The 50th Anniversary of Algol 60

1. The 50th Anniversary of Algol 60 Or, why long dead programming languages matter

2. Preliminary Remarks • My motivation? • Developer with an interest in PL design & implementation, paradigms, and history. • Interested in questions like: is PL X suitable for task Y? • Lisp’s 50th 2 years ago: Clojure talk at AJUG in 2008 • Before this talk I hadn’t written any Algol; lots of Pascal • Have you? • Contribute anecdotes… • This is a BIG topic. • The amount of material available re: Algol is overwhelming • Have done my best to extract the key points

3. Agenda • Algol: the Demon Star • Algol: the ALgOrithmic Language • IAL & Algol 60 • Motivations • Features and contributions • Problems • Implementations (then and now) • The Children of Algol 60 • “Algol-like” languages • Algol is Dead: Long Live Algol • The failure of Algol 60 • The legacy of Algol 60

4. Algol: the Demon Star • Distance: 92.8 light years • Orbital Period: 2.87 days • Separation: 0.06 AU • Variability recorded in 1667; • probably known much earlier • Algol: Beta Persei • Arabic ra's al-ghūl from head of the demon/ogre/ghoul (Gorgon Medusa) • Also: Demon Star or Blinking Demon Algol images x 2 taken from this video: http://www.citizensky.org/forum/cs-video-and-planetarium-show-wtimothy-ferris

5. Algol: The ALgOrithmic Language “One member of the ALGOL committee ruefully noted that the name ALGOL, a contraction of Algorithmic Language, was also the name of a star whose English translation was “the Ghoul”. Whatever the reason for its ill-fate, ALGOL nonetheless was influential on later languages.” (Ceruzzi, p 95)

8. Algol: The ALgOrithmic Language “Lisp and Algol, are built around a kernel that seems as natural as a branch of mathematics.” (Metamagical Themas, Douglas Hofstadter) "Here is a language so far ahead of its time that it was not only an improvement on its predecessors but also on nearly all its successors.” (1980 Turing Award Lecture, C.A.R. Hoare) “Algol 60 lives on in the genes of Scheme and Pascal.” (SICP, Abelson & Sussman)

9. Algol 60 Example: Factorial begin integer result; integer procedure fact(n); value n; integer n; fact := if n = 0 then 1 else n * fact(n-1); result := fact(5); outinteger (1, result); outstring (1, "\n") end A familiar style for Pascal, Delphi, Modula-2 programmers.

10. Early History • Zuse published details of an algorithmic language (Plankalkül) in 1948; did not receive much attention for decades. • Rutihauser showed in-principle translation by computer of simple formulae and loop control in 1951. • Boehm published method to translate algebraic formulae in computer notation in 1954. • Similar attempts were made by Glennie (1952, UK) and Liapunov (1959, Russia). • Adams and Laning presented an algorithmic language in 1954; first actually used (Europe). • IBM released Fortran in 1954. Previously in the US, machine-specific assembly languages had been in use. • An automatic computing symposium was held in 1955 in Darmstadt • translation of algorithmic languages into machine code was discussed; • Rutihauser and others stressed the need for a universal, machine-independent algorithmic language. • Flow-Matic (1958): pre-cursor of COBOL; others by Grace Murray Hopper • Math-Matic (1957): intended as improvement over Fortran; “Algol” contender

12. Motivations for Algol • By mid 1950’s, need for a universal machine independent PL was becoming apparent. • Key issue: portability • Passing of time aggravated problem: more PLs and more programs written in those languages • GAMM (a European association for applied mathematics and mechanics) created a committee to look at developing such a language. • In 1957, ACM and user groups such as SHARE formed a committee to “…study and recommend action for the creation of a universal programming language.” (MacLennan, p 99) • In October 1957, GAMM proposed to ACM a joint effort.

13. Algol 58 Report: Guiding Principles • It should be as close as possible to standard mathematical notation and be readable without too much additional explanation. • Naur thinks Algol (in general) failed on this count. • It should be possible to use it for the description of computing processes in publications. • It set the standard for at least next 30 years. • Naur thinks this is the most frequent use of Algol 60. • It should be mechanically translatable into machine programs. • BNF and syntax-directed compiler tools

14. Algol 58, officially IAL • IAL: International Algebraic Language • Created in Zurich meeting over 8 days by 8 people. • Many IBM users suggested abandoning Fortran, backing Algol • IBM decided against that (understandably) • Implementations for many dialects appeared, e.g. • JOVIAL (by Jule’s Schwartz) • Jule’s Own Version of the International Algebraic Language • Still in use by USAF; for military aircraft embedded systems • NELIAC: Navy Electronic Laboratories International Algol Compiler • MAD: Michigan Algorithm Decoder (IBM, UNIVAC). • Early dialects diminished value of universal language committing users to obsolete versions.

15. Quote from Algol 60 Report “What can be said at all can be said clearly; and what we cannot talk about we must pass over in silence.” [Was sich überhaupt sagen läßt, läßt sich klar sagen; und wovon man nicht reden kann, darüber muß man schweigen.] (Ludwig Wittgensten, Tractatus Logico Philosophicus, 1921)

16. Algol 60 Report • Peter Naur (Editor) • Backus, Bauer, Green, Katz, McCarthy, Perlis, Rutihauser, Samelson, Vauquois, Wegstein, van Wijngaarden, Woodger • Formal syntax via Backus-Naur Form (BNF) • Informal, semantics and examples for each syntactic construct • Short: 15 pages! Common Lisp: ~450 pages • In 1970s, Scheme paid tribute to Algol 60 with the Report on the Algorithmic Language Scheme • BNF and formal (operational) semantics • Started out short; even in its 6th revision, still only 90 pages

17. 1958: Preliminary report on Algol, Zürich meeting • intention was to collect comments/criticisms until final November 1959 design meeting; meant to be an interim report • 1958: Informal meeting in Mainz (40 people) • 1959: Copenhagen Algol implementation meeting • led to creation of regular “Algol Bulletin” by Naur • November 1959: Paris UNESCO Algol conference • Backus presented syntax of Algol in a formal notation he had developed; based in part on his work in Fortran • November 1959: ACM Committee • considered comments sent to Communications of the ACM • industry reps, e.g. SHARE, USE participated • December 1959: Preparatory Boston meeting for 7 US delegates • January 1960: International Algol meeting • 7 European representatives selected from Paris • various language changes discussed (see Perlis, 1974) • May 1960: Report on the Algorithmic Language Algol 60 • 1962: Meeting in Rome to resolve remaining errors & ambiguities • 1963: Revised Report on the Algorithmic Language Algol 60

18. Key Features and Contributions • Focus: machine independence and portability • Strong, static typing (no implicits) • integer, real, Boolean, arrays, string literals as parameters • Block structure and compound statements • Lexical (static) vs dynamic scope, Fortran COMMON • Free vs fixed format due to input device variety: • 80 or 90 column punched cards; • Teletype; • Punched paper tape; • Forced programmers to think about code structure. • By contrast: Fortran constrains format, Python enforces structure. • Fortran format and I/O heavily influenced by IBM 704.

19. Algol 60 usagecontext circa 1975 Watson, 1975

20. Key Features and Contributions • Arrays • Upper and lower limits can be specified • Arbitrary number of dimensions • Arbitrary subscript expressions • Bounds checked at run-time • Procedures • Call-by-value and Call-by-name • Recursion (in Lisp previously, not imperative PLs) • Implicit use of stacks for scopes, activation records • Generalised Fortran’s control constructs (if, do) • own keyword ala static variables in C

21. Key Features and Contributions • Backus-Naur Form to describe syntax • Character set independence via different representations • More on these later

22. Free Format & Block Structure begin integer result; integer procedure fact(n); value n; integer n; begin if n = 0 then fact := 1 else fact := n * fact(n-1) end; result := fact(5); print(result) end begin integer result; integer procedure fact(n); value n; integer n; begin if n = 0 then fact := 1 else fact := n * fact(n-1) end; result := fact(5); print(result) end

23. Fortran COMMON blocks c Main program … c common arrays real*8 tvec,xvec common/datapts/tvec(1000000),xvec(1000000) … integer n … subroutine average c common arrays real*8 tvec,xvec common/datapts/tvec(1000000),xvec(1000000) …

24. Lexical vs Dynamic Scope • In dynamic scoping the meanings of statements and expressions are determined by the dynamic structure of the computations evolving in time. • In lexical scoping the meanings of statements and expressions are determined by the static structure of the program. (MacLennan, p 119) • Early versions of Lisp used dynamic scoping. • Before version 5, Perl only had the local keyword which denoted dynamic scoping. Perl 5 added static scoping via my keyword.

25. Example: Lexical Scope begin real procedure f(x); value x; real x; f := x**2 + 1; real procedure sum(n, m, inc); value n,m,inc; real n,m,inc; begin real s,x; s := 0; for x := n step inc until m do s := s + f(x); sum := s end; print(sum(1, 10, 0.1)) end

26. Example: Dynamic Scope begin real procedure sum(n, m, inc); value n,m,inc; real n,m,inc; begin real s,x; s := 0; for x := n step inc until m do s := s + f(x); sum := s end; begin real procedure f(x); value x; real x; f := x**2 + 1; print(sum(1, 10, 0.1)) end end

27. A Problem with Dynamic Scope begin real procedure helper(x); value x; real x; comment Does something expected for f; helper :=x*x; real procedure f(n); value n; real n; f := helper(n) + 1; begin real procedure helper(x); value x; real x; comment Does something else…; helper := x*x*x; print(f(5)) end end lexical: 26 dynamic: 126

28. Call-by-name Parameters n ∑ 1 x = — i i=1

29. Call-by-name: Jensen's Device begin comment Jensen's device using Algol 60's default Call-by-name parameter passing convention.; integer idx; real procedure sum (i, lo, hi, term); value lo, hi; integer i, lo, hi; real term; comment: term and i are passed by name; begin real temp; temp := 0; for i := lo step 1 until hi do temp := temp + term; sum := temp end; print(sum(idx, 1, 100, 1/idx)) end

30. Jensen's Device Inlined begin comment Inlined Jensen's Device.; integer lo, hi; integer idx; real sum; begin real temp; lo := 1; hi := 100; temp := 0; for idx := 1 step lo until hi do temp := temp + 1/idx; sum := temp end; print(sum); end

31. Call-by-name and "thunks" • It is as if the function’s formal parameters are bound to the named entity , e.g. term is bound to ‘1/idx’. • The usual implementation is via so-called thunks. • http://en.wikipedia.org/wiki/Thunk • Parameterless, anonymous, unseen function. • “…delays the computation of a function's argument, and the function forces the thunk to obtain the actual value.” • Value returned is lvalue or rvalue depending upon whether on left or right hand side of an assignment. • Thunks are still used by functional languages such as Haskell and Scala to implement code blocks whose invocation is to be delayed and re-evaluated.

32. Example: Call-by-name: Problem begin integer k; integer array A[1:5]; procedure inc2(i, j); integer i, j; begin i := i+1; j := j+1 end; k := 1; A[k] := 2; A[2] := 0; inc2(k, A[k]); print(k, A[k]) end begin integer k; integer array A[1:5]; k := 1; A[k] := 2; A[2] := 0; begin k := k+1; A[k] := A[k]+1 end; print(k, A[k]) end call-by-reference: 2 3 call-by-name: 2 1

33. Example: Call-by-name is Broken begin integer i; integer array a[1:27]; procedure swap(x,y); integer x,y; begin integer temp; temp := x; x := y; y := temp end; a[1] := 27; i := 1; swap(i, a[i]) end begin integer temp; temp := 1; i := 27; a[27] := temp end; Instead of (i, a[1]) being swapped from (1, 27) to (27, 1) we get: (27, 27) since: i = 27 a[1] = 27 (unchanged) a[27] = 1 begin integer temp; temp := i; i := a[i]; a[i] := temp end;

34. Call-by-name is Broken • It is not possible to write a general swap procedure in Algol 60. Call-by-name (aka call-by-name) has odd side effects. Some combinations of parameters work. • Algol 68 and other languages since Algol 60 have abandoned call-by-name in favour of the now familiar (and less expensive) call-by-reference in which the address of the object is passed. • Thunks still have a role to play, as mentioned.

35. Backus-Naur Form (BNF) “Peter Naur, then the editor of the Algol Bulletin, was surprised because Backus’s definition of Algol-58 did not agree with his interpretation of the Algol-58 report. He took this as an indication that a more precise method of describing syntax was required and prepared some samples of a variant of the Backus notation. As a result, this notation was adopted for the Algol-60 report…” (MacLennan, p 101)

36. Chomsky's Language Types • Chomsky type 0: recursively enumerable • Chomsky type 1: context-sensitive • Chomsky type 2: context-free • Chomsky type 3: regular • BNF corresponds directly to Chomsky type 2. • Enabled mathematical analysis of PL grammar. • Led to development of automatic parser generators • e.g. META II used for VALGOL implementation • later, extended BNF • current tools, e.g. yacc, ANTLR

37. Example: BNF from Algol 60 Report <for list element> ::= <arithmetic expression> | <arithmetic expression> step <arithmetic expression> until <arithmetic expression> | <arithmetic expression> while <Boolean expression> <for list> ::= <for list element> | <for list> , <for list element> <for clause> ::= for <variable> := <for list> do <for statement> ::= <for clause> <statement> | <label>: <for statement>

38. Example: BNF from Algol 60 Report • Examples of for loop syntax based upon BNF: for q:=1 step s until n do A[q]:=B[q] for i := 1 step 1 until 100 do begin x := i * 2; A[i] := x end for k:=1,V12 while V1<N do for days := 31, if leap then 29 else 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 do

39. GOTO Considered Harmful • Edsger Dijkstra wrote a letter to the editor of the March 1968 CACM that appeared as “GoTo-Statement Considered Harmful”: “For a number of years I have been familiar with the observation that the quality of programmers is a decreasing function of the frequency of go to statements in the programs they produce.” (Ceruzzi, p 100) • This resulted in a “goto controversy” that lasted a few years. • Added to the push towards Structured Programming.

40. GOTO Considered Harmful • Diminished need for GOTO in Algol vs Fortran: • Block structure • Improved control constructs • However, still present in Algol • The expense of GOTO in Algol 60 • GOTO statements can be complex, e.g. • goto if a<b then exit else L[if b<=3 then b else 4]; • Popping of activation records

41. begin integer a, b; switch L := first, second, third, exit; inreal(0, a); inreal(0, b); goto if a<b then exit else L[if b<=3 then b else 4]; first: print("first"); goto exit; second: print("second"); goto exit; third: print("third"); goto exit; exit: end ERK!

42. GOTO can be expensive a: begin b: begin integer i; for i:=1 step 1 until 100 do begin outinteger(1, i*i); if i*i > 9 then goto exit end end; comment activation records must be popped to get here; worse if goto from within recursive procedure.; exit: outstring(1, "exited\n") end

43. Implementations • Now • marst compiler (under GPL): generates C • A60 interpreter • In the decade following the Algol 60 Report • Elliot Algol (Hoare, Dijkstra) • Burroughs Algol • VALGOL via META-II compiler-compiler • http://en.wikipedia.org/wiki/ALGOL_60

44. The three forms of Algol 60 • Reference Language • The working language of the committee • The defining language; used in Algol 60 Report • Character set chosen to illuminate • Basic reference and guide for compiler builders • Publication Language • To be used for communicating algorithms • Character set may be richer, may vary by country • Hardware Representation • Constrained by input device and character set of target computer • Must specify rules to map from reference language • See Algol 60 Report (Naur 1960)

46. The Children of Algol: Simula class Complex(x,y); real x,y; begin real re, im; real procedure RealPart; begin RealPart := re; end; … procedure Add(other); begin re := re + other.RealPart; …

47. Algol is Dead: Long Live Algol: Failures • Algol was competing against Fortran in the scientific languages arena. • European committee members more committed to cause of single universal language than US members who were engaged in other language projects. (Perlis, 1974) • Nofre (2010) suggests Algol’s lack of adoption had more to do with diversity vs uniformity: • UNCOL (SHARE sponsored) vs Algol 60

48. Algol is Dead: Long Live Algol: Failures • Lack of standard I/O or library writing capability • fixed in Algol 68 (transput) • Call-by-name problem • fixed in Algol 68 (call-by-ref) • Paucity of types, e.g. no complex type, strings as 2nd class citizens • fixed in Simula, Algol 68 • Baroque language features: for, switch/goto • No separate compilation • Fixed in Algol 68 (modules), Ada (packages), OO langs

49. Algol is Dead: Long Live Algol: Legacy • We’ve talked about Algol’s contributions: • Language innovations • Influenced most subsequent languages • BNF, syntax directed compiler design • Algol 60 as a publication language • set the standard for years • Also influenced machine architecture, e.g. dynamic allocation of storage for variables (ICL, Burroughs). • Many processors today have machine instructions that cater for Algol-style activation records. • But, experience with machine implementation was also leading this way.

50. Algol is Dead: Long Live Algol: Legacy “Algol’s is the linguistic form most widely used in describing the new exotic algorithms…Where important new linguistic inventions have occurred, they have been imbedded usually within an Algol framework, e.g. records, classes, definitions of types and their operations,…,modules. Algol has become so ingrained in our thinking about programming that we almost automatically base investigations in abstract programming on an Algol representation to isolate, define, and explicate our ideas…It was a noble begin but never intended to be a satisfactory end.” (Perlis, 1978)