one can formulate and solve mixed systems of
numeric and symbolic constraints. Together, these two levels augment Common
Lisp with practically all of the functionality of both Prolog and constraint
logic programming languages such as CHiP and CLP(R). Furthermore, Screamer is
fully integrated with Common Lisp. Screamer programs can coexist and
interoperate with other extensions to as CLIM and Iterate.

In several ways Screamer is more efficient than other implementations of
backtracking languages. First, Screamer code is transformed into Common Lisp
which can be compiled by the underlying Common Lisp system. Many competing
implementations of nondeterministic Lisp are interpreters and thus are far
less efficient than Screamer. Second, the backtracking primitives require
fairly low overhead in Screamer. Finally, this overhead to support
backtracking is only paid for by those portions of the program which use the
backtracking primitives. Deterministic portions of user programs pass through
the Screamer-to-Common-Lisp transformation unchanged. Since in practise, only
small portions of typical programs utilize the backtracking primitives,
Screamer can produce more efficient code than compilers for languages in which
backtracking is more pervasive.

Screamer was written by Jeffrey Mark Siskind and David Allen McAllester, see
file LICENSE for licensing information.

This version of Screamer is based on released version 3.20, and is
being dragged kicking -- and screaming -- into the 21st century:

* Support for Symbolics, AKCL, etc. has been stripped.

* It has been modified to run in ANSI Common Lisp, as opposed to CLtL1/CLtL2.

* Ongoing maintenance work: original screamer.lisp has been split into
  package.lisp and screamer.lisp, ChangeLog.old, and some information
  has been moved over to TODO.

* Ongoing development and documentation work: a new manual is in the making,
  and add support for missing Common Lisp special forms in nondeterministic
  context is planned.

See TODO for the current task list.

Source files part of the distribution not currently referenced in the

screams.lisp - A file containing all of the examples from the Screamer manual
and the two papers ircs-93-03 and aaai93. To use, first compile and load
Screamer and Iterate, compile and load this file, and then type

equations.lisp - A file containing some equations for testing Screamer's
numeric constraint satisfaction procedures. To use, first compile and load
Screamer, compile and load this file, and then type (IN-PACKAGE :SCREAMS).

iscream.el - If you run Lisp on Unix under GNUEmacs using ILisp you can load
this Emacs Lisp file (preferably byte compiled first). You must also then
set the variable SCREAMER:*ISCREAM?* to T. This will enable the Screamer
macro LOCAL-OUTPUT and improve the behavior of Y-OR-N-P and PRINT-VALUES
under ILisp.

Subdirectory papers/ contains the original Screamer manual and papers:

screamer.ps - DVI and Postscript versions of an outdated manual for Screamer.
The code in this manual has some bugs but corrected versions are included in

ircs-93-03.ps - DVI and Postscript versions of a paper describing the
fundamentals of nondeterministic CommonLisp. This paper is available at
Technical Report 93-03 of the University of Pennsylvania Institute for
Research in Cognitive Science. The appropriate BibTeX entry is:
\newcommand{\Screamer}{{\mbox{\sc Screamer}}}
\newcommand{\CommonLisp}{{\mbox{\sc Common Lisp}}}
@string{IRCS = {University of Pennsylvania Institute for Research in Cognitive
	author = {Jeffrey Mark Siskind and David Allen McAllester},
	title = {{\Screamer:} A Portable Efficient Implementation of
		Nondeterministic {\CommonLisp}},
	institution = IRCS,
	year = 1993,
        number = {IRCS--93--03}}
The code in this paper is included in screams.lisp.

aaai93.ps - DVI and Postscript versions of a paper describing the constraint
package included with Screamer. This paper will appear in the Proceedings of
AAAI-93. The appropriate BibTeX entry is:
The code in this paper is also included in screams.lisp.
\newcommand{\Lisp}{{\mbox{\sc Lisp}}}
@string{AAAI93 = {Proceedings of the Eleventh National Conference on
	Artifical Intelligence}}
	author = {Jeffrey Mark Siskind and David Allen McAllester},
	title = {Nondeterministic {\Lisp} as a Substrate for Constraint Logic
	booktitle = AAAI93,
	year = 1993,
	month = jul}

Following are old notes regarding incompatibilities between Screamer 3.20 and
Screamer 2.4 (as eg. described in papers/screamer.ps):

Screamer 3.20 contains numerous bug fixes, performance enhancements and novel
features over Screamer 2.4, the prior widely released version. I do not have
the time to describe all such improvements. Until the completion of a new
Screamer manual you must resort to looking at the source code. At the beginning
of the file there is a fairly extensive change log.

A small number of incompatibilities have been introduced in the transition
from Screamer 2.4 to Screamer 3.20. These are summarized below. Those already
familiar with Screamer should have no difficulty modifying their code modulo
these changes.

1. All Screamer code must be recompiled. The Screamer 3.20 runtime is
incompatibile with the Screamer 2.4 compiler.

2. The function MAP-VALUES has been removed. An expression such as:
(MAP-VALUES function expression)
can be rewritten using the new FOR-EFFECTS macro as follows:
(FOR-EFFECTS (FUNCALL function expression))
The new syntax is every bit as powerful as the old syntax. In fact it is more
powerfull. MAP-VALUES used to require that the function argument be a
deterministic expression while the new syntax makes no such requirement. (Note
that FUNCALL still requires that its first argument evaluate to a deterministic

3. You no longer need to reload Screamer after doing an UNWEDGE-SCREAMER since
Screamer keeps track of which functions are intrinsic and UNWEDGE-SCREAMER
does not purge those functions.

4. The following functions have been renamed:
The original names were inconsistent with the naming convention that every
function ending in V names a lifted version of the function name without the V.
I.e. NUMBERV would have been a lifted version of a function NUMBER but there
is no ground function. NUMBERV was really a lifted version of NUMBERP and thus
should have been named NUMBERPV.

5. A new naming convention has been introduced. All nondeterministic
`generators' now begin with the prefix A- or AN-. This results in the
following name changes:
FLIP            -> A-BOOLEAN
Furthermore, `lifted generators' both begin with A- or AN- and end with V.
This results in the following name changes:

6. The variable *FUZZ* has been eliminated. The functionality of this variable
has been replaced by additional arguments to the REORDER function.

7. REORDER now takes four arguments:
instead of one. The FORCE-FUNCTION is the same as the prior lone argument.
The COST-FUNCTION is a function to be applied to each VARIABLE at each
reordering step to return its cost. Typical values for COST-FUNCTION are
#'DOMAIN-SIZE or #'RANGE-SIZE. The COST-FUNCTION can return NIL which causes
REORDER to not consider that variable for further forcing. ORDER is a two
argument predicate applied to the non-NIL cost functions computed for the
variables at each reordering step. Typical values are #'<, to choose the least
cost, and #'>, to choose the greatest cost variable to force next. TERMINATE?
is a one argument predicate applied to the (non-NIL) cost function computed
for the variable chosen to force next. If TERMINATE? returns T then the
variable reordering and forcing terminates. The following is a typical call
to REORDER used to solve numerical constraints:

         #'(LAMBDA (X) (< X 1E-6))

The following is a typical call to REORDER used to solve symbolic constraints:

         #'(LAMBDA (X) (DECLARE (IGNORE X)) NIL)

8. Instead of the standard Screamer file preamble which used to be:

(IN-PACKAGE :<my-package>)

there is now a different standard preamble. Loading Screamer creates a
predefined package SCREAMER-USER which is useful for small student and
demonstration programs. If you wish your file to be in the SCREAMER-USER
package the single line:


should be placed at the top of the file. In addition:


should be typed to the Listener after loading Screamer. More complex programs
typically reside in their own package. You can place a program in its own
package by using the following preamble to your file:

 <optional defpackage arguments>)