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1. The ALGOL Family
CSE341
Programming Languages

2. Overview
 Historical perspective
 ALGOL Goals
 BNF (Backus-Naur Form)
 ALGOL 58
 ALGOL 60
 ALGOL 68
 Success and Failure of ALGOL
 Conclusion

3. History
 ALGOrithmic Language
 Developed in Europe by an international group,
consisting of 7 different countries, in the late 1950’s
 Very similar to FORTRAN
 Peter Naur and J.W. Backus worked on the project.
Was the debut of the BNF syntax.
 Designed specifically for programming scientific
computations

4. History (Continued)
 Never became as commercially popular as FORTRAN
or COBOL
 Was not compatible with IBM
 Is considered the most important programming language
in terms of influence on later language development
 Many similar languages can (and are) referred to as
“ALGOL-like”
 JAVA, C, C++, Pascal, Ada, FORTRAN, etc.

5. ALGOL Goals
 To be as close as possible to standard math notation
 Also very readable without much more explanation
 Should be possible to use it to describe algorithms in publications
 A form of it is still used today
 Should be mechanically translatable into machine language
programs

6. BNF (Backus-Naur Form)
 Was first to use BNF (Backus-Naur Form)
 Same Backus that created Fortran
 He also was one of the main creators of Algol
 And he created functional programming
 And won the Turing award in ’77
 Right. Back to BNF
 BNF example:
 <value> := <number> | <variable> | <expression>
 <number> := <integer> | <float>
 <integer> := <integer><digit> | <digit>
 <digit> := 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
 Language was designed by committee
 Report is a “paradigm of brevity and clarity”
 Most languages today require 1000’s of pages
 Brevity and clarity contributed to reputation as simple, elegant language

7. Three Language Versions
 Reference language
 Used by the committee, described in the report, and used in
official Algol publications
 Publication language
 Allowed for differences in the character set for different
languages
 Europeans and Americans couldn’t decide on which character to
use for the decimal point!
 Hardware representations
 Condensed languages for machine input

8. ALGOL 58
The beginning
 June 1957 – ACM requested a committee to
look into a universal programming language
 European and American groups got together in
Zurich.
 No current language covers everything
 Creating more non-ideal languages doesn’t help the
situation
 Each passing month more people start using other
languages
 How can the logical structure of existing languages be
adjusted

9. ALGOL 58
New Features
 Types
 Integer
 Real
 Boolean
 Formal vs Actual parameters
 Declaration
 for statements
 Switch
 Compound statements
 Begin end delimiters
 Three level language description
 Call by name

10. ALGOL 58
Function vs Procedure
 Nature of the body
 Expression
 Compound statement
 Parameter evaluation mechanism
 Value of any actual parameter
 Text of any actual parameter
 Identifier collision
 Non-parameter identifiers are global to the definition
 Communication only through parameters
 Place of use
 An operand in an expression
 A statement or an operand in an expression

11. Call by name vs. by value
 Call by name is default
 Call by name: re-evaluate the actual parameter on every
use
 For actual parameters that are simple variables, it’s
the same as call by reference
 For actual parameters that are expressions, the
expression is re-evaluated on each access
 No other language ever used call by name…

12. Call by name
begin
integer n;
procedure p (k: integer) parameter is n+10 (not just 10)
begin
print (k); prints n+10, which is 10
n := n+1; n is still 0; thus, n becomes 1
print (k); prints n+10, which is 11
end;
n is set to 0
n := 0; parameter is n+10 (not just 10)
p (n+10);
end;

13. Example
 Computing Illustrate ( Compute The Mean ):
// the main program (this is a comment)
begin
integer N;
Read Int(N);
begin
real array Data[1:N];
real sum, avg;
integer i; sum:=0;
for i:=1 step 1 until N do
begin real val;
Read Real(val);
Data[i]:=if val<0 then -val else val
end;
for i:=1 step 1 until N do
sum:=sum + Data[i];
avg:=sum/N;
Print Real(avg)
end
end

14. Example(Continued)
 I/O Illustrate: because ALGOL had no IO facilities. The following
code could run on an ALGOL implementation for a Burroughs A-
Series mainframe.
BEGIN
FILE F (KIND=REMOTE);
EBCDIC ARRAY E [0:11];
REPLACE E BY "HELLO WORLD!";
WHILE TRUE DO
BEGIN
WRITE (F, *, E);
END;
END.

15. Problems with Algol58
 Didn’t include a I/O library
 Thus, each implementation had a different
means for I/O
 This caused compatibility problems
 And no standard way to write a Hello World
program

16. ALGOL 60
 Basic Language of 1960
 Simple imperative language + functions
 Successful syntax, BNF -- used by many successors
 statement oriented
 begin … end blocks (like C { … } )
 if … then … else
Recursive functions and stack storage allocation
 Type discipline was improved by later languages
 Very influential but not widely used in US
 Tony Hoare: “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 of its successors.”

17. ALGOL 60
Proposed Changes
 The empty statement was adopted
 Modifications to operation hierarchy
 Unary operators having higher precedence than binary operators was adopted
 Implied multiplication was rejected
 Relation sequences were rejected
 The else clause was adopted
 Strings, lists, trees, matrices, and complex numbers were rejected as types
 Multiple assignment statements were adopted
 Created a new problem, A[i] := i := e
 Recursion became possible in obvious ways
 Constants were rejected

18. ALGOL 60
What’s New
 Block
 Call by value/name
 Typed procedures
 Declaration scope
 Dynamic arrays
 Side effects
 Global and local variables
 Step, until, while, if then else
 Activation records
 Recursive decent parsers
 No I/O

19. Algol 60 Sample
real procedure average(A,n);
real array A; integer n; no array bounds
begin
real sum; sum := 0;
for i = 1 step 1 until n do
sum := sum + A[i];
average := sum/n no ; here
end;
set procedure return value by assignment

20. Algol Oddity
 Question:
 Is x := x equivalent to doing nothing?
 Interesting answer in Algol
integer procedure p;
begin
….
p := p
….
end;
 Assignment here is actually a recursive call

21. Problems with Algol60
 Holes in type discipline
 Parameter types can be arrays, but
 No array bounds
 Parameter types can be procedures, but
 No argument or return types for procedure parameters
 Problems with parameter passing mechanisms
 Pass-by-name “Copy rule” duplicates code, interacting
badly with side effects
 Pass-by-value expensive for arrays
 Some awkward control issues
 goto out of block requires memory management

22. ALGOL 68
 Considered difficult to understand
 Idiosyncratic terminology
 Types were called “modes”
 Arrays were called “multiple values”
 Used vW grammars instead of BNF
 Context-sensitive grammar invented by van Wijngaarden
 Elaborate type system
 Complicated type conversions
 Fixed some problems of Algol 60
 Eliminated pass-by-name
 Not widely adopted

23. ALGOL 68
What’s New modes
Many new types, called
 Primatives
 Bits
 Bytes
 String
 Sema (a semaphore)
 Complex
 File
 Pipe
 Channel
 format
 Additional
 Flex (Flexible array)
 Heap (space on the heap)
 Loc (local space on the stack)
 Ref (pointer)
 Long (bigger ints/reals)
 Short (smaller ints/reals)
 Declaration of custom types/modes

24. Algol 68 Modes
Primitive modes Compound
Modes
 int --arrays
 Real --structures
 Char --procedures
 bool --sets
 string --pointers
 Compl(complex)
 bits
 bytes
Rich, structured, and orthogonal
type system is a major contribution
 Sema (semaphore)
of Algol 68.
 Format (I/O)
 file

25. Other Features of Algol 68
 Storage management
 Local storage on stack
 Heap storage and garbage collection
 Parameter passing
 Pass-by-value
 Use pointer types to obtain pass-by-reference
 Assignable procedure variables

26. Structure
 ALGOL68 is block structured w/ static scope rules
 ALGOL68's model of computation:
 static
 stack: block/procedure AR's; local data objects
 heap: “heap” -- dynamic-- data objects
 ALGOL68 is an expression-oriented language

27. Basic Syntax
 Addition : “ + ”
 Subtraction : “ - ”
 Multiplication : “ * ”
 Division : “ / ”
 Exponentiation : “ ** ”
 Assignment : “ := ”
 Boolean Expressions
 = , > , < , <= , >= , /=

28. Recursion
 Algol 68 Supports recursion
Example:
real procedure factorial (n);
begin
if n = 1 then
factorial := 1;
else
factorial := n* factorial(n-1);
end;

29. Arrays
 Three types of arrays: real, integer, Boolean
 Each array must contain all the same types
 All arrays are of type real unless specified
 Can have multidimensional arrays
 Declarations:
 array name1[1:100]; (1D array of type real)
 real array name2(-3:6,20:40); (2D array of type real)
 integer array name3,name4(1:46);(2 1D arrays of type integer)
 Boolean array name5(-10:n); (1D array of type Boolean)
(Allocated Dynamically)

30. Block Structure
 First language to implement a block structure
 Similar in form to pascal
begin
…..
end;
 Each block can have its own variables, visible only to
that block (local variables). After the block is exited the
values of all the local variables are lost.

31. Block Structure example
 Example:
begin
own integer i; integer j,k;
i := j + k;
end;
 The integer i will have the value of j+k stored the next time
the block is entered
 By using the “own” statement the variable will retain its
value for the next time the block is entered

32. Parameter Passing
 Two types of parameter passing: by Value, by Name
 Pass by Value works the same as in most other languages
 Pass by Name is similar to pass by reference, but it adds flexibility
 All parameters are pass by name unless otherwise specified
 Example: can make a call “sum(i,2,5,x+6)” to the procedure sum
procedure sum(i,j,k,l); value i,j,k;
begin
i := i + j + k + l
end;
(will execute as i := i + 2 + 5 + (x+6))

33. ALGOL
Successes and Failures
 Programming computers – Partial Success
 Core language is strong, no I/O is a serious shortcoming
 Publication of algorithms – Very Successful
 Stimulus to compiler design – Success with a seed of
corruption
 It only stimulated compiler design because it was difficult
 Stimulated formal language research – Success
 Not the goal of the ALGOL effort
 Description of ALGOL 60 – Failure
 Difficult to understand for the uninitiated reader
 Needs an informal explanation

34. Conclusion
 General purpose algorithmic language with a clean
consistent and unambiguous syntax.
 Comprehensive fully-checked type-system covering
structures, unions, pointers, arrays and procedures.
 Procedures may be nested inside procedures and can
deliver values of any type without you having to worry
about where the storage is coming from.
 User-defined operators including user-defined operator
symbols.
 Powerful control structures can deliver values of any
type.

35. Conclusion (Continued)
 Dynamic sized arrays know their current bounds.
 Array and structure displays can be used in any context.
 Parallel programming with semaphores.
 Complex arithmetic.
 Declarations can be interleaved with statements.
 Clear distinction between value semantics and reference
semantics.
 No distinction between compile-time constants and run-
time values.

36. References
 Lindsey, C.H., “A History of ALGOL 68.” History of Programming
Languages, ACM Press. 1993. 97-132
 Naur, Peter, “Successes and failures of the ALGOL effort”, ALGOL
Bulletin #28, Computer History Museum. 1986. 58-62
 Thomson, C.M., “Algol 68 as a Living Language”, ALGOL Bulletin
#47, Computer History Museum. 1981. 21-24
 Perlis, Alan, “The American side of the development of Algol”, The
first ACM SIGPLAN conference on History of programming
languages, ACM Press. 1978. 3-14
 Naur, Peter. “The European side of the last phase of the
development of ALGOL 60”, The first ACM SIGPLAN conference on
History of programming languages, ACM Press. 1978. 15-44