Scheme (programming language): Difference between revisions

Revision as of 04:55, 6 November 2023

Revision as of 21:21, 12 November 2023

Line 27:

The Scheme language is standardized in the official [[Institute of Electrical and Electronics Engineers]] (IEEE) standard<ref name="ieee1178">1178-1990 (Reaff 2008) IEEE Standard for the Scheme Programming Language. IEEE part number STDPD14209, [http://standards.ieee.org/board/rev/308minutes.html unanimously reaffirmed] at a meeting of the IEEE-SA Standards Board Standards Review Committee (RevCom), March 26, 2008 (item 6.3 on minutes), reaffirmation minutes accessed October 2009. This document is available from IEEE for purchase only, and not online at time of writing: 2009.</ref> <!--

-->and a ''de facto'' standard called the ''Revised{{padlsup|n}} Report on the Algorithmic Language Scheme'' (R''n''RS). A widely implemented standard is R5RS (1998).<ref name="r5rs">{{Cite journal |lastlast1=Richard Kelsey |last2=William Clinger |last3=Jonathan Rees |last4=Rozas |first4=G.J. |last5=Adams Iv |first5=N.I. |last6=Friedman |first6=D.P. |last7=Kohlbecker |first7=E. |last8=Steele Jr. |first8=G.L. |last9=Bartley |first9=D.H. |display-authors=3 |date=August 1998 |title=Revised⁵ Report on the Algorithmic Language Scheme |url=http://www.schemers.org/Documents/Standards/R5RS/ |journal=Higher-Order and Symbolic Computation |volume=11 |issue=1 |pages=7–105 |doi=10.1023/A:1010051815785 |access-date=2012-08-09 |s2cid=14069423}}</ref> <!--

-->The most recently ratified standard of Scheme<!--

--> is "R7RS-small" (2013).<ref name="r7rs">{{Cite web |date=2013-07-06 |title=R7RS final available |url=http://trac.sacrideo.us/wg/raw-attachment/wiki/WikiStart/r7rs.pdf}}</ref> The more expansive and modular R6RS was ratified in 2007.<ref name="r6rs">{{Cite web |lastlast1=Sperber |firstfirst1=Michael |last2=Dybvig |first2=R. Kent |last3=Flatt |first3=Matthew |last4=Van Straaten |first4=Anton |display-authors=etal |date=August 2007 |title=Revised⁶ Report on the Algorithmic Language Scheme (R6RS) |url=http://www.r6rs.org |access-date=2011-09-13 |publisher=Scheme Steering Committee}}</ref> Both trace their descent from R5RS; the timeline below reflects the chronological order of ratification.<!--

-->

Line 36:

===Origins===

Scheme started in the 1970s as an attempt to understand [[Carl Hewitt]]'s [[Actor model]], for which purpose Steele and Sussman wrote a "tiny Lisp interpreter" using [[Maclisp]] and then "added mechanisms for creating actors and sending messages".<ref name="revisited">{{Cite journal |lastlast1=Sussman |firstfirst1=Gerald Jay |last2=Steele |first2=Guy L. |date=1 December 1998 |title=The First Report on Scheme Revisited |journal=Higher-Order and Symbolic Computation |volume=11 |issue=4 |pages=399–404 |doi=10.1023/A:1010079421970 |s2cid=7704398}}</ref> Scheme was originally called "Schemer", in the tradition of other [[Lisp (programming language)|Lisp]]-derived languages such as [[Planner (programming language)|Planner]] or ''Conniver''. The current name resulted from the authors' use of the [[Incompatible Timesharing System|ITS operating system]], which limited filenames to two components of at most six characters each. Currently, "Schemer" is commonly used to refer to a Scheme programmer.

===R6RS===

Line 45:

Currently the newest releases of various Scheme implementations<ref name="rs6s_Implementations">{{Cite web |title=R6RS Implementations |url=http://www.r6rs.org/implementations.html |access-date=2017-11-24 |publisher=r6rs.org}}</ref> support the R6RS standard. There is a portable reference implementation of the proposed implicitly phased libraries for R6RS, called psyntax, which loads and bootstraps itself properly on various older Scheme implementations.<ref name="psyntax">{{Cite web |last=Abdulaziz Ghuloum |date=2007-10-27 |title=R6RS Libraries and syntax-case system (psyntax) |url=https://www.cs.indiana.edu/~aghuloum/r6rs-libraries/ |access-date=2009-10-20 |publisher=Ikarus Scheme}}</ref>

A feature of R6RS is the record-type descriptor (RTD). When an RTD is created and used, the record type representation can show the memory layout. It also calculated object field bit mask and mutable Scheme object field bit masks, and helped the garbage collector know what to do with the fields without traversing the whole fields list that are saved in the RTD. RTD allows users to expand the basic RTD to create a new record system.<ref>{{Cite journal |lastlast1=Keep |firstfirst1=Andrew W. |last2=Dybvig |first2=R. Kent |date=November 2014 |title=A run-time representation of scheme record types |journal=Journal of Functional Programming |volume=24 |issue=6 |pages=675–716 |doi=10.1017/S0956796814000203 |s2cid=40001845|doi-access=free}}</ref>

R6RS introduces numerous significant changes to the language.<ref name="r6rs_Language_changes">{{Cite web |date=2007-09-26 |title=Revised^6 Report on the Algorithmic Language Scheme, Appendix E: language changes |url=http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-19.html#node_chap_E |access-date=2009-10-20 |publisher=Scheme Steering Committee}}</ref> The source code is now specified in [[Unicode]], and a large subset of Unicode characters may now appear in Scheme symbols and [[identifier]]s, and there are other minor changes to the lexical rules. Character data is also now specified in Unicode. Many standard procedures have been moved to the new standard libraries, which themselves form a large expansion of the standard, containing procedures and syntactic forms that were formerly not part of the standard. A new module system has been introduced, and systems for exception handling are now standardized. Syntax-rules has been replaced with a more expressive syntactic abstraction facility (syntax-case) which allows the use of all of Scheme at macro expansion time. Compliant implementations are now ''required'' to support Scheme's full [[Numerical tower|numeric tower]], and the semantics of numbers have been expanded, mainly in the direction of support for the [[IEEE 754-1985|IEEE 754]] standard for floating point numerical representation.

Line 67:

===Minimalism===

{{Main|Minimalism (computing)}}

Scheme is a very simple language, much easier to implement than many other languages of comparable [[Expressive power (computer science)|expressive power]].<ref name="easy_to_implement_scheme48">The [[Scheme 48]] implementation is so-named because the interpreter was written by Richard Kelsey and Jonathan Rees in 48 hours (August 6th{{spndash}}7th, 1986. See {{Cite web |lastlast1=Richard Kelsey |last2=Jonathan Rees |last3=Mike Sperber |date=2008-01-10 |title=The Incomplete Scheme 48 Reference Manual for release 1.8 |url=http://s48.org/1.8/manual/manual.html |access-date=2012-08-09 |publisher=Jonathan Rees, s48.org}}</ref> This ease is attributable to the use of [[lambda calculus]] to derive much of the syntax of the language from more primitive forms. For instance of the 23 s-expression-based syntactic constructs defined in the R5RS Scheme standard, 14 are classed as derived or library forms, which can be written as macros involving more fundamental forms, principally lambda. As R5RS (§3.1) says: "The most fundamental of the variable binding constructs is the lambda expression, because all other variable binding constructs can be explained in terms of lambda expressions."<ref name="r5rs"/>

: '''Fundamental forms''': define, lambda, quote, if, define-syntax, let-syntax, letrec-syntax, syntax-rules, set!

Line 91:

The impetus to incorporate lexical scoping, which was an unusual scoping model in the early 1970s, into their new version of Lisp, came from Sussman's studies of [[ALGOL]]. He suggested that [[Block (programming)|ALGOL-like lexical scoping mechanisms]] would help to realize their initial goal of implementing [[Carl Hewitt#Actor model|Hewitt's Actor model]] in Lisp.<ref name="revisited"/>

The key insights on how to introduce lexical scoping into a Lisp dialect were popularized in Sussman and Steele's 1975 Lambda Paper, "Scheme: An Interpreter for Extended Lambda Calculus",<ref name="lambda_paper_1">{{Cite journal |lastlast1=Gerald Jay Sussman |last2=Guy Lewis Steele Jr. |name-list-style=amp |date=December 1975 |title=Scheme: An Interpreter for Extended Lambda Calculus |url=https://dspace.mit.edu/bitstream/handle/1721.1/5794/AIM-349.pdf |journal=AI Memos |publisher=[[MIT Computer Science and Artificial Intelligence Laboratory|MIT AI Lab]] |volume=AIM-349 |access-date=23 December 2021 |hdl=1721.1/5794}}</ref> where they adopted the concept of the [[Closure (computer science)|lexical closure]] (on page 21), which had been described in an [[AI Memo]] in 1970 by [[Joel Moses]], who attributed the idea to [[Peter J. Landin]].<!--

--><ref name="Moses">{{Citation |last=Joel Moses |title=The Function of FUNCTION in LISP, or Why the FUNARG Problem Should Be Called the Environment Problem |date=June 1970 |id=[[AI Memo]] 199 |quote=A useful metaphor for the difference between FUNCTION and QUOTE in LISP is to think of QUOTE as a porous or an open covering of the function since free variables escape to the current environment. FUNCTION acts as a closed or nonporous covering (hence the term "closure" used by Landin). Thus we talk of "open" Lambda expressions (functions in LISP are usually Lambda expressions) and "closed" Lambda expressions. [...] My interest in the environment problem began while Landin, who had a deep understanding of the problem, visited MIT during 1966-67. I then realized the correspondence between the FUNARG lists which are the results of the evaluation of "closed" Lambda expressions in [[LISP 1.5|LISP]] and [[ISWIM]]'s Lambda Closures. |author-link=Joel Moses |hdl=1721.1/5854}}</ref>

Line 98:

[[Alonzo Church]]'s mathematical notation, the lambda calculus, has inspired Lisp's use of "lambda" as a keyword for introducing a procedure, as well as influencing the development of [[functional programming]] techniques involving the use of [[higher-order function]]s in Lisp. But early Lisps were not suitable expressions of the lambda calculus because of their treatment of [[Free variables and bound variables|free variables]].<ref name="revisited"/>

A formal lambda system has axioms and a complete calculation rule. It is helpful for the analysis using mathematical logic and tools. In this system, calculation can be seen as a directional deduction. The syntax of lambda calculus follows the recursive expressions from x, y, z, ...,parentheses, spaces, the period and the symbol λ.<ref>{{Cite journal |last=van Tonder |first=André |date=1 January 2004 |title=A Lambda Calculus for Quantum Computation |journal=SIAM Journal on Computing |volume=33 |issue=5 |pages=1109–1135 |arxiv=quant-ph/0307150 |doi=10.1137/S0097539703432165 |s2cid=613571}}</ref> The function of lambda calculation includes: First, serve as a starting point of powerful mathematical logic. Second, it can reduce the requirement of programmers to consider the implementation details, because it can be used to imitate machine evaluation. Finally, the lambda calculation created a substantial meta-theory.<ref>{{Cite journal |lastlast1=Niehren |firstfirst1=J. |last2=Schwinghammer |first2=J. |last3=Smolka |first3=G. |date=November 2006 |title=A concurrent lambda calculus with futures |url=https://hal.inria.fr/inria-00090434/file/0.pdf |journal=Theoretical Computer Science |volume=364 |issue=3 |pages=338–356 |doi=10.1016/j.tcs.2006.08.016}}</ref>

The introduction of lexical scope resolved the problem by making an equivalence between some forms of lambda notation and their practical expression in a working programming language. Sussman and Steele showed that the new language could be used to elegantly derive all the imperative and declarative semantics of other programming languages including ALGOL and [[Fortran]], and the dynamic scope of other Lisps, by using lambda expressions not as simple procedure instantiations but as "control structures and environment modifiers".<ref name="lambda_paper_2">{{Cite journal |lastlast1=Gerald Jay Sussman |last2=Guy Lewis Steele Jr. |name-list-style=amp |date=March 1976 |title=Lambda: The Ultimate Imperative |url=http://library.readscheme.org/page1.html |url-status=dead |format=postscript or PDF |journal=AI Memos |publisher=[[MIT Computer Science and Artificial Intelligence Laboratory|MIT AI Lab]] |volume=AIM-353 |archive-url=https://web.archive.org/web/20160510140804/http://library.readscheme.org/page1.html |archive-date=2016-05-10 |access-date=2012-08-09}}</ref> They introduced [[continuation-passing style]] along with their first description of Scheme in the first of the Lambda Papers, and in subsequent papers, they proceeded to demonstrate the raw power of this practical use of lambda calculus.

===Block structure===

Line 223:

===Shared namespace for procedures and variables===

In contrast to Common Lisp, all data and procedures in Scheme share a common namespace, whereas in Common Lisp [[Common Lisp#The function namespace|functions and data have separate namespaces]] making it possible for a function and a variable to have the same name, and requiring special notation for referring to a function as a value. This is sometimes known as the "[[Lisp-1 vs. Lisp-2]]" distinction, referring to the unified namespace of Scheme and the separate namespaces of Common Lisp.<ref>{{Cite news |lastlast1=Gabriel |firstfirst1=Richard P. |author-link=Richard P. Gabriel |last2=Pitman |first2=Kent |author-link2=Kent Pitman |year=1988 |title=Technical Issues of Separation in Function Cells and Value Cells |volume=1 |pages=81–101 |work=LISP and Symbolic Computation |issue=1 |publication-date=June 1988 |url=http://www.nhplace.com/kent/Papers/Technical-Issues.html |access-date=2012-08-09 |doi=10.1007/BF01806178}}</ref>

In Scheme, the same primitives that are used to manipulate and bind data can be used to bind procedures. There is no equivalent of Common Lisp's <code>defun</code> and <code>#'</code> primitives.

Line 492:

The form of the titles of the standards documents since R3RS, "Revisedⁿ Report on the Algorithmic Language Scheme", is a reference to the title of the [[ALGOL|ALGOL 60]] standard document, "Revised Report on the Algorithmic Language Algol 60," The Summary page of R3RS is closely modeled on the Summary page of the ALGOL 60 Report.<!--

--><ref name="algol_report">{{Cite journal |lastlast1=J.W. Backus |last2=F.L. Bauer |last3=J.Green |last4=C. Katz |last5=J. McCarthy P. Naur |display-authors=etal |date=January–April 1960 |title=Revised Report on the Algorithmic Language Algol 60 |url=http://www.masswerk.at/algol60/report.htm |journal=Numerische Mathematik, Communications of the ACM, and Journal of the British Computer Society |access-date=2012-08-09}}</ref><ref name="r3rs">{{Cite journal |date=December 1986 |editor2-last=William Clinger |title=Revised(3) Report on the Algorithmic Language Scheme (Dedicated to the Memory of ALGOL 60) |url=http://groups.csail.mit.edu/mac/ftpdir/scheme-reports/r3rs-html/r3rs_toc.html |journal=ACM SIGPLAN Notices |volume=21 |issue=12 |pages=37–79 |citeseerx=10.1.1.29.3015 |doi=10.1145/15042.15043 |access-date=2012-08-09 |editor=Jonathan Rees |hdl=1721.1/6424 |s2cid=43884422}}</ref>

== Review of standard forms and procedures ==

Line 649:

== Usage ==

Scheme is widely used by several<ref name="schemers_inc">{{Cite web |last=Ed Martin |date=2009-07-20 |title=List of Scheme-using schools |url=http://www.schemers.com/schools.html |access-date=2009-10-20 |publisher=Schemers Inc.}}</ref> schools; in particular, several introductory [[computer science]] courses use Scheme in conjunction with the textbook ''[[Structure and Interpretation of Computer Programs]]'' (SICP).<ref name="sicp_adopters">{{Cite web |date=1999-01-26 |title=List of SICP-using schools |url=http://mitpress.mit.edu/sicp/adopt-list.html |access-date=2009-10-20 |publisher=MIT Press}}</ref> For the past 12 years, [[Racket (programming language)|PLT]] has run the [[ProgramByDesign]] (formerly TeachScheme!) project, which has exposed close to 600 high school teachers and thousands of high school students to rudimentary Scheme programming. [[MIT]]'s old introductory programming class 6.001 was taught in Scheme,<ref name="6.001">{{Cite web |last=Eric Grimson |author-link=Eric Grimson |date=Spring 2005 |title=6.001 Structure and Interpretation of Computer Programs |url=http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-001Spring-2005/CourseHome/index.htm |access-date=2009-10-20 |publisher=MIT Open Courseware}}</ref> Although 6.001 has been replaced by more modern courses, SICP continues to be taught at MIT.<ref name="6.001_zombies">{{Cite web |lastlast1=Alex Vandiver |last2=Nelson Elhage |display-authors=etal |date=January 2009 |title=6.184 - Zombies drink caffeinated 6.001 |url=http://web.mit.edu/alexmv/6.001/ |access-date=2009-10-20 |publisher=MIT CSAIL}}</ref> Likewise, the introductory class at [[UC Berkeley]], CS 61A, was until 2011 taught entirely in Scheme, save minor diversions into [[Logo (programming language)|Logo]] to demonstrate dynamic scope. Today, like MIT, Berkeley has replaced the syllabus with a more modern version that is primarily taught in [[Python (programming language)|Python 3]], but the current syllabus is still based on the old curriculum, and parts of the class are still taught in Scheme.<ref name="61A">{{Cite web |last=John DeNero |date=Fall 2019 |title=Computer Science 61A, Berkeley |url=https://cs61a.org/articles/about.html |access-date=2019-12-17 |publisher=Department of Electrical Engineering and Computer Sciences, Berkeley}}</ref>

The textbook ''[[How to Design Programs]]'' by Matthias Felleisen, currently at Northeastern University, is used by some institutes of higher education for their introductory computer science courses. Both [[Northeastern University]] and [[Worcester Polytechnic Institute]] use Scheme exclusively for their introductory courses Fundamentals of Computer Science (CS2500) and Introduction to Program Design (CS1101), respectively.<ref name="neu">[http://www.ccs.neu.edu/course/cs2500/ CS 2500: Fundamentals of Computer Science I], [[Northeastern University]]</ref><ref name="wpi">[http://web.cs.wpi.edu/~cs1101/a05/details.html#software CS 1101: Introduction to Program Design (A05): course software], [[Worcester Polytechnic Institute]]</ref> [[Rose-Hulman]] uses Scheme in its more advanced Programming Language Concepts course.<ref name="rhit">{{Cite web |title=CSSE 304: Programming Language Concepts |url=https://www.rose-hulman.edu/Users/faculty/young/CS-Classes/csse304/syllabus.html |publisher=[[Rose-Hulman Institute of Technology]]}}</ref> [[Brandeis University]]'s core course, Structure and Interpretations of Computer Programs (COSI121b), is also taught exclusively in Scheme by theoretical computer scientist [[Harry Mairson]].<ref name="brandeis">{{Cite web |title=Spring 2021 CS121b Syllabus |url=https://moodle2.brandeis.edu/syllabus/public/1202dcdfe19c0860e226f6447f66875c.pdf |publisher=[[Brandeis University]]}}</ref> [[Indiana University]]'s introductory class, C211, is taught entirely in Scheme. A self-paced version of the course, CS 61AS, continues to use Scheme.<ref>{{cite web |url=https://berkeley-cs61as.github.io/ |title=Home |website=berkeley-cs61as.github.io}}</ref> The introductory computer science courses at [[Yale]] and [[Grinnell College]] are also taught in Scheme.<ref name="yale_cs201">{{Cite web |last=Dana Angluin |date=Fall 2009 |title=Introduction to Computer Science (CPSC 201) |url=http://zoo.cs.yale.edu/classes/cs201/ |access-date=2009-10-20 |publisher=The Zoo, Yale University Computer Science Department}}</ref> Programming Design Paradigms,<ref name="neu2">{{Cite web |date=Fall 2009 |title=Programming Design Paradigms CSG107 Course Readings |url=http://www.ccs.neu.edu/home/matthias/107-f08/readings.html |access-date=2012-08-09 |publisher=Northeastern University College of Computer and Information Science}}</ref> a mandatory course for the Computer science Graduate Students at [[Northeastern University]], also extensively uses Scheme.

Line 658:

* The [[Document Style Semantics and Specification Language]] (DSSSL), which provides a method of specifying [[SGML]] [[Style sheet (web development)|stylesheet]]s, uses a Scheme subset.<ref name="dsssl">{{Cite web |last=Robin Cover |date=2002-02-25 |title=DSSSL - Document Style Semantics and Specification Language. ISO/IEC 10179:1996 |url=http://xml.coverpages.org/dsssl.html |access-date=2012-08-09 |publisher=Cover Pages}}</ref>

* The well-known [[Open-source software|open source]] [[raster graphics editor]] [[GIMP]] uses [[TinyScheme]] as a [[scripting language]].<ref name="gimp">"''The major scripting language for the GIMP that has been attached to it today is Scheme.''" From {{Cite web |last=Dov Grobgeld |year=2002 |title=The GIMP Basic Scheme Tutorial |url=http://www.gimp.org/tutorials/Basic_Scheme/ |access-date=2012-08-09 |publisher=The GIMP Team}}</ref>

* [[GNU Guile|Guile]] has been adopted by [[GNU]] project as its official scripting language, and that implementation of Scheme is embedded in such applications as [[GNU LilyPond]] and [[GnuCash]] as a scripting language for extensions. Likewise, Guile used to be the scripting language for the [[desktop environment]] [[GNOME]],<ref name="archive_gnomefaq">{{Cite web |lastlast1=Todd Graham Lewis |last2=David Zoll |last3=Julian Missig |year=2002 |title=GNOME FAQ from Internet Archive |url=http://www.gnome.org/gnomefaq/html/x930.html |archive-url=https://web.archive.org/web/20000522010523/http://www.gnome.org/gnomefaq/html/x930.html |archive-date=2000-05-22 |access-date=2012-08-09 |publisher=The Gnome Team, gnome.org}}</ref> and GNOME still has a project that provides Guile bindings to its library stack.<ref name="live_gnome">{{Cite web |title=guile-gnome |url=https://www.gnu.org/software/guile-gnome/ |access-date=2012-08-09 |publisher=Free Software Foundation}}</ref> There is a project to incorporate Guile into [[GNU Emacs]], GNU's flagship program, replacing the current [[Emacs Lisp]] interpreter.{{Citation needed |reason=Where is the source? |date=May 2019}}

* [[Extension Language Kit|Elk Scheme]] is used by [[Synopsys]] as a scripting language for its [[Technology CAD|technology CAD (TCAD)]] tools.<ref name="tcad">{{Cite web |last=Laurence Brevard |date=2006-11-09 |title=Synopsys MAP-in^SM Program Update: EDA Interoperability Developers' Forum |url=http://www.synopsys.com/community/interoperability/documents/devforum_pres/2006nov/milkywaysession_mapin_overview.pdf |access-date=2012-08-09 |publisher=Synopsis Inc}}</ref>

* Shiro Kawai, senior programmer on the movie ''[[Final Fantasy: The Spirits Within]]'', used Scheme as a scripting language for managing the real-time rendering engine.<ref name="shiro_kawai">{{Cite journal |last=Kawai |first=Shiro |date=October 2002 |title=Gluing Things Together - Scheme in the Real-time CG Content Production |url=http://practical-scheme.net/docs/ILC2002.html |journal=Proceedings of the First International Lisp Conference, San Francisco |pages=342–348 |access-date=2012-08-09}}</ref>

* [[Google App Inventor]] for [[Android (operating system)|Android]] uses Scheme, where [[Kawa (Scheme implementation)|Kawa]] is used to compile the Scheme code down to bytecodes for the [[Java virtual machine]] running on Android devices.<ref name="android">{{Cite web |lastlast1=Bill Magnuson |last2=Hal Abelson |last3=Mark Friedman |name-list-style=amp |date=2009-08-11 |title=Under the Hood of App Inventor for Android |url=http://googleresearch.blogspot.com/2009/08/under-hood-of-app-inventor-for-android.html |access-date=2012-08-09 |publisher=Google Inc, Official Google Research blog}}</ref>

== See also ==

Line 673:

* [ftp://ftp.cs.utexas.edu/pub/garbage/cs345/schintro-v14/schintro_toc.html An Introduction to Scheme and its Implementation] ([http://icem-www.folkwang-hochschule.de/~finnendahl/cm_kurse/doc/schintro/schintro_toc.html a mirror])

* {{Cite web |last=Christopher T. Haynes |date=1999-06-22 |title=The Scheme Programming Language Standardization Experience |url=http://acm.org/tsc/sstd.html}}

* {{Cite web |last=[[Guy L. Steele Jr.]], [[Richard P. Gabriel]] |title=The Evolution of Lisp |url=http://www.dreamsongs.org/Files/HOPL2-Uncut.pdf|archive-url=https://web.archive.org/web/20160611055324/http://www.dreamsongs.org/Files/HOPL2-Uncut.pdf |archive-date=2016-06-11 }}

* {{cite wikisource |author1=Gerald Jay Sussman|author1-link=Gerald Sussman |author2=Guy Lewis Steele Jr.|author2-link=Guy Steele|name-list-style=amp |title=Scheme: An Interpreter for Extended Lambda Calculus |publisher=[[MIT Computer Science and Artificial Intelligence Laboratory|MIT Artificial Intelligence Lab]] |volume=AI Memo 349 |date=December 1975|citeseerx=10.1.1.128.80}}