Or, why long dead programming languages matter

The 50th Anniversary of Algol 60

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 PascalHave you? Contribute anecdotes…

This is a BIG topic.The amount of material available re: Algol is overwhelmingHave done my best to extract the key points

Preliminary Remarks

Algol: the Demon StarAlgol: the ALgOrithmic Language

IAL & Algol 60MotivationsFeatures and contributionsProblemsImplementations (then and now)

The Children of Algol 60“Algol-like” languages

Algol is Dead: Long Live AlgolThe failure of Algol 60The legacy of Algol 60

Agenda

Algol: the Demon Star

• Algol: Beta Persei

• Arabic ra's al-ghūl from head of the demon/ogre/ghoul (Gorgon Medusa)

• Also: Demon Star or Blinking Demon

• Distance: 92.8 light years

• Orbital Period: 2.87 days• Separation: 0.06 AU• Variability recorded in

1667; • probably known much

earlier

Algol images x 2 taken from this video: http://www.citizensky.org/forum/cs-video-and-planetarium-show-wtimothy-ferris

“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)

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)

Algol: The ALgOrithmic Language

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.

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

Early History

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.

Motivations for Algol

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

Algol 58 Report: Guiding Principles

IAL: International Algebraic LanguageCreated 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.

Algol 58, officially IAL

“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)

Quote from 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

Algol 60 Report

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

Focus: machine independence and portabilityStrong, static typing (no implicits)

integer, real, Boolean, arrays, string literals as parametersBlock structure and compound statementsLexical (static) vs dynamic scope, Fortran COMMONFree 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.

Key Features and Contributions

Algol 60 usagecontext circa 1975

Watson, 1975

ArraysUpper and lower limits can be specifiedArbitrary number of dimensionsArbitrary subscript expressionsBounds checked at run-time

ProceduresCall-by-value and Call-by-nameRecursion (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

Key Features and Contributions

Backus-Naur Form to describe syntax

Character set independence via different representations

More on these later

Key Features and Contributions

Free Format & Block Structurebegininteger result;integer procedure fact(n);value n;integer n;beginif n = 0 thenfact := 1elsefact := 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

Fortran COMMON blocksc 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) …

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.

Lexical vs Dynamic Scope

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

Example: Dynamic Scopebegin 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)) endend

A Problem with Dynamic Scopebegin 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)) endend

dynamic: 126

lexical: 26

Call-by-name Parameters

∑n

x =

i=1

1

i —

Call-by-name: Jensen's Devicebegin 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

Jensen's Device Inlinedbegin 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

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/ThunkParameterless, 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.

Call-by-name and "thunks"

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 call-by-name: 2 1

call-by-reference: 2 3

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

Example: Call-by-name is Brokenbegin 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 := i; i := a[i]; a[i] := tempend;

begin integer temp; temp := 1; i := 27; a[27] := tempend; Instead of (i, a[1])

being swapped from(1, 27) to (27, 1) we get:(27, 27) since:

i = 27a[1] = 27 (unchanged)a[27] = 1

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.

Call-by-name is Broken

“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)

Backus-Naur Form (BNF)

Chomsky type 0: recursively enumerableChomsky type 1: context-sensitiveChomsky type 2: context-freeChomsky 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 BNFcurrent tools, e.g. yacc, ANTLR

Chomsky's Language Types

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>

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 dobegin x := i * 2; A[i] := xend

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

Example: BNF from Algol 60 Report

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.

GOTO Considered Harmful

Diminished need for GOTO in Algol vs Fortran:Block structureImproved control constructs

However, still present in Algol

The expense of GOTO in Algol 60GOTO 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

GOTO Considered Harmful

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!

GOTO can be expensivea: 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

Nowmarst compiler (under GPL): generates CA60 interpreter

In the decade following the Algol 60 ReportElliot Algol (Hoare, Dijkstra)Burroughs AlgolVALGOL via META-II compiler-compilerhttp://en.wikipedia.org/wiki/ALGOL_60

Implementations

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)

The three forms of Algol 60

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;

…

The Children of Algol: Simula

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

Algol is Dead: Long Live Algol: Failures

Lack of standard I/O or library writing capabilityfixed in Algol 68 (transput)

Call-by-name problemfixed in Algol 68 (call-by-ref)

Paucity of types, e.g. no complex type, strings as 2nd class citizensfixed in Simula, Algol 68

Baroque language features: for, switch/gotoNo separate compilation

Fixed in Algol 68 (modules), Ada (packages), OO langs

Algol is Dead: Long Live Algol: Failures

We’ve talked about Algol’s contributions:Language innovations

Influenced most subsequent languagesBNF, syntax directed compiler designAlgol 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.

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)

Algol is Dead: Long Live Algol: Legacy

Knuth, D., Backus Normal Form vs Backus Naur Form, CACM, Vol. 7, No. 12, Dec 1964

Naur, P. (Ed), Report on the Algorithmic Language ALGOL 60, CACM, Vol. 3 No. 5, May 1960, p 299–314

Naur, P. (Ed), Revised Report on the Algorithmic Language ALGOL 60, CACM, 1962

Naur, P., The European side of the last phase of the development of Algol 60, ACM SIGPLAN Notices, Vol. 13, No. 6, Aug 1976

Nofre, D., 2010, Unravelling Algol: US, Europe, and the creation of a Programming Language, IEEE Annals of the History of Computing

Perlis, A.J., The American side of the development of Algol, Vol. 13, No. 8, Aug 1978

References

Ceruzzi, P., 2003, A History of Modern Computing, 2nd ed., MIT Press

ACM Turing Award Lectures 1966–1985, Addison-Wesley

MacLennan, 1983, B.J., Principles of Programming Languages: Evaluation and Implementation, HRW

Louden, K.C., 1993, Programming Languages: Principles and Practice, PWS-Kent

Watson, 1975, An Introduction to Algol, Bell

References

http://en.wikipedia.org/wiki/Algol http://stars.astro.illinois.edu/sow/algol.html http://www.constellationsofwords.com/stars/Algol.html http://en.wikipedia.org/wiki/ALGOL http://en.wikipedia.org/wiki/ALGOL_60 http://en.wikipedia.org/wiki/META_II http

://www.citizensky.org/forum/cs-video-and-planetarium-show-wtimothy-ferris

http://rosettacode.org/wiki/ALGOL_68_Genie http://www.masswerk.at/algol60/algol60-sample.htm http://en.wikipedia.org/wiki/Thunk http://www.slideworld.com/slideshows.aspx/Algol-and-Haskell-p

pt-775501 Whetstone Algol:

http://www.mcjones.org/dustydecks/archives/2010/05/16/159/

Links